3D thermoreflectance (TR) imaging utilizing light field camera is proposed and developed in this study. With the capability of resolving light direction and intensity simultaneously, we are able to measure directional TR signal from tilted or curved surfaces through the light field camera. Once the directional TR coefficients of the target surface is collected, we can then estimate the temperature distributions of tilted or curved surfaces based on the measured directional TR signal. This new 3D TR imaging approach has the potential to measure temperature of surfaces with different configurations as long as the TR signal is strong enough. In this technical paper, we first present basic principles of 3D TR imaging techniques based on light field theory. The construction and calibration procedures of the resulting 3D TR imaging system are then presented. At the end of the technical paper we present steady-state surface temperature measurements of microscale electronic devices including an infrared laser diode and a gold wire with the current 3D TR imaging system. The obtained temperature distributions of the microscale samples showing high temperature and spatial resolutions in real-time temperature measurement on the 3D surfaces that cannot be achieved with traditional 2D TR imaging techniques.

The phenomenon of bubble growth on a heated surface is of fundamental importance in many scientific and engineering applications, including boiling heat transfer. Although the growth of a homogeneous bubble in a pool of hot liquid is well understood, bubbles growing on hot solid surfaces involve evaporation from a three-phase contact line and therefore exhibit several peculiar features. One of these is the effect of surface texture and wetting properties on the size and timing of bubbles that form and depart from a uniformly heated surface. Here, we present pool boiling experimental results elucidating this important phenomenon. Using high-speed optical imaging, we perform a comparative study of the process of growth and departure of bubbles on plain and rough surfaces and explore the different factors that dictate this behavior. Using scaling analysis, we analyze the primary forces acting on a growing bubble and show that the effect of surface roughness on bubble behavior can be explained in terms of the dependence of these forces on the rate of bubble growth and in-turn on the rate of thin-film evaporation from the three-phase contact line of the bubble.

The development of hydrophilic surface coatings for enhanced wetting characteristics has led to improvement in heat transfer metrics like impinging droplet vaporization time and the heat transfer coefficient. Hydrothermal synthesis, a method of developing hydrophilic surfaces, has been previously shown to produce high performing heat transfer surfaces on copper substrates [1]. Our study applied this production method to aluminum substrates, which have the advantage of being cheaper, lighter, and a more widely used for heat sinks than copper. Previous experiments have shown that water droplets on ZnO nanostructure coated surfaces, at low superheats, evaporate via thin film evaporation rather than nucleate boiling. This leads to heat transfer coefficients as much as three times higher than nucleate boiling models for the same superheat. Our nanocoated aluminum surfaces exhibit superhydrophilicity with an average droplet liquid film thickness of 20–30 microns, which can produce heat transfer coefficients of over 25 kW/m 2 K. This study discusses characterization of ZnO nanostructured aluminum surfaces to better understand the related mechanisms which lead to such high heat transfer performance. All ZnO nanostructured aluminum surfaces produced for this study exhibited superhydrophilicity, with sessile droplet contact angles of less than 5 degrees. The challenge of achieving accuracy for such low contact angles led to the development of a new wetting metric related to the droplet’s wetted area on a surface rather than the contact angle. This new metric is predicated on the the fact that heat transfer performance is directly related to this wetted area, thickens, and shape of the expanding droplet footprint. Shape irregularity of droplets on these superhydrophilic surfaces is discussed in this study, where there appears to be advantages to irregular spreading compared with surfaces that produce symmetric radial spreading. One form of irregular spreading consists of liquid droplets spreading out both on top of the surface and within the microstructure of the surface coating. The liquid within the microstructure forms films less than 5 microns thick, making local heat transfer coefficients of greater than 100 kW/m 2 K possible. SEM microscope imaging provided additional insight to the underlying mechanisms which cause these surfaces to produce such exceptional spreading as well as irregular spreading, resulting in very good heat transfer performance. Experimental work was coupled with computational analysis to model the contact line of the droplet footprint. Image processing of experimental photos helps to analyze spreading characteristics, which can be directly related to heat transfer due to film thickness at various points during spreading. Approaches used to characterize these superhydrophilic surfaces advance understanding of the connections between nanoscale structural elements and macroscale performance characteristics in heat transfer. This understanding can reveal key insights for developing even better high performance surfaces for a broad range of applications.

