Surface roughness is one of the most important factors to determine the flow and heat transfer characteristics of the microchannel. This paper experimentally and theoretically investigated the effects of surface roughness for the flow and heat transfer behavior within the circular microchannel. The stainless steel circular microchannels were fabricated by electrical spark-erosion perforating and drilling separately to control the relative roughness of the surface which is 1% for drilling method and 1.5% for electrical spark-erosion perforating method. Each test piece includes 44 identical circular microchannels in parallel with diameter of 0.4 mm. In the experiments, the air flowed through the circular microchannels with Reynolds number changing from 200 to 2600. The results showed that the surface roughness in microchannels has a remarkable effect on the performance of flow behavior and heat transfer within the circular microchannel. The values of Poiseuille number and Nusselt number are higher when the surface relative roughness is larger. At the same time, the flow behavior is inconsistent with the behavior within the macrochannel. For the flow behavior, Poiseuille number increases monotonously with the increase of Reynolds number, and is higher than the constant theoretical value. The Reynolds number for the transition from laminar to turbulent flow is between 1400 and 1600. For the heat transfer property, Nusselt number also increases as the increase of the Reynolds number.

The manufacture method based on the silicon etching process is one of the most important methods to fabricate micro mechanical structure, e.g. micro-engine. In the processing, the high aspect ratio silicon etch process (HARSE process) is very important to improve the efficiency of structure. At the same time, the surface morphology should be controlled exactly to keep the performance of structure. In this paper, the feasibilities of controlling the surface morphology and Si etch rates were experimentally investigated. In the experiments, the width of structure changes from 15um to 1500um and the depth changes from 50um to 500um. The parameters of surface morphology including sidewall angle, surface roughness, and so on were measured and compared. The influence mechanisms of etching parameters were analyzed. The etching process were completed in a surface technology system (STS) multiplex advanced silicon etcher inductively coupled plasma (ICP) system with SF6/O2 plasma as etching plasma and C4F8 as passivation plasma. In the experiments, the etching experiments were conducted in a low pressure (5–50mTorr), high density, inductively coupled plasma etching reactor (ICP) with a planar coil. The Si etches rates and sidewall angle were investigated as a function of chamber pressure, cathode RF-power, and gas flow. The results indicated that the increasing of total etching time results in an acceleration in etch rate as well as the decrease in sidewall angle (the top width of trench is narrow than the bottom width). Meanwhile, the total passivation time has an opposite effect in the influence of etch rate and sidewall angle. All the experiments indicate that the quick shift between etch and passivation period leads to a smoother surface. An interesting phenomenon were discovered that the etch rate will not change with the changing of width parameter in most of the high aspect ratio silicon etch recipes when the width-depth ratio is upper than 0.34. An experiential function formula were fitted based on four parameters, including width and depth of the structure, and total etching and passivation time.

This paper developes a model to predict the thermal contact resistance of the vertically aligned carbon nanotubes (VACNTs). The model includes the effects of CNT array properties and surface roughness, with the aim of providing useful information for optimizing CNT array thermal contact resistance. The contact resistance is consisted of two parts: interfacial thermal resistance and constriction thermal resistance. The carbon nanotube (CNT) is treated as a thin elastic rod and macroscopic mechanical is used to calculate the mechanical properties of CNT. Greenwood-Williamson (GW) model is used to describe the roughness. The interfacial thermal resistance is calculated by molecular dynamics. The calculated values are in good agreement with experimental data. The interfacial thermal resistance is the domain major factor.

For enhancement of absorption and transmission at a specified wavelength using magnetic polariton (MP) resonance, it is necessary to determine the accurate geometry parameters at the corresponding resonance condition. In this work, the feature of the geometry design problem is analyzed and a method is presented for accurately determining the geometry parameters for specified MP resonance mode, which combines the LC circuit model for MP and inverse technique. The LC circuit model is used to give an initial rough design of geometric parameters and parameters range for the inverse algorithm. The particle swarm optimization (PSO) algorithm is used to minimize the objective function and determine the optimized geometric parameters. The forward problem to evaluate the objective function is solved using rigorous method. The presented method is demonstrated to have good performance in the geometry design of MP resonance structure using several example cases.

Thermochromic coating on yttria stabilized zirconia (YSZ) substrate was prepared by the sol-gel La 0.825 Sr 0.175 MnO 3 nanoparticles and the binder composed of terpineol and ethyl cellulose. The surface morphology and the variable emittance properties of coating in the infrared range was evaluated from infrared reflectance spectra at various temperatures. LSMO nanoparticles with size 200 nm are obtained by the sol-gel process. The coating sample shows a thermochromic behavior with the rise of temperature. The emittance of coating increases from 0.62 at 173 K to 0.88 at 373 K. When the sample surface root-mean-square roughness is 122 nm, its emittance variation increases from 0.43 at 173 K to 0.73 at 373 K.

