## Abstract

Laser-induced forward transfer (LIFT) is a well-established, versatile additive manufacturing technology for orifice-free printing of highly viscous solutions and suspensions. In order to improve the efficiency of point-wise LIFT printing, an optical scanner is integrated into the laser printing system to enable the formation of overlapping adjacent jets used for deposition. The objective of this study is to evaluate the ejection behavior and deposition performance under such conditions during LIFT printing for further improvement. The effects of the overlap of adjacent jets are investigated in terms of jet formation and material deposition processes, capturing the jet tilting phenomenon caused by the perturbance induced by previously formed jet(s). The feasibility of optical scanner-assisted LIFT printing of viscous metal-based ink suspension has been successfully demonstrated during conductive line printing with induced overlapping jets. Investigation of various overlap ratios of adjacent jets found that a 30% jet overlap and a 133 µs time interval between laser pulses are optimal, in terms of deposition quality and ejection stability, even when a tilted jet ejection is present for the laser and material system in this study. Furthermore, multilayer polygonal and interdigitated structures are successfully deposited under these identified printing conditions. With the inclusion of an optical scanner, LIFT printing efficiency for viscous inks can be improved as the usage of higher laser frequencies is enabled, providing a faster orifice-free laser printing methodology.

## 1 Introduction

Laser-induced forward transfer (LIFT) [1,2], as a material jetting process for additive manufacturing (AM), has been extensively implemented for a wide range of printing applications: from micro-electronics [3] to bio-materials used in living cells [46], to name a few. The droplet-on-demand LIFT technology utilizes orifice-free dispensing, which also enables printing of higher viscosity materials than other nozzle-based processes such as those used in inkjet printing [7,8]. During the LIFT printing process, the material being transferred is first coated in a form of a thin layer on a transparent donor substrate, which is collectively known as a ribbon. A laser pulse is then focused on the interface between the substrate surface and the material coating. As this material absorbs the incident laser energy, a bubble can form and expand which can lead to the formation of a jet off the coated material. This jet can also break apart, under certain conditions, into one or several droplets [9] which can be studied and utilized as potential building blocks for further printing applications. Although LIFT provides a powerful and broad approach to printing various materials, the point-wise process being utilized, as well as the efficiency of the processes, can be improved upon to facilitate wider adoption within the additive manufacturing field.

During conventional LIFT, the laser position is stationary and the receiving substrate is mechanically moved to build a three-dimensional (3D) structure. Unfortunately, the inertia of printed structures on the receiving substrate might jeopardize their structural integrity under fast and abrupt motions. Also, this method cannot be used for plane projection, as in stereolithography, because the laser pulse lacks sufficient power to induce the material-laser interaction for material transfer across planes larger than that of the laser spot itself [10]. Therefore, moving the receiving substrate quickly is not recommended. Rather, the use of a dynamic laser is an alternative which overcomes these issues. The integration of an optical scanner into the laser printing system, to guide the reflection of the incident laser beam, improves the efficiency of point-wise LIFT printing by enabling the formation of overlapping adjacent jets. Although various studies have investigated the ejection patterns of adjacent jets from an experimental perspective [1113], none have investigated, either qualitatively or quantitively, the effects of the overlap ratio between adjacent jets and its impact upon deposition quality. This is evident in that the distance between jets has been commonly considered to be large enough, from a modeling perspective, that consecutive cavitation bubbles do not overlap with each other when modeling of the formation of a second jet [14]. Although there is limited knowledge available about the interaction mechanisms and morphologies between two adjacent jets or how the interactions of these adjacent jets affect printing quality, this knowledge is key to unlocking improved printing performance within the AM environment and is the subject of this study.

The objective of this research is to evaluate the effect of overlapping adjacent jets on the jetting behavior and deposition performance during an optical scanner-assisted LIFT printing of viscous inks. Specifically, this research investigates the jet formation process under perturbation from previously formed jet(s) and further assess the effect of jet overlap ratio on the jetting and printing performance. The deposition performance for these conditions is analyzed as a quality metric. For this study, silver-based suspensions have been used due to their potential applications in fabricating metallic structures and conductive patterns. The efficacy of optical scanner-assisted laser printing is demonstrated by the printing of conductive lines using silver particle-laden ink suspension. Although silver suspensions are being used in this study, the results and conclusions obtained would be applicable to other, similar particle-laden inks.

This paper is organized as follows. First, the preparation of inks as well as the laser printing experimental setup is introduced. Second, the jet formation process of silver inks is investigated using a high-speed camera. Third, the effects of the overlap of two adjacent jets on the jet formation and deposition dynamics are studied, and some continuous conductive lines are printed under the identified optimal conditions for demonstration purposes. Moreover, the jet morphology in this study is discussed, and a phase diagram is constructed using the Weber number, jet overlap ratio, and laser pulse time interval to better classify the jetting behavior. Finally, key conclusions are drawn regarding the proposed optical scanner-assisted LIFT printing technology.