Lasers are widely used as high-accuracy tools for material processing. Different types of lasers such as CO 2 , Nd:YAG, and excimer lasers are used in different operating modes such as continuous wave, pulsed or Q-switched. Volumes of materials and their composition, structure, and properties can be controlled or modified by varying laser pulses. In this research, by using laser as a material processing tool, an experimental method was developed for laser induced implantation doping of glass substrates with conductive metals. Experiments were performed on glass samples using Q-switched Nd:YAG lasers. Gold, silver, and copper were used as conductive dopant materials. Initial experiments were performed using nickel as a catalyst. Effect of the catalyst on the composition of implanted dopant material was observed using Large Area Rapid Imaging Analytical Tool (LARIAT). Through further experiment, the effect of several parameters such as beam fluence, scanning speed, pulse repetition frequency, wavelength, substrate temperature, dopant material, and glass substrate material on the morphology of heat affected zones were investigated by optical microscopy (OM). Depth of penetration in doped glass samples was measured for different substrate temperatures by means of a laser displacement sensor. The effect of beam fluence and glass substrate thickness on depth of penetration was investigated. The results of these non-destructive measurements were verified using scanning electron microscopy (SEM). Based on optical observations, morphological characteristics of the heat affected zone were assessed in order to obtain the best parameter settings in different experiments. These settings were defined by factors such as the number and size of cracks in glass substrates, and the quality of the distribution of dopant metal over the scanned pattern. While using a catalyst with substrates at room temperature, the best parameter settings were obtained at wavelength of 532 nm, pulse repetition frequency of 6 kHz, beam fluence of 0.36 J/cm 2 , and scanning speed of 0.10 m/s. By removing the catalyst, these settings were changed to 355 nm, 10 kHz, 0.09 J/cm 2 , and 0.01 m/s for gold sputtered soda-lime glass substrate at 500 °C. For beam fluence values ranging from 0.06 J/cm 2 to 0.38 J/cm 2 , the obtained values for average depth of penetration were 255 μm and 187 μm in 1 mm and 3 mm thick soda-lime glass substrates respectively. Further development of this implantation method could lead to implantation of electronic circuits in transparent substrates, inspiring the evolution of transparent electronic devices such as transparent smart phones, smart windows and displays, and lighting products in the future.

The thermal imaging technique was applied in this work to measure the transient temperature fields during melting of a phase change material (PCM) in a metal foam. A paraffin wax was used as the PCM that was filled in an open-celled copper foam. Melting of a paraffin wax in the presence of copper foam was studied in a rectangular cavity that was heated from one lateral side wall, while the top surface was exposed to an infrared (IR) camera. A thermocouple (TC) was also employed to validate the accuracy of temperature measurements by IR thermal imaging. The relative deviation of measured temperature by the TC and IR camera was found to be under 2% in steady state and under 4% during the entire course of melting. The resolution of IR thermal imaging with the aid of a macro lens allowed for temperature measurements at pore-scale of the copper foam. Local thermal imaging was captured through a minor window on the top plate of the container. Three points (Sp1–3) inside a selected individual pore were marked to quantify the temperature variations of melting process within metal foam/PCM at pore-scale. The average temperature differences between Sp1 and Sp2, Sp3 were found to be about 1 °C over the entire course of melting, and the maximum value was up to nearly 10 °C around the melting point. These preliminary results clearly highlighted the effect of metal ligaments on the temperature distributions at pore-scale.

The utilization of porous media can enhance the heat transfer process due to its large heat transfer area within limited space. The natural convection in porous media widely exists in various heat transfer equipment and the related flow and heat transfer in porous spaces is one complicated transport phenomenon, for which the accurate prediction is challenging. Pore-scale models can predict transport phenomena in porous media in pore space, which can be used in the modeling of flow and heat transfer in porous media under local thermal non-equilibrium condition. The pore-scale study includes the reconstruction of porous structure and the direct numerical simulation of transport phenomena in the pore spaces. In this paper, the geometrical reconstruction approach was developed to generate the porous region using the tomographic reconstruction, which is one nondestructive imaging technique. The porous sample was scanned on a micro-CT scanner with micrometer resolution. 2D sliced scan images were obtained and then stacked to reconstruct the 3D porous geometry. A double-population thermal lattice Boltzmann model was established to predict the natural convection in reconstructed porous media at pore scale.

