Accepted Manuscripts

Yu Feng, Jie Cao, Xin Li, Silong Zhang, Jiang Qin and Yu Rao
J. Heat Transfer   doi: 10.1115/1.4037086
An idea of using dimples as heat transfer enhancement device in a regenerative cooling passage is proposed to extend the cooling limits for liquid-propellant rocket and scramjet. Numerical studies have been conducted to investigate the flow and heat transfer characteristics of supercritical hydrocarbon fuel in a rectangular cooling channel with dimples applied to the bottom wall. The numerical model is validated through experimental data and accounts for real fuel properties at supercritical pressures. The study shows that the dimples can significantly enhance the convective heat transfer and reduce the heated wall temperature. The average heat transfer rate of the dimpled channel is 1.64 times higher than that of its smooth counterpart. While the pressure drop in the dimpled channel is only 1.33 times higher than that of the smooth channel. Furthermore, the thermal stratification in a regenerative cooling channel is alleviated by using dimples. Although heat transfer deterioration of supercritical fluid flow in the trans-critical region cannot be eliminated in the dimpled channel, it can be postponed and greatly weakened. The strong variations of fuel properties are responsible for the local acceleration of fuel and variation of heat transfer performance along the cooling channel.
Ammar Alsabery, Ali J. Chamkha, Ishak Hashim and Pradeep G. Siddheshwar
J. Heat Transfer   doi: 10.1115/1.4037087
The effects of non-uniform heating and a finite wall thickness on natural convection in a square porous cavity based on the local thermal non-equilibrium (LTNE) model are studied numerically using the finite difference method. The finite thickness horizontal wall of the cavity is heated either uniformly or non-uniformly and the vertical walls are maintained at constant cold temperatures. The top horizontal insulated wall allows no heat transfer to the surrounding. The Darcy law is used along with the Boussinesq approximation for the flow. The results of this study are obtained for various parametric values of the Rayleigh number, thermal conductivity ratio, ratio of the wall thickness to its height and the modified conductivity ratio. Comparisons with previously published work verify good agreement with the proposed method. The effects of the various parameters on the streamlines, isotherms, and the weighted average heat transfer are shown graphically. It is shown that a thicker bottom solid wall clearly inhibits the temperature gradient which then leads to the thermal equilibrium case. Further, the overall heat transfer is highly affected by the presence of the solid wall. The results have possible applications in the heat-storage fluid-saturated porous systems and the applications of the high power heat transfer.
John Tencer, Kevin Carlberg, Marvin Larsen and Roy E. Hogan
J. Heat Transfer   doi: 10.1115/1.4037098
Radiation heat transfer is an important phenomenon in many physical systems of practical interest. When participating media is important, the radiative transfer equation (RTE) must be solved for the radiative intensity as a function of location, time, direction, and wavelength. In many heat-transfer applications, a quasi-steady assumption is valid, thereby removing time dependence. The dependence on wavelength is often treated through a weighted sum of gray gases (WSGG) approach. The discrete ordinates method (DOM) is one of the most common methods for approximating the angular (i.e., directional) dependence. The DOM exactly solves for the radiative intensity for a finite number of discrete ordinate directions and computes approximations to integrals over the angular space using a quadrature rule; the chosen ordinate directions correspond to the nodes of this quadrature rule. This work applies a projection-based model-reduction approach to make high-order quadrature computationally feasible for the DOM for purely absorbing applications. First, the proposed approach constructs a reduced basis from (high-fidelity) solutions of the radiative intensity computed at a relatively small number of ordinate directions. Then, the method computes inexpensive approximations of the radiative intensity at the (remaining) quadrature points of a high-order quadrature using a reduced-order model constructed from the reduced basis. This results in a much more accurate solution than might have been achieved using only the ordinate directions used to compute the reduced basis. One- and three-dimensional test problems highlight the efficiency of the proposed method.
TOPICS: Approximation, Heat transfer, Wavelength, Gases, Radiation (Physics), Radiative heat transfer
Technical Brief  
Akihiro Kishimoto, Hideki Moriai, Kenichiro Takenaka, Takayuki Nishiie, Masaki Adachi, Akira Ogawara and Ryoichi Kurose
J. Heat Transfer   doi: 10.1115/1.4037099
A new non-adiabatic procedure of the flamelet/progress-variable approach (NA-FPV approach) is proposed, and the validity is assessed by performing a large-eddy simulation (LES) employing the NA-FPV approach for an O2/H2 combustion field in a single element coaxial combustor under a pressurized condition. The results show that the LES employing the NA-FPV approach can successfully predict the heat flux and capture the effects of heat loss through the cooled walls on the combustion characteristics. This procedure is quite useful especially for the numerical simulations of combustion fields with high temperatures, where there remain reactive radicals (e.g. OH, CH) with high concentrations, such as pressurized combustion, supercritical combustion, and oxygen combustion.
TOPICS: Combustion, Computer simulation, Combustion chambers, Heat losses, Oxygen, Large eddy simulation, Heat flux, High temperature
Qi Yao, Michael J. Stubblebine and Ivan Catton
J. Heat Transfer   doi: 10.1115/1.4037079
An Inorganic Aqueous Solution, known as IAS, has shown its compatibility with aluminum phase change heat transfer devices. When using IAS in aluminum devices, aluminum prefers to react with the two oxidizers, permanganate and chromate, rather than water to generate a thin and compact layer of aluminum oxide, which protects the aluminum surface and prevents further reactions. In addition, an electrochemical theory of aluminum passivation is introduced, and the existence of an electrochemical cycle is demonstrated by an aluminum thermo-syphon test. The electrochemistry cycle, built up by liquid back flow and tube wall, allows the oxidizers to passivate the aluminum surface inside the device without being directly in contact with it. However, failure was detected while using IAS in thermo-syphons with air natural convection cooling. The importance of a continuous liquid back flow to aluminum passivation in phase change heat transfer devices is pointed out, and a vertical thermo-syphon test with natural convection cooling is used to demonstrate that a discontinuous liquid back flow is the main reason of the failures.
TOPICS: Aluminum, Heat transfer, Flow (Dynamics), Cooling, Natural convection, Cycles, Failure, Water, Electrochemistry
Qiang Zhang and Liang-Bi Wang
J. Heat Transfer   doi: 10.1115/1.4037080
To find the reasons of heat transfer enhancement of a laminar convective heat transfer process in a channel at uniform heat flux boundary when a pair of the longitudinal vortex generators is mounted on the bottom wall, the laminar convective heat transfer process in the channel is investigated numerically in a frame built up by the convective transport equation of the heat flux. The results show that longitudinal vortices greatly increase the local convection contribution terms that determine the local intensity of the convective transport of the heat flux component in the span direction, and that the increased local contribution terms intensify the local convective transport of the heat flux component in the same direction. This process increases the convection contribution terms that determine the convective transports of the heat flux components in the main stream direction and in the normal direction of the channel walls. The increase of these convection contribution terms results in an enhancement of the convective heat transfer capacity on the channel walls, and then the heat transfer is enhanced by longitudinal vortices. When the span averaged characteristic is numerically counted, longitudinal vortices are found to have no contribution on the span averaged convective transport of the heat flux in the span direction.
TOPICS: Heat transfer, Channel flow, Vortices, Generators, Heat flux, Convection
Te Pi, Kevin Cole and James Beck
J. Heat