## 2 Materials and Methods

### 2.1 Optical Scanner-Assisted Laser-Induced Forward Transfer System.

For illustration, the schematic of the printing system used herein is shown in Fig. 1(a). A diode-pumped solid-state (DPSS) Nd:YAG laser (Navigator I, Spectra-Physics, Santa Clara, CA) was used for this LIFT study. The output wavelength was 1064 nm with an available average laser power of 0.35–7.00 W. The laser frequency can be adjusted from 20 Hz to 100 kHz while the nominal pulse width remained constant at 20 ns. The laser spot size was measured to be around 75 μm in diameter. The irradiated laser fluence was measured using an Ophir sensor (PE-50-DIF-C, Israel) both at the theta-lens level and at the laser spot, where the energy delivery loss through air was negligible. An optical scanner with f-theta lens (JD2203, Sino-Galvo Technology, Jiangsu, China) was utilized to focus the laser beam and provide a flat field at the image plane of the scanning system, which is capable of displacing the laser beam on the x- and y-axes in a controlled manner. While previous studies [6,15] delivered a static laser pulse onto a movable ribbon, this research was based on a different approach: a dynamic laser beam was delivered onto a static ribbon. Through tuning the scanning speed of the optical scanner, the distances between two consecutive laser spots, also named transverse feed, L, were adjusted. This allowed different overlap ratios between two adjacent jets to be obtained while considering the jet diameter, D, as illustrated in Fig. 1(b). The maximum transverse speed was set at 1000 mm/s as limited by the optical scanner capability. The laser beam was delivered onto a ribbon, which consisted of a microscope glass slide (Fischer Scientific, Hampton, NH) and the ink to be transferred. The same glass slides were used as receiving substrates. Additionally, the transmittance loss of the glass slide was measured to be approximately 5% at the wavelength of 1064 nm. The stand-off distance between the donor and the receiving substrate, also known as direct writing height (DWH), was adjusted by a manually controlled z-stage (Thorlabs, Newton, NJ). The DWH was set to 300 µm and 500 µm for the deposition analysis. A DWH of 300 µm was used for the printing of the two-dimensional (2D) and two-and-a-half-dimensional (2.5D) elements with DWH adjusted after consecutively depositing 35 to 40 lines, being equivalent to an approximate thickness of 30 µm.

Fig. 1
Fig. 1
Close modal

The jet formation process was captured using a Photron high-speed camera (Fastcam SA5, Photron, San Diego, CA). Given the high temporal resolution required for this study, the maximum recording speed of 10,000 frames per second at a resolution of 896 × 848 pixels was chosen. Due to the lack of a common trigger signal between the laser and camera system, the image capturing was manually synchronized, and all time stamps are based on the temporal resolution of 100 µs that is the time interval between two consecutive frames. While a higher temporal resolution may be of interest in accurately tracking the evolution of jets such as in Refs. [16,17], the used timeframe of 100 µs is able to capture the tilting of the adjacent jets being formed and their ejection speed. Figure 1(c) illustrates the specific setup used in this research.

### 2.2 Silver-Based Ink Preparation.

Silver-based inks are a common hard particle-laden material used in printing electronics due to its conductive nature. Since they allow fast and easy assessment [18,19] of the quality of the deposited material, silver-based inks were chosen for this laser printing study. There are generally two ways to prepare donor substrate when using LIFT to print metal structures or patterns. One method uses an intact metal coating on the glass slide applied by magnetron sputtering [20,21], and another uses coated metal-containing inks applied by spin coating or blade coating [22], which was used in this study. Although the first method of using an intact metal layer for printing might avoid donor layer thickness variations because of the good uniformity control gained in sputtering, the relatively high cost and low efficiency of using magnetron sputtering in preparing a large amount of donor substrates may become a bottleneck for further adoption of such methods in large-scale structure or pattern printing. Also, as the solvent in the metal-containing ink can serve as an energy absorption matrix in the LIFT process to a given wavelength, it usually requires less laser energy to transfer metals when using metal-containing ink. Therefore, this study used silver inks as a model material for metal structure and pattern printing for its potential in large-scale printing.

Given the interest of this research on investigating the effect of overlapping jets, the utilized inks were required to have a viscosity low enough to enable the ink’s flow back to refill the cavity produced after the material is ejected. As such, the silver-based inks for this study were custom-made from a silver paste (viscosity >100 Pa · s) mixed with an additional amount of a solvent material to reduce the resulting ink’s viscosity. Specifically, silver paste Metalon HPS-FG77 (NovaCentrix, Austin, TX) was chosen, presenting a silver (Ag) content of 85 (wt%) and a specific gravity of 4.21 g/cm3 with the particle size ranging from 190 to 550 nm. The silver paste was further mixed with diethylene glycol monobutyl ether (Sigma-Aldrich, St. Louis, MO) in order to reduce its viscosity as this was the same solvent used in the Metalon HPS-FG77. A similar approach was used [23], in which continuous lines were deposited using an inkjet printer by mixing nano-silver particles with water and diethylene glycol as cosolvents, due to the nozzle-based nature of the inkjet technology requiring lower viscosity values. In this study, two different custom ink formulations were made with ratios of paste vs. continuous phase of 2:1, named low-concentration, and 3:1, named high-concentration. The designation of the low- and high-concentration inks is adopted based on their relative ratio of the paste used. Greater presence of solvent has not been considered due to the low-powder density in the resulting ink. The main properties of the conductive inks are listed in Table 1. In this study, after both materials were vortexed thoroughly for 5 min, and 5, 7, 9, 11, and 13 μL of the resulting ink was pipetted onto the glass substrate with a limited coating area of 5 × 25 mm as they were selected to be the optimum coating thicknesses for this study of 40, 56, 72, 88, and 104 µm, respectively. Lower and higher coating thicknesses were not considered due to the impossibility of obtaining a homogeneous-coated surface. The ribbon was later smoothed using a blade coater (MTI, Richmond, CA) to ensure an even surface. The surface tension was measured using a tensiometer (Attension ThetaLite 101, Biolin Scientific, Sweden) and the viscosity using a viscometer (DV-I Prime, Brookfield Engineering, Middleboro, MA) at 12 and 60 rpm, respectively. All measurements were done at room temperature (25 °C).