Boiling heat transfer impacts the performance of various industrial processes like quenching, desalination and steam generation. At high temperatures, boiling heat transfer is limited by the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect), where the low thermal conductivity of the vapor layer inhibits heat transfer. Interfacial electrowetting (EW) fields can disrupt this vapor layer to promote liquid-surface wetting. This concept works for a variety of quenching media including water and organic solvents. We experimentally analyze EW-induced disruption of the vapor layer, and measure the resulting enhanced cooling during quenching. Imaging is employed to visualize the fluid-surface interactions and understand boiling patterns in the presence of an electrical voltage. It is seen that EW fundamentally changes the boiling pattern, wherein, a stable vapor layer is replaced by intermittent wetting of the surface. This switch in the heat transfer mode substantially reduces the cool down time. An order of magnitude increase in the cooling rate is observed. An analytical model is developed to extract instantaneous voltage dependent heat transfer rates from the cooling curve. The results show that electric fields can alter and tune the traditional cooling curve. Overall, this study presents a new concept to control the mechanical properties and metallurgy, by electrical control of the quench rate.

This paper summarizes the results of computational and experimental studies of an enhanced thin film solar structure. The cell structure consists of a reflective aluminum layer beneath an 80nm absorbing layer of amorphous silicon, coated with a top layer of transparent and conductive indium tin oxide (ITO). The structure is mounted on a glass substrate. We first use constrained optimization techniques along with numerical solvers of the electromagnetic equations to specify the layer thicknesses of the design for maximized efficiency. Numerical analysis suggests that solar absorptivity in the thin film silicon can be enhanced by a factor of 2. The proposed design is then fabricated using Plasma Enhanced Chemical Vapor Deposition techniques, along with a control sample of bare silicon absorber for comparison. AFM imaging and spectrophotometry experiments are applied to estimate the realized thin film dimensions, deposition error, unwanted oxidation volume and the resulting reflectivity spectra. Comparisons of the measured and simulated reflectivity spectra of the fabricated cells, as well as Monte Carlo simulations based on incorporating random geometry errors in the numerical simulations suggest that the measured spectra are in accordance with the expected curves from simulations.

Development of electric and hybrid electric vehicles is of great interest to the transportation industry due to increased demand and cost of imported fuel, uncertainty in the steady supply of oil, and increased standards for reduced emissions. Lithium-ion batteries are considered as one of the leading types for the battery systems to be employed in electric vehicles (EVs) or hybrid electric vehicles (HEVs). Using a regenerative braking system and storing it in battery stacks and using it later for propulsion and acceleration can improve the overall efficiency and reduction of fuel consumption. The objective of this study is to evaluate experimentally the battery performance considering different discharge and charge rates, and investigate the thermal behavior and thermal management requirements of the batteries under a variety of environmental conditions. An experimental test facility has been developed to evaluate thermal performance during charging and discharging modes. Environmental temperatures were varied in environmental chamber to analyze their effects on the charging and discharging patterns of the battery by using the CADEX battery analyzer in order to find the temperature range for optimum battery performance. The batteries were monitored with thermal sensors and a thermal imaging camera while they were run through different load scenarios. In the present study, lithium-ion batteries have been tested and battery performance in terms of polarization curves and discharge capacity were measured using a computerized battery analyzer system for different discharge and charge rates, and over a range of ambient temperatures. Results indicate that at higher discharge and charge rates battery performance decreases due to increased polarization losses, which results in increased internal heat generation and temperature of the battery. Battery performance also depends strongly on the ambient temperature conditions.