Despite that using surface-roughness-induced superhydrophobic surface as a solution for ice/snow accretion issues has achieved extensive progresses, its icephobicity breaks down in case of condensation frosting, while the high aspect ratio structure brings more concerns on its durability and sustainability. In this work we investigated condensate frosting on substrates fabricated with patterned micropillars having a small aspect ratio, and studied the freezing propagation with different pattern sizes. The results show that a coarse patterned substrate can effectively suppress the freeing propagation while a fine patterned one can drastically promote the freezing propagation. Frost coverage can also be reduced with proper pattern design. A theoretical model was developed to explain the mechanism of surface ice propagation, and agrees well in tendency with experiment measurements. The aim of this study is to provide some new insights on the influence of surface morphology on ice growth.

In modern microprocessors, thermal management has become one of the main hurdles in continued performance enhancement. Cooling schemes utilizing single phase microfluidics have been investigated extensively for enhanced heat dissipation from microprocessors. However, two-phase fluidic cooling devices are becoming a promising approach, and are less understood. This study aims to examine two-phase flow and heat transfer within a pin-fin enhanced micro-gap. The pin-fin array covered an area of 1cm × 1cm and had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, (aspect ratio of 1.33). Heating from two upstream heaters was considered. The working fluid used was R245fa. The average heat transfer coefficient was evaluated for a range of heat fluxes and flow rates. Flow regime visualization was performed using high-speed imaging. Results indicate a sharp transition to convective flow boiling mechanism. Unique, conically-shaped two-phase wakes are recorded, demonstrating 2D spreading capability of the device. Surface roughness features are also discussed.

Non-equilibrium molecular dynamics method is applied to determine the slip length of fluid in the vicinity of the boundaries in Poiseuille flow in a nano-channel with and without nanoscale roughness, in which pressure is imposed to the fluid. Our simulations reveal the boundary roughness effect on the interfacial interaction between the fluid and boundary, which will result in the change of fluid velocity, and this phenomenon is more distinct with boundary roughness height. Bonding energy between solid boundary and fluid atoms is also calculated to make a comparison with that of pure fluid. Roughness changes with highness and its shape, so the key factors controlling fluid flow are presented. The simulation results show that there’s a relationship between the bonding energy at fluid–solid boundary interface and the fluid kinetic properties. In the case of rough surfaces, the bonding strength between liquid and solid atoms has a strong signature in the fluid velocity close to the boundary. In addition, temperature effect is also considered in our simulations, and we find that temperature also affect the hydrodynamic properties of fluid flowing in nano-channel significantly.

In this work, an experimental heat transfer investigation was carried out to investigate the combined influence of both amorphous carbon (a-C) layer thickness and carbon nanofibers (CNFs) on the convective heat transfer behavior. Synthesis of these carbon nano structures was achieved using catalytic chemical vapor deposition process (CCVD) on a 50 μm nickel wire at 650°C. Due to their extremely high thermal conductivity, CNFs are used to augment/modify heat transfer surface. However, the inevitable layer of a-C that occurs during the synthesis of the CNFs layer exhibit low thermal conductivity which may result in insulating the surface. In contrast, the amorphous layer helps in supporting and mechanical stabilizing of the CNFs layer attachment to the polycrystalline nickel (Ni270) substrate material. To better understand the influences of these two layer on heat transfer, the growth mechanism of the CNFs layer and the layer of carbon is investigated and growth model is proposed. The combined impact of both a-C and CNFs layer on heat transfer performance is studied on three different samples which were synthesized by varying the deposition period (16 min, 23min and 30 min). The micro wire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the micro wire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Maximum heat transfer enhancement of 18% was achieved. This enhancement is mainly attributed to the surface roughness and surface area increase of the samples with moderate CNFs surface area coverage on the sample.

Non-uniform heat flux generated by microchips can create “hot spots” in localized areas on the microchip surface. This research presents an improved design of an active cooling electro-osmotic flow (EOF) based micro-pump for hot spots thermal management. The design of the micro-pump was simpler and more practical for the application compared to designs presented in literature. Most micro-channel heat sink devices presented in literature were silicon based. Though silicon has better thermal conductivity when compared to polymers used in micro-devices fabrication, processes of silicon fabrication are complicated and time consuming. Also, most micro-channel fabrication processes use silicon etching which leads to rough walls within the micro-channel. An improved design, which uses a combination of silicon and Polydimethylsiloxane (PDMS), is being developed and tested. The main idea of this design is to utilize the favorable thermal properties of silicon while achieving both smoother charged surfaces and ease of fabrication of PDMS material. The EOF micro-pump was tested for four cooling fluid namely, DI water, distilled water, borax buffer, and Al 2 O 3 nano-particles suspended in water solution. A maximum flow rate of 31.2 μL/min was achieved using distilled water at 500 V of EOF voltage. Such micro-pump with this flow rate range can be implemented in a closed loop heat rejection system for hot spot thermal management. Moreover, it can be used in Lap-on-chip and uTAS application for sample transport.