Table 1

Ink properties

InkPaste–solvent ratioAg concentration (wt%)Ink density ρ (g/cm3)Surface tension σ (mN/m)Viscosity η0 (mPa s)Oh
Low-concentration2:1≈56≈3.1245.7940.30.55
High-concentration3:1≈63≈3.4046.5755.60.72
InkPaste–solvent ratioAg concentration (wt%)Ink density ρ (g/cm3)Surface tension σ (mN/m)Viscosity η0 (mPa s)Oh
Low-concentration2:1≈56≈3.1245.7940.30.55
High-concentration3:1≈63≈3.4046.5755.60.72

### 2.3 Jet Formation and Printing.

Different ink coating thicknesses of the two ink compositions were tested delivering various laser fluences at different laser frequencies. Table 2 details the materials and printing conditions adopted in this study. In order to experimentally capture a single jet formation mechanism, the lowest frequency available of 20 Hz in the system was used in stationary conditions (transverse speed of 0 mm/s) using both inks, whose coating thicknesses ranged from 40 to 104 µm. The laser fluence levels shown in Table 2 were selected based on preliminary studies that empirically determined to be the laser fluence range under which a single well-defined jet was obtained: values lower than 0.2 J/cm2 do not generate any material transfer for any given coating thickness from both inks while bubbles remained at the base of the jet, if the laser fluence emitted was above 0.2 and 0.3 J/cm2 for the low-and high-concentration ink, respectively. A single jet formation is analyzed in Sec. 3.1 and its results enable the formation of multiple adjacent jets when using frequencies of 6250–11,000 Hz with transverse speeds within the range of 500–1000 mm/s, as described in Sec. 3.2, where both inks were used for a 56 µm thick coating under a fluence of 0.2 J/cm2. The validity of the proposed overlap values is proven by printing continuous lines by using a 56 µm low-concentration ink coating and laser fluence of 0.2 J/cm2. In order to shorten the ink curing time, the substrate was heated between printing cycles to sustain the substrate at an elevated temperature by using a positive temperature coefficient (PTC) heater (28 W, Mayitr, Guangzhou, China). The substrate was heated to 185 °C using the positive temperature coefficient element. At such high temperatures, the lines are instantly cured by having all the solvent evaporated. The substrate was experimentally determined to be able to maintain a temperature of 185 °C for 10 to 15 s, after removal from contact with the PTC heating element. Additionally, this was determined to be enough time to consecutively print five lines. After each five lines of printing, the substrate was heated to 185 °C prior to starting the process again. The heater was not kept within the proximity of the printing area considering that such a small DWH in place may compromise the donor integrity due to the high level of heat radiation. The printed line’s height and surface images were obtained using a profilometer (SurfTest SJ-400, Mitutoyo, Japan) and a scanning electron microscope (SEM, NanoScience Phemon Pro X, Thermo Fisher Scientific, Waltham, USA). The electrical conductivity of the printed structures was measured using a multimeter (Fluke Corporation, Everett, USA).

Table 2

Design of laser-induced forward transfer experiments

InkInk coating (µm)Laser frequency (Hz)Transverse speed (mm/s)Laser fluence (J/cm2)
Single jet analysisLow-concentration40–1042000.2
High-concentration40–1042000.2 and 0.3
Multiple jet analysisLow-concentration566250–11,000500–10000.2
High-concentration
Pattern printingLow-concentration5675008000.2
InkInk coating (µm)Laser frequency (Hz)Transverse speed (mm/s)Laser fluence (J/cm2)
Single jet analysisLow-concentration40–1042000.2
High-concentration40–1042000.2 and 0.3
Multiple jet analysisLow-concentration566250–11,000500–10000.2
High-concentration
Pattern printingLow-concentration5675008000.2

## 3 Experimental Results

In this section, the characterization of the single jet formation and the induced interference between adjacent jets are presented. The effect of the overlap ratio between adjacent jets is studied in terms of ejection dynamics and deposition performance, identifying the optimal overlap level for both concentrations of silver ink. Some printed lines using the proposed optimized overlap are shown for illustration purposes.