Heat exchangers employing falling films are relevant to a multitude of industrial applications using water-based liquids. In the present study, periodic, two-dimensional waves are imposed by excitation on a vertically falling film of water, which is then heated by a uniform heat flux, within the laminar and transitional flow range (39<Re<200). Liquid-film thickness is measured by confocal chromatic imaging and surface temperature is measured by high-speed IR thermography. As the 2D waves travel downstream they destabilize in the spanwise direction and evolve 3D structures (bumps). Further wave destabilization, under relatively low heating, was observed to coincide with the appearance of local thermal flows (“hot streaks”), though no deformation of the liquid surface could be measured. These flows are understood to be induced by thermo-capillary forces, which in extreme cases are known to lead to the formation of rivulets, film rupture and heater burnout. Understanding these initial stages of thermo-capillary flow is crucial to its suppression. Analysis of the thermal images reveals several significant streamwise length scales: a thermal inlet length based on the emergence of the thermal boundary layer ( Lt ), a thermal inlet length based on reaching thermally developed conditions ( Lh ), and the length at which “hot-streaks” first appear ( Ls ). In addition the dominant (most unstable) spanwise wavelength of the hot streaks, Lz , was identified through FFT analysis of the thermal profile beyond Ls . First the independence of the thermal inlet lengths from the heat-flux was established. Next, the influence of the nominal flow conditions (Reynolds number and excitation frequency) on Lt , Lh and Lz was examined — thereby extending the range of previous studies to higher Reynolds numbers. The thermal inlet lengths Lt and Lh were found to increase with flow rate, whereas they had opposing trends with regard to frequency. Lz consistently decreased with an increase of the flow rate, as smaller (turbulent) scales became more dominant, and it was found to be indifferent to excitation frequency over a wide range. Some future directions and methods of hot streak suppression are discussed, as well.

Studies published over the past decade have established the importance of sliding bubbles in enhancing the heat transfer in subcooled two-phase flow in channels small enough to confine the bubbles. Recent work in this laboratory (Ozer et al., 2011, 2012) proposed that the primary enhancement mechanism is a single-phase convective mechanism: the transport of cold fluid nearer the wall due to the mixing behind the bubble. This is in contrast to two phase-change mechanisms: distributed bubble nucleation and the evaporation of the liquid microlayer between a sliding bubble and the surface. The work reported here explores this hypothesis by comparing the heat transfer enhancement produced by injected air bubbles to Ozer’s measurements obtained with naturally nucleated vapor bubbles. Data were collected under similar conditions in a highly subcooled laminar flow of Novec 649 in a horizontal rectangular minichannel of 1.21 to 1.484 mm channel spacing. The channel was formed by an electrically heated metallic upper wall and an unheated transparent lower wall. For the air/liquid flow, bubbles were injected at either a single point on the lower wall or through a sintered metal plug. The latter system produced a more channel-filling distribution of bubbles. A high-speed imaging system recorded the bubble motion and liquid crystal thermography recorded time-averaged surface temperature data. The comparison is presented in the form of the streamwise evolution of surface temperatures and the enhancement in time-averaged Nusselt number. Also, results for the passage of a single air bubble are presented. The air/liquid flow produced a Nu enhancement of between 120–350% compared to a single-phase flow at the same conditions. The passage of the single gas bubble produced a decrease in the wall temperature directly behind the bubble of 2–5 ° C . The Nu enhancement produced by the air/liquid data and the nucleated vapor data is well correlated to appropriate dimensionless groups involving bubble diameter and frequency. The results from both data sets support the contention that a transient transport/mixing model developed previously for the vapor/liquid case captures the dominant single-phase convective mechanism in sliding bubble flows in highly confined channels.

We report the results of experiments using high resolution imaging of transient frost melting to obtain quantitative information on frost thickness and porosity. A highly controlled test chamber and surface are described along with visual and digital methods extract quantitative data. When the frost layer approaches the fully permeated state and melting proceeds to liquid droplets, we report a digital method for analysis of droplet size and geometry. For each of the measurements, an error analysis is presented. We find that the measurement techniques employed in the present study provide faster data acquisition and higher accuracy than traditional approaches. The present data are utilized successfully in a companion paper in connection with an empirically based model of frost growth.

Sulfur poisoning can deactivate nickel catalysts in solid oxide fuel cells (SOFCs), resulting in a significant drop in fuel cell performance. In this paper, a Ni/YSZ SOFC anode exposed to 100 ppm H 2 S was examined using x-ray absorption contrast imaging for its microstructure and x-ray fluorescence (XRF) spectroscopy to probe and map Ni, S, and YSZ phases. It was observed that S was frequently found to be collocated with Ni, with higher concentrations being located on or near the surface of the Ni particles exposed to gas, while little S was found near the YSZ phase.