Thermal and optical engineering applications of electromagnetic wave reflectance from rough surfaces include temperature measurement, radiation heating process, etc. Most of the surfaces are random roughness and often with coating material different from the substrate. This paper presented a novel hybrid partial coherence and geometry optics (HPCGO) model to improve the generic geometry optics (GO) method by incorporating a previously developed partial coherence reflectance equation. In this way, HPCGO expands the applicable region of GO model, and largely reduces the computation time of integrating different wavelength results in the regular hybrid model that considers coherence effect only. First, the HPCGO model is validated by more rigorous Maxwell equations solvers, for example, the finite-difference time-domain (FDTD) method and integral equation (IE) method. Then, the HPCGO model is applied to study the coherent effect of directional-hemispherical reflectance from coated rough surfaces. It is found the roughness of coated rough surface can cause partial or non-coherent scattered light even if the incident light source is coherent. It also shows the coherence effect reduces with increased incident wave-number bandwidth.

The bidirectional reflectance distribution function (BRDF) describes spatial distribution of light scattered from rough surfaces and can be used to calculate other radiative properties. There have been many researches focus on the BRDF of rough surface structures. A number of approximate techniques such as perturbation theory, the Kirchhoff approximation, and geometric optics approximation (GOA) are widely used due to the complexity of the electromagnetic theory. However, these approximate techniques neglect some phenomenon such as surface plasmon polaritons (SPP), the diffraction effect, cavity resonance etc., which can be efficiently studied by the finite-difference time-domain (FDTD) method. As the increasing improvement of the computational science, it is feasible to simulate the radiative properties of complex surface structures by electromagnetic theory. Silicon tends to oxidize naturally, which hence changes the radiative properties of the rough surface structure. The focus of this study is on the effect of oxide films on BRDF of two-dimensional randomly rough surface of silicon using the FDTD method. The BRDF of different surface roughness of silicon substrate coated with oxide film has been investigated. The results show that, the BRDF of the rough surface of silicon will be affected by the RMS roughness (σ) and correlation length (τ). The energy of the reflection peaks increase as the roughness decreases (σ), when the correlation length (τ) remains the same. And the energy of the reflection peaks decrease as the correlation length decreases, when the roughness remains the same. The calculation domain and time increase as the rate of στ/λ increases. Moreover, when the rough surface of silicon covered with oxide film, the reflection peak will be weakened obviously, which is mainly due to that the absorbing oxide film.

Monte Carlo model is used to study the roughness effects on lattice thermal conductivity of silicon nanowires. Based on the Matthiessen’s rule, a roughness influence coefficient is adopted to amend the relaxation time. Individual silicon nanowires growing by vapour-liquid-solid method are different from those making by aqueous electroless etching method in the surface roughness, which are both simulated over a temperature range of 15–315K with various equivalent diameters. The results are close to the laboratory data, proving the validity of the MC model and showing the effects of surface roughness on thermal transport decrease with the increasing of equivalent diameter.

This paper studies the roughness effect of the two-dimensional micro Poiseuille gas flows in a curved channel by a modified lattice Boltzmann model. A method relating to the Knudsen number ( Kn ) with the relaxation time is discussed. In addition, to capture the slip velocity on a solid boundary more accurately, a combined boundary scheme (CBC), which combines the no-slip bounce-back and the free-slip specular reflection schemes, is applied to boundary condition treatment. The rough wall of the micro-channel is described by uniformly distributed rectangular or triangular rough elements. The simulation results show that the roughness and the geometries of the channel have great effects not only on velocity distribution but also on pressure distribution.

The fractal characterization of the topography of rough surfaces by using Cantor set structures is introduced in this paper. Based on the fractal Cantor surface, a model of laminar flow in rough microchannels is developed and numerically analyzed to study the characterization of surface roughness effects on laminar flow. The effects of Reynolds number, relative roughness, and fractal dimension on laminar flow are all discussed. The results indicate that the presence of roughness leads to the form of the detachment, and eddy generation is observed at the shadow of the roughness elements. The pressure drop in the rough channel along the flow direction is no longer in a linear fashion and larger than that in the smooth channel. The fluctuation characteristic of pressure drop along the stream, which is due to the vortex formation at the wall, is found. Differing from the smooth channel, the Poiseuille number for laminar flow in rough microchannels is no longer only dependent on the cross-sectional shape of the channel, but also strongly influenced by the Reynolds number, relative roughness and fractal dimension of the surface.