### 3.1 Single Jet Characterization.

The formation process of a single jet can be seen in Figs. 2(a1)–2(a7) by irradiating a 0.2 J/cm2 laser pulse onto a 56 µm thick coating of the high-concentration ink as a jet formation example. Once the coated ink initiates the energy absorption delivered by the laser pulse, the fast-growing bubble ejects material in form of a jet. A picture of the formed jet, after approximately 100 µs of laser pulse delivery, can be seen in Figs. 2(a1) and 2(a2) with the previous frame showing that the ribbon is unaffected at the time stamp of t = 0 µs. Figure 2(a3) shows the formed single jet continues to grow in length after 500 µs. Multiple breakups of the jet into several droplets are captured at 900 µs in Fig. 2(a4), and these droplets flying downward toward the substrate are seen at t = 1.4 ms in Fig. 2(a5). The morphology of these droplets is not round, as the surface tension of the ink is considered to be rather low [24]. Upon jet breakup that usually occurs after 400–600 µs depending on the ink composition, coating thickness, and laser fluence, the material at the base of the jet recoils back to the donor. At t = 1.9 ms, the donor surface is observed at the laser spot to be flat with a material coating thickness identical to prior the material ejection (Fig. 2(a6)). Figure 2(a7) represents a magnified view of t = 1.9 ms after the laser pulse which confirms no protuberance is observed at the laser spot. The time elapsed from the laser emission (t = 0 s) until the laser spot area flattens back is considered the recovery time.

Fig. 2
Fig. 2
Close modal

The jet is further characterized in terms of the measured diameter, D, of the base of a jet, and error bars in this study represent the ±1σ variation. Figure 2(b) plots the relationship between the jet diameter, D, coating thickness for a given emitted pulse fluence of 0.2 and 0.3 J/cm2. As can be seen, thicker coatings result in larger jets: for the low-concentration ratio, the jet diameter increases from approximately 110–160 μm when the coating thickness is increased from 40 to 72 µm, respectively. Interestingly, all measured diameters are larger than the laser spot diameter of 75 µm. Moreover, under the same conditions (fluence and coating thickness), the high-concentration ink develops larger jets than the low-concentration ink. This is driven by the higher concentration of silver particles in the ink, as at the wavelength of 1064 nm used in this study, silver has a higher absorption coefficient, α, than that of ethylene glycol, (9.0 × 105 cm−1 and 1.9 × 10−1 cm−1, respectively) [25,26]. Therefore, more energy is likely absorbed by the high-concentration ink, leading to a larger bubble and jet formation.

As such, greater contents of silver particles within the ink lead to higher energy levels absorbed by the coating, and therefore, the bigger the expanding bubble at the laser-matter interface is expected. The bigger expanding bubble translates into larger jets being formed. For a given coating thickness, larger jets are also obtained through the delivery of higher energy fluences as concluded in a previous study [15]. Hence, the jet diameter increases linearly with the increasing coating thickness and increasing emitted laser fluence as observed in other studies [11,15]. According to Fig. 2(c), there is a direct correlation between the time the material donor flattens after the jet column recoils back (recovery time tr) and the jet diameter size: the larger the jet diameter is, the longer it takes to recover. Because there is a linear relationship between the coating thickness and recovery time, as a greater amount of ink is perturbed by the laser-induced interaction, there has to be a longer recovery time. Specifically, it can be seen from Fig. 2(c) that the lowest recovery time of 0.8 ms is obtained under the thinnest coating of the low-concentration ink (40 μm) at the lowest level of irradiated laser fluence (0.2 J/cm2) while the longest recovery time of 8.0 ms is obtained by using the high-concentration ink on its thickest coating (104 μm) at the highest level of laser irradiation (0.3 J/cm2). The longer recovery times seen in Fig. 2(c) of the high-concentration ink are attributed mainly to its higher values of viscosity since both the inks have similar surface tension values. An analysis of variance (ANOVA) test is carried out (RStudio, Boston, MA), and the obtained p-values are less than 0.05, indicating the statistically significant influence of the ink type, coating thickness, and laser fluence on the resulting jet diameter as discussed before [15,17,27,28]. The values of each ink’s recovery time are used in Sec. 3.2 in order to identify the starting point for the analysis of printing with adjacent jets.

Furthermore, two nondimensional numbers are used to construct a 2D phase diagram on the single jetting behavior: a process dynamics-related Weber number and a material properties-related Ohnesorge number. Herein, the velocity U of a jet is determined by measuring the distance travelled by the tip of the jet during a given time period [27,28]. The Weber number $We=ρRU2σ$ is obtained by using the U at the specific time of t = 100 µs since the jetting conditions are defined by the initial velocity and the highest jet velocity is usually found during the early stage of jet formation [27]. In addition, R is the characteristic length, which in this study is considered the laser spot radius (37.5 µm). The material properties are characterized by using the Ohnesorge number $Oh=η0ρσR$, which relates the viscous force to inertial and surface tension forces, where η0 is the zero-shear viscosity. As noted in Table 1, the Oh numbers for the low- and high-concentration inks are 0.55 and 0.72, respectively. Based on the calculated dimensionless numbers, three regimes during single jetting are mapped out in a two-dimensional phase diagram in a (We, Oh) space as shown in Fig. 3: no material transferring, well-defined jetting, and not well-defined jetting. As seen in Fig. 3, both inks present similar behavior: for lower We (<∼60), there is no material transferred since the viscous and surface tension forces may still dominate; for medium We (∼60–1000), a well-defined jet is formed, which is desirable for this study; and for high We (>1000), not well-defined jets are formed, which present a visible bubble at the jet bottom.