An alternative to air-cooling of high performance computing equipment is presented. Heat removal via pool boiling in FC-72 was tested. Tests were conducted on a multichip module using 1.8 cm × 1.8 cm test die with multiple thermal test cells with temperature sensing capability. Measurements with the bare silicon die in direct contact with the fluid are reported. Additional testing included the test die directly indium-attached to copper heat spreaders having surface treatments. A screen-printed sintered boiling-enhanced surface (4 cm × 4 cm) was evaluated. Tests were conducted on an array of five die. Parameters tested include heat flux levels, dielectric liquid pool conditions (saturated or subcooled), and effect of neighboring die. Information was gathered on surface temperatures for a range of heat flux values up to 12 W/cm 2 . The highest heat dissipated from a circuit board with five bare die was 195 W (39 W per die). Addition of the heat spreader allowed heat dissipation of up to 740 W (from a five-die array). High-speed imaging was also acquired to help examine detailed information on the boiling process. Numerical modeling indicated that placing multiple boards in close proximity to each other did not degrade performance until board spacing was reduced to 3 mm.

A well-established method for determining the thermal diffusivity of materials is the laser flash method. The work presented here compares two analysis methods for flash heating tests on anisotropic carbon bonded carbon fiber (CBCF). This material exhibits a higher conductivity in the direction in which the fibers are oriented than in the direction perpendicular to the fiber orientation. The two analysis methods being compared in this experiment use different portions of the data in obtaining results. One method utilizes the temperature data from the entire surface of the sample by examining 201 temperature histories simultaneously, with each temperature history originating from an individual pixel within a line across the middle of the sample. The other analysis method utilizes only the temperature history from a single pixel in the center of the sample, similar to the data which is traditionally generated using the classical flash diffusivity method. Both analysis methods include accommodations for modeling the penetration of the laser flash into the porous surface of the CBCF material. Additionally, both models include a parameter which accounts for the non-uniform heating of the sample surface from the flash. Although the sample surface is ostensibly heated uniformly in flash diffusivity experiments, the heating has been found to be somewhat non-uniform, with more energy deposited more heavily in the center of the sample. This affects the analysis results, particularly in tests on anisotropic materials. The results in this work show very little difference between the thermal parameters arising from the two methods. The robustness of the method using the single-pixel temperature history shows that anisotropic thermal diffusivity can be measured using standard flash diffusivity instruments, avoiding the additional complexity associated with a thermal imaging camera.

Cryosurgery is also called as cryoablation or cryoleision. The third generation of cryo-machine use argon gas for cooling and helium for rewarming to destroy cancer cells. The probes may be put into the tumor during surgery or through the skin (percutaneously). After cryosurgery, the frozen tissue thaws and is either naturally absorbed by the body (for internal tumors), or it dissolves and forms a scab (for external tumors). The main purpose of this paper is to establish a preliminary computer assisted simulation in prostate tumor cryosurgery. A radiologist and an urologist in a medical center in addition to the engineering specialist from the university participated in this interdisciplinary research program. The first step of this simulation protocol is to trim hundreds of two-dimensional medical imaging photos from a patient through the imaging reconstructive software into building a three-dimensional solid modeling. The image data for each patient can be obtained from the x-ray computed tomography (CT), or magnetic resonance imaging (MRI) in the imaging department of hospital. It has successfully built up the related knowledge to overcome the complicacy between the medical imaging modalities and engineering graphic solid modeling with high resolution. It is worthy to mention here that the present solid modeling of prostate can demonstrate the variable diameters and courses of the prostate urethra in vivo. The second step focuses on thermal calculation. So far, there has been no existing commercial software for the specific purpose of the bioheat transfer problem. Hence, user subroutines must be added to the existing commercial software to simulate the clinical situation of cryosurgery. For example, the occurrence of phase change during some specified temperature range and the latent heat of fusion are also incorporated into bio-heat transfer model. It has successfully incorporated bioheat transfer model into the software program to fit the reality in thermal medicine. The third step supplies the data and knowledge concerned with the position of a tumor and the related mechanism of metabolism of living tissue and vessels. The number of probes, the position of each probe, and the operating time of each probe will be explored to ensure a complete killing of the tumor tissue while saving as much healthy surrounding tissue as possible. In this study, the three-dimensional transient temperature distributions based on cryosurgery for prostate tumors have been performed for several cases to find the optimal operating conditions. Different cryoprobes with different freezing time are considered to find the temperature distribution. The simulation results for cryosurgery of prostate tumors will be supplied for practicing physicians as reference to greatly improve the effectiveness of cryosurgery.