Superhydrophobic surfaces, especially lotus leaf surface, have been largely explored due to their great importance in fundamental research and abundant potential applications. However, many efforts have been focused on investigating the superhydrophobic surfaces in air instead of in water environment, which are rather crucial to industrial separation progress. A novel air bubble bursting effect on lotus leaf surface was firstly discovered and the underlying mechanism was believed to be related to the micro/nano-hierarchical rough structures. Inspired by air bubble bursting effect on lotus leaf, a superhydrophobic “artificial lotus leaf” with similar micro/nano-hierarchical rough structures was successfully constructed by photolithography and wet etching and also achieved air bubble bursting effect. Smooth and rough silicon surface with the ordered nano-structure or patterned micro-structure were utilized to study the contribution of the micro/nano-hierarchical structures to air bubble bursting, and it was found that air bubble could burst on the superhydrophobic surfaces with micro-structure, but more time was required, while nano-structure could accelerate air bubble bursting. Moreover, the height, width, and spacing of hierarchical structures also affected air bubble bursting, and the effect of the height was more obvious. When the height of hierarchical structures was around the height of lotus papillae, the width and spacing were significant for air bubble bursting. Eventually, an original model was proposed to further evaluate the reason that the micro/nano-hierarchical rough structures had an excellent air bubble bursting effect, and its validity was theoretically demonstrated. It was believed that these findings should spark further theoretical study of some bubble-related interfacial phenomena and find its wide applications in the industrial separation process without any accessional energy and other additives, such as mineral flotation, food processing, textile dyeing, and fermentation.

In this study, we explore the clear idea of drag change when the surface contains roughness. For that purpose, we performed a simple terminal velocity experiment using small solid spheres. The terminal velocity of Cu spheres is measured before and after they are surface treated to form superhydrophilic or superhydrophobic nanostructures and appreciable increase in the terminal velocity for both cases is observed. An analytic solution is derived to evaluate the corresponding slip length in an external flow and the result is comparable to values reported in previous studies of lithographically patterned superhydrophobic surfaces. To gain useful physical insight, incompressible Navier-Stokes equations are solved. From the solution, a simple explanation for the experimental observation is developed using the concept of friction drag and form drag. The total drag can be reduced when the reduction of the friction drag is larger than the increase of the form drag.

Roughness effects on flow and heat transfer in flat microchannels has been numerically simulated by using CFD with fluid-solid conjugate heat transfer techniques, the surface roughness has been modeled through a series triangular toothed roughness cells. In this paper, the influence for roughness on the entrance length of flow and heat transfer has been emphasized, the influence for relative roughness on transitional Reynolds number has been also analyzed at the same time.

Electrokinetic transport in nanofluidics has attracted many attentions in recent years due to its potential applications in biomedical analysis and energy conversion systems. The biggest challenge to model the electrokinetic transport using atomistic simulations is the extensive cost in calculation of the long-range electrostatic force between ions. In this work, we develop an efficient molecular approach to simulate electrokinetic transport in nano-scale. The long-range Coulombic interactions are treated using the Particle-Particle Particle-Mesh (P 3 M) scheme and the Poisson equation for electric potential is solved in physical space using an iterative multi-grid technique. We implement this approach to systematically investigate two examples: the electroosmotic flow in random rough channels and the electrowetting on dielectric (EWOD). Both cases pose important engineering applications as a mechanism for transporting small amount of liquid in micro and nano devices.

Newfound attention has been given to solute transport in nanochannels. Because the electric double layer (EDL) thickness is comparable to characteristic channel dimensions, nanochannels have been used to separate ionic species with a constant charge-to-size ratio (i.e., electrophoretic mobility) that otherwise cannot be separated in electroosmotic or pressure-driven flow along microchannels. In nanochannels, the electrical fields within the EDL cause transverse ion distributions and thus yield charge-dependent mean ion speeds in the flow. Surface roughness is usually inevitable during microfabrication of microchannels or nanochannels. Surface roughness is usually inevitable during the fabrication of nanochannels. In the present study, we develop a numerical model to investigate the transport of charged solutes in nanochannels with hundreds of roughness-like structures. The model is based on continuum theory that couples Navier-Stokes equations for flows, Poisson-Boltzmann equation for electrical fields, and Nernst-Planck equation for solute transports. Different operating conditions are considered and the solute transport patterns in rough channels are compared with those in smooth channels. Results indicate that solutes move slower in rough nanochannels than in smooth ones for both pressure-driven and electroosmotic flows. Moreover, solute separation can be significantly improved by surface roughness under certain circumstances.