Fig. 3
Fig. 3
Close modal

### 3.2 Characterization and Printing Using Multiple Jet Ejection.

In this section, the ejection of multiple jets and their interference is discussed from an ejection perspective using high-speed camera images and from a deposition perspective analyzing the lines deposited onto the substrate.

#### 3.2.1 Effect of Overlap on Jetting.

Since the goal is to analyze adjacent jet printing, the transverse speed effect is further investigated by taking into consideration the jet diameter. As shown in Fig. 1(b), transverse speed and overlap ratio between adjacent jets can be calculated using Eqs. (1) and (2), respectively:
$v=Lf$
(1)
$λ(%)=(1−LD)100=(1−1000fD)100$
(2)
where v is the x/y-transverse speed in mm/s, f is the laser frequency, and λ is the overlap ratio percentage. For a chosen overlap ratio and maximum transverse speed (1000 mm/s), the laser frequency f can be easily determined based on Eq. (2). Herein, the jet diameter under single jet conditions is considered the baseline for the overlap calculation. In order to create overlapping adjacent jets scenarios, frequencies higher than 2500 Hz are utilized since the typical jet break-up time is approximately 400 µs in this study. In addition, the identical conditions of coating thickness of 56 µm and laser fluence of 0.2 J/cm2 are chosen for the adjacent jets analysis, as both inks present well-defined jetting under such conditions. This allows for a better assessment of the effects of the time interval and overlapping conditions into the jetting behavior.

Figure 4 presents some time-lapse images of the ejected material by irradiating 0.2 J/cm2 laser pulses onto a 56 μm thick coating of the low-concentration ink under a speed v of 1000 mm/s. The expected overlap using Eq. (2) results in 0% as the jet diameter, D, is considered approximately 130 μm (Fig. 2(b)). As the frequency f used is 7500 Hz, the resultant time interval between the laser pulses is 133 µs. Multiple jets can be observed: the left jet is the one being created, while the two in the middle are well-formed jets, and the fourth jet is breaking up. Some flying droplets can also be observed on the right part as the jet recoiled back. The observed time-lapse also confirms the formation of jets right next to each other. Similar jet morphology, consisting of simultaneous jets formed parallel to each other, has been reported [12], although the reported results were of smaller magnitude, approximately by a factor of 200, as the used parameters were a laser pulse length of 30 ps, laser spot diameter of 33 µm, and ink coating thickness of 3.8 µm. Such differences are reviewed further in detail in Sec. 4.

Fig. 4
Fig. 4
Close modal

In order to analyze the effect of the overlap between adjacent jets, the ejection mechanism of jets with different overlaps can be obtained by tuning a given laser frequency, f, at the maximum scanning speed of 1000 mm/s. For illustration, an approximate 30% overlap is obtained by delivering a laser fluence of 0.2 J/cm2 onto a coating of 56 μm thick of the high-concentration ink, as the jet diameter, D, is measured as 160 μm (Fig. 2(b)) if a time interval between laser pulses of 111 µs (laser frequency f of 9000 Hz) and a transverse feed L of 1000 mm/s is used. The ejection angle γ is defined as the angle formed by the ejected jet axis compared to the vertical axis as seen in Fig. 5(a). Figure 5(b1) shows how the jets do not interfere significantly with each other with no overlap (0%) if a time interval between laser pulses of 160 µs is used, leading to the ejection angle being somehow straight compared to the vertical axis (γ ≈ 0 deg). Larger overlap ratios (15–20% and 30–35%) are obtained by decreasing the time interval to 128 and 111 µs, respectively. It should be noted that the resulting jet diameter variation may affect slightly the estimated overlap ratio, which is ignored in this study. This results in a tilted ejection angle (Figs. 5(b2) and 5(b3)) toward the laser motion of approximately 5–10 deg and 15–20 deg, respectively. The greater the overlap ratio between jets, the greater the incline of ejection angles with respect to the vertical axis. This phenomenon is also reported in Ref. [14]. This is caused by the newly forming jet having less material available in the surrounding area of the laser spot, as the previous jet is still undergoing formation. From an ejection perspective, overlaps of greater than 50% do not show any material transfer. This is seen in Fig. 5(b4) when using a time interval of 95 µs (f of 10,500 Hz) results in a smaller jet which is forced to recoil back to enable the next jet to be formed, as surface tension minimizes the free surface in the jet, without depositing any material. This is further analyzed in Sec. 4.2.

Fig. 5
Fig. 5
Close modal

For both inks tested using a coating thickness of 56 μm under a laser fluence of 0.2 J/cm2, a strong correlation between the measured ejection angle and overlap ratio percentage can be seen from Fig. 6. The transverse feed L is determined based on the measured jet diameters, which are 650 and 800 mm/s for a 30% overlap when using the low-and high-concentration inks, respectively, and the time interval between laser pulses is 133 µs. A maximum ejection angle of 30 deg is obtained with an overlap ratio of approximately 50% using the low-concentration ink, while for the high-concentration ink the maximum ejection angle is approximately 20 deg with an overlap value of 45%. The high-concentration ink shows smaller ejection angles over the entire domain compared to those of the low-concentration ink. These results can be attributed to the different rheological properties of both inks. Previous research indicates that a major factor affecting the tilt of the adjacent jet [14] is the traveling capillary wave induced by the first jet formation and the resulting normal interface where the second jet is formed. The capillary wave progression is heavily influenced by the time delay, the distance between pulses, and the physical and rheological properties of the ink.