An experimental investigation on the effects of condenser temperatures, heating modes and heat inputs on a miniature, three dimensional flat-plate oscillating heat pipe (3D FP-OHP) was conducted visually and thermally. The 3D FP-OHP was charged with acetone at a filling ratio of 0.80, had dimensions of 101.60 × 63.50 × 2.54 mm 3 , possessed 30 total turns, and had square channels on both sides of the device with a hydraulic diameter of 0.762 mm. Unlike traditional flat-plate designs, this new three-dimensional, compact design allows for multiple heating arrangements and higher heat fluxes. Transient and steady-state temperature measurements were collected at various heat inputs and the activation/start-up was clearly observed for both bottom and side heating modes during reception of its excitation power for this miniature 3D FP-OHP. The neutron imaging technology was simultaneously employed to observe the internal working fluid flow for all tests directly through the heat pipe’s copper wall. The activation was accompanied with a pronounced temperature field relaxation and the onset of chaotic thermal oscillations — all occurring with the same general oscillatory pattern at locations all around the 3D FP-OHP. Qualitative and quantitative analysis of these thermal oscillations, along with the presentation of the average temperature difference and thermal resistance, for all experimental conditions are provided. The novelty of the three-dimensional OHP design is its ability to still produce the oscillating motions of liquid plugs and vapor bubbles and, more importantly, its ability to remove higher heat fluxes.

The implementation of in vivo imaging technologies, such as digital photography, dermoscopy and confocal scanning laser reflectance microscopy (CSLM) in dermatology has led to recent improvements in recognizing skin lesions. Specifically, in the case of skin cancer, a key issue is that the rate of cancerous tissue growth and changes in its spatial extent with time are linked to the energy released locally by these uncontrolled metabolic processes. We believe that with a properly designed infrared (IR) imaging and measurement system combined with thermal analysis, one can characterize healthy and diseased tissue. This paper augments our previous work, in which we introduced a computational model to estimate the location and size of lesions using IR imaging data. In this paper, we focus on calibrating the IR camera and correcting its inherent artifacts. Calibration and corrections are first performed on a blackbody object and then on human skin images in order to acquire accurate surface temperature distributions. As future work, in addition to these correction steps, several other steps, such as accounting for emissivity variations will be developed for clinical studies. In addition to IR imaging, images acquired by in vivo confocal scanning laser microscopy are used to examine the structure of the human skin for different skin types. Our aim is to generate additional data necessary for the IR imaging model by further analyzing the 3D structure of healthy tissue and the lesion. Specifically, in clinical studies, confocal images will be used to describe thermal associations with skin lesion and its blood supply in order to refine our transient thermal model of skin tissue.

One of the most important functions of an electronic package is thermal management, as package is responsible from removing the heat generated by the transistors to ensure reliability. The quality of the package is very important for proper thermal management and it is important to have minimal flaws that increase thermal resistance of the package. Therefore, detection of flaws in the multi-layered package is critical during the assembly process development to monitor the package quality. This is achieved by techniques such as computerized tomography (CT) using x-rays, or scanning acoustic microscopy (SAM), all of which require very expensive equipment and significant processing time. Thermal diffusion tomography (TDT) can be used for detecting the flaws as a lower cost alternative to these imaging techniques. The feasibility of TDT as a fault detection technique for electronic packages with IR thermometry is considered in the current study. Two reconstruction algorithms considered; an iterative perturbation approach and Levenberg-Marquard method were found to be capable of detecting the flaws in the thermal interface layer.

A Finite Difference Time Domain (FDTD) simulation is employed to calculate electromagnetic field and charge density distributions at the junction between a gold nanoparticle (NP) tip of a scanning ZnO nanowire and gold NP bio-markers. This three-dimensional simulation calculates the magnetic and electric field components in a large matrix of Yee cells by solving Maxwell’s curl equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber. In the specific simulations considered here, a laser pulse of single wavelength is incident on the backside of a transparent silicon dioxide micro-cantilever, and coupled into a ZnO nanowire grown from an opening on a metal coating of the front side of the cantilever. The simulation results reveal local field enhancement between the gold NP tip of the nanowire and only one of three 20 nm gold NPs with a 28 nm empty spacing between two adjacent NPs. The charge density distributions in the gold tip and the gold NP are calculated and correlated with the local field enhancement, which makes the gold tip of the scanning nanowire waveguide attractive for use in imaging gold NP bio-labels on cell membranes.