Fig. 6
Fig. 6
Close modal

The differences in surface tension and viscosity values for each of the ink concentrations used are the main factors that affect the induced wave, leading to the different tilted ejections: higher viscosity and surface tension values are reported to act as resistance in the expansion of the bubble [29]; so, in the case of the formation of a second jet, such resistance is presented in a form of a less tilted ejection. As such, the longer time delay between pulses and the higher viscosity leads to a less pronounced slope of the bubble being formed as presented in Fig. 6, where the distinctive different morphologies of the bases of the jets can be seen. The same tilted ejection angle of approximately 12–15 deg is obtained by using an overlap ratio of 20% and 42% for the low- and for the high-concentration inks, respectively. Higher overlap ratio values result on jets, which are closer to each other. Moreover, in Fig. 7(a), it can be seen that the base of the jet of the low-concentration ink presents a triangular-like bubble morphology, presenting a higher tilted normal surface at the coating and bubble interface than that in Fig. 7(b). This is attributed to the viscosity and density differences that result in the slope difference of the bubble surface where the second jet formation takes place. This different pattern allows the second jet to present a less tilted ejection, due to the less disturbed slope of the coating/bubble in the second laser spot when printing with the high-concentration ink.

Fig. 7
Fig. 7
Close modal

Nevertheless, it must be noted that both overlap values are obtained under different conditions, using higher frequencies for the low-concentration ink, as this presents smaller jets under the given laser fluence and shorter recovery times. As such, the use of low-concentration inks is preferable as this makes it possible to obtain faster depositions, as higher working laser frequencies are enabled.

#### 3.2.2 Effect of Overlap on Deposition.

The jet overlap-dependent deposition performance is qualitatively assessed by the printed line morphology and line width of the low-concentration ink coated with a ribbon coating thickness of 56 µm under a laser fluence of 0.2 J/cm2. Such values were experimentally chosen due to the good deposition quality (few satellite droplets). Furthermore, the low-concentration ink is chosen because of the greater amount of solvent and resulting in compositional stability of the ink at room temperature (25 °C). The DWH values were set at 300 and 500 μm to ensure that deposition is mainly jet-impingement based [9]. Larger DWH values lead to the deposition being droplet-based, increasing the probability of a splashy deposition with an increased number of satellite droplets.

Figure 8 presents the overlap-dependent printing results obtained using the different DWHs of 300 and 500 μm. An increase of overlap results in wider lines being deposited, and this increase of line width is also related to the DWH adopted. Lines with satellite droplets appear at higher overlap values at a DHW of 300 µm compared to those at the lower DWH of 500 µm. Moreover, it has been reported about the tendency of the second jet to be smaller due to the mass continuity under the jet formation [14], which is translated into the shortening of the pinch off location of the jet. As such, larger DWHs present an increased space for a jet to break up into multiple droplets, and under adjacent jet conditions, a tilted angle during ejection is also likely to influence the trajectory of satellite droplets, affecting them to land onto different off-line positions. The early jet break-up, induced by larger overlap ratios, contributes to the formation of wider lines, that are likely to present an undesirable spreading morphology. This is corroborated by the deposited line morphology when satellite droplets appear, as a wider area with greater amount of satellite droplets can be seen within the vicinity of the printed line. If a large DWH is needed for some applications, a small overlap ratio is preferred in order to maintain the same line width.

Fig. 8
Fig. 8
Close modal

Generally, overlaps greater than 30% will result in discontinuous line printing and satellite droplet deposition for the specific setup and materials used herein. Under the optimized conditions (56 µm coating of the low-concentration ink, laser emittance of 0.2 J/cm2, laser frequency of 7500 Hz and transverse speed of 800 mm/s), 2D (one layer only) and 2.5D (multiple layers) continuous lines have been printed in order to establish that the deposition quality is not compromised when a 25–30% overlap is adopted for long operation times. For long-term jetting stability during printing, the overlap value is controlled through decreasing the scanning speed instead of increasing the laser frequency. It should be pointed out that the print quality is affected by the low viscosity of the inks [27] used in this study, substrate surface preparation, and jetting instability [28], which may be further enhanced by using viscous inks with more suitable rheological properties [30]. The 2D deposition shown in Fig. 9(a) illustrates the acronym (UF) for the University of Florida. Punctual discontinuities of the deposited line and undesired satellite droplets are caused by local imperfections on the ribbon coating surface and thickness were noted. In addition, 2.5D structures of polygonal and interdigitated patterns have been successfully printed. The build-up on the vertical direction was achieved with a layer-by-layer deposition of printed materials. In particular, the printing substrate was controlled at 185 °C through the use of an auxiliary heater during the printing process in order to allow layer stacking. Also, the DWH during the printing process is adjusted after every 35–40 layers (approximately equivalent to 30 µm) of printing to maintain constant printing quality throughout the whole process. Figure 9(b1) presents a closed circuit, whose lines were obtained from stacking 100 layers. The measured line height is approximately 85 µm as seen in Fig. 9(b2), where the shaded area represents the standard deviation. The line width is measured to be approximately 700 µm, showing a clear conical cross section. The line resistivity is measured to be 0.3 ohms. In order to evaluate the electrical conductivity, 1 mm long cuts were done to the deposited structure in Fig. 9(b1) to avoid any short circuits as seen in Fig. 9(c1). Figure 9(c2) shows how a light-emitting diode (LED) is lighted when 15 V is applied to the modified circuit. Figure 9(d1) presents a printed interdigitated battery pattern. Furthermore, Fig. 9(d2) exhibits an SEM image, showing the surface morphology of the printed interdigitated structure. A clear, densely packed silver nanoparticle granular texture can be observed, as boundaries of some individual silver particles are visible which indicates the 185 °C temperature used, successfully evaporates the glycol-solvent phase of the silver-based ink. Additionally, the exposure time of the substrate to the high temperature is not long enough to initiate the sintering process of the silver particles as seen in the previous published work [23]. As such, future temperature adjustments may be a potential factor inducing further particle bonding and, if needed, electrical conductivity.

Fig. 9
Fig. 9
Close modal

## 4 Discussion

This work addresses the interactions between a newly forming jet, adjacent to a previously formed one. Some key phenomena are discussed herein in terms of the jet morphology and jetting phase diagram.

### 4.1 Jet Morphology.

First, it must be noted that the resulting jets are wider (Fig. 2(b)) than the laser spot of 75 µm for all the coating thickness ranges (40–104 µm). In this study, larger coating thicknesses/laser spot ratios of 0.5–1.4 are used as compared to that (0.04–0.14) of a previous study elsewhere [13], which had a jet diameter of 30 µm when using a 34 µm laser spot. For a thick coating configuration, it takes a longer time for the bubble formation process [31,32] to travel along the coating thickness direction to form a jet, meaning more time for heat to transfer along the planar direction of the coating and resulting in a larger heat affected zone (HAZ). As a result, the obtained jet diameter may be larger than the laser spot itself.

Second, a different, adjacent jet morphology is also observed in this study as shown in Fig. 10. The interaction between the successive expanding bubbles while inducing jets being formed next to each other was reported as a unique big bubble [13]. This big bubble is the result of the fusion of consecutive bubbles, as a new bubble merges during the expansion phase with the previously formed one as presented in Fig. 10(a). The bubble is only present near the current laser spot and the previous one, leaving a continuous ink line connecting all the successive jets while being ejected towards the substrate. This results in depositing continuous lines with some microdroplets surrounding the line due to the splashy process induced by the breakdown of some jets. However, a continuous bubble can be observed in this study (Fig. 10(b)) that travels along the laser feed direction (Figs. 10(b1)10(b4)). Conversely, the remaining bubble, at the ink interface, affects the jet ejection trajectory with a tilted angle as discussed in Fig. 7 The main factor attributed to the distinctive multi-jet formation, with tilted ejections, is the higher levels of viscosity (∼40–55 mPa · s versus 10–13 mPa · s) [13], surface tension (∼46 mN/m versus 28–31 mN/m) [13], and coating thickness (40–104 µm versus. 1.5–4.9 µm) [13]) used, as these contribute to pack the bubble, near the laser spot, and tend to minimize the free surface area of the bubble. This allows tilting of the jet while keeping the integrity of the bubble, as both higher viscosities and surface tension acts as resistance to forming bubbles [29]. Although a longer laser pulse used herein (nano-second) is almost 103 times longer than in some previous studies [1315], it is noted that the differences of the tilting ejections are not attributed to the pulse length duration, since there is no pronounced difference on the jet morphology [33].

Fig. 10
Fig. 10
Close modal

### 4.2 Phase Diagram on Jetting Behavior.

For further illustration, a 3D phase diagram is presented in Fig. 11 in a (We, overlap ratio, time interval between laser pulses) space to classify the jetting behavior under overlapping adjacent jets with a spatial overlap ratio and a temporal time interval between. Examples of overlapping adjacent jets are included to illustrate different jetting behavior in place: from well-defined jets to no material transfer conditions. Specifically, Figs. 11(a) and 11(b) is under an approximately 5% overlap ratio and 133 µs time interval of two laser pulses, Figs. 11(c) and 11(d) an approximately 33% overlap ratio and 133 µs time interval, Figs. 11(e) and 11(f) an approximately 52% overlap ratio and 133 µs time interval, and Figs. 11(g) and 11(h) an approximately 33% overlap ratio and 95 µs time interval. The elapsed time is measured based on the current nth jet. Under well-defined jetting conditions (Figs. 11(a)11(c)), two adjacent jets (nth and (n−1)th jets) do not significantly affect the velocity of their counterpart, and their velocity does not vary too much during the measurement window. It is noted that the magnitude of such jetting velocity is greatly affected by the overlap ratio: while the jetting velocities under a small overlap ratio (∼5%) is estimated to be around 14.7 m/s (Fig. 11(b)), the velocity under a larger overlap (∼33%) may be down to 9.2 m/s (Fig. 11(d)). From an energy conservation perspective during LIFT [34], it is concluded that the closer a forming jet is from the adjacent one, either spatially by an increase of overlap or temporally by a reduction of the time interval between laser pulses, the more the kinetic energy diminished based on the reduced velocity. Furthermore, the velocity evolution of two adjacent jets under the condition of no material transferring is conducted. It is noted that this scenario of no material transferring is investigated based on the identified well-defined jetting conditions for single jets. It is found that a current jet (nth jet) may significantly slow down its previous jet ((n−1)th jet) as seen from Figs. 11(f)11(h). Under these combinations of overlap ratios and time intervals, the previous jet recoils and returns to the original coating state upon the interaction with the consecutive jet. In particular, the nth jet presents a higher jetting speed than its predecessor ((n−1)th jet) for the first elapsed 200 µs. However, as the previous jet shows a velocity of 0.5 m/s and the velocity of the current jet is approximately 3 m/s at the first 100 µs, the previous jet is forced to recoil back due to the surface tension and viscous resistance, preventing it from material transferring.

Fig. 11
Fig. 11
Close modal

Due to the significance of the We number in LIFT [15,35], Fig. 12 further illustrates two-phase diagrams in the (We, time interval) plane under two given overlap ratios of approximately 20% and 33%, and Fig. 13 presents a phase diagram in the (We, overlap ratio) plane under a given time interval of 133 µs in order to better visualize the time interval and overlap effects on the jetting conditions. In addition, some regime boundaries are included for illustration purposes only. Figure 12(a) represents the different jetting behaviors for a given overlap ratio of approximately 20%. A decrease of the time interval between laser pulses induces a transition of well-defined jetting to severely tilted jetting, and this transition occurs around 110 µs for the given laser and material system. The transition may occur at a longer time interval under a larger overlap ratio of approximately 33% (Fig. 12(b)) since longer time intervals are required to better form jets. Furthermore, if the time interval under a given overlap ratio is not guaranteed, no material transferring is expected. In addition, the time interval required to induce well-defined jetting increases along with an increasing overlap ratio. As seen in Fig. 13, the Weber number needed for severely tilted jetting decreases with the overlap ratio, and for overlap ratios larger than approximately 30% under certain process conditions, the jetting is severely tilted as discussed in Figs. 5 and 6. Furthermore, it is expected there is a boundary limiting the jetting regime to the overlap ratio lower and higher than a certain percentage such as 50% (133 µs time interval) in this study, and no material is transferred under such conditions.

Fig. 12
Fig. 12
Close modal
Fig. 13
Fig. 13
Close modal

## 5 Conclusions and Future Work

This paper presents a jet formation study of silver particle-laden inks during an optical scanner-assisted LIFT printing-induced overlapping jets. As a result, improved laser printing conditions are studied and discussed. Some optimal printing conditions have been implemented to successfully deposit continuous conductive lines with the initials of the University of Florida and 2.5D conductive interdigitated structures.

The main conclusions based on the laser and material system in this study are drawn as follows: (1) using an optical scanner, the overlap between adjacent jets can be controlled by adjusting the scanning speed and/or the laser frequency. In this study, two approaches are used to obtain different overlap ratios: varying the laser frequency when the transverse speed is set to its maximum allowable value (1000 mm/s) and varying the transverse speed from 500 to 1000 mm/s for a given laser frequency; (2) a larger overlap leads to an increase of the material ejection angle. Specifically, the tilted angle may be up to 20 deg for the low- and 30 deg for the high-concentration inks with a coating thickness of 56 µm under a fluence of 0.2 J/cm2 and time interval between laser pulses of 133 µs, which may have a maximum, achievable overlap ratio of approximately 50%. However, the use of overlap ratios greater than 50% may lead to no jet ejection. It should be noted that the aforementioned tilted angles may change with other process conditions; (3) from a deposition perspective, a 30% jet overlap ratio is found to produce continuous lines without satellite droplets at a width of 250 μm and 300 μm, under typical DWH of 300 μm and 500 µm, respectively. The 30% overlap ratio, if implemented, may lead to increased printing efficiency while maintaining similar printing quality, and (4) these identified printing conditions have been implemented to successfully deposit continuous lines with the initials of the University of Florida and 2.5D conductive structures.

For better applications of optical scanner-based laser-induced forward transfer printing of particle-laden inks, some future work is proposed as follows: first, the printing process should be modeled to identify optimal printing conditions to study the effects of higher viscosity ink compositions on jet formation and interaction which will enable faster, more controllable material deposition; second, suitable statistical tools may be implemented to determine the most significant factors and/or their interactions for better printing performance under jet overlapping and the controllability and repeatability of forming jets; third, a multi-dimensional phase diagrams should be designed and constructed to draw general conclusions for process development and optimization; and finally, the current ejection pattern presented in this research, should be used as the cornerstone for process feedback control and automation. As an example, process parameters such as the transverse speed and laser frequency can be adjusted, based on real-time data of the ejection behavior (i.e., ejection angle, droplet formation, etc.).

## Acknowledgment

The study was partially supported by the US National Science Foundation (CMMI-1537956). The scanning electron microscopy support provided by Dr. Nancy J. Ruzycki and the discussion with Jeffrey M. Fitzgerald of BAE Systems are highly appreciated.

## Conflict of Interest

There are no conflicts of interest.

## Data Availability Statement

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request. The authors attest that all data for this study are included in the paper. Data provided by a third party are listed in Acknowledgments.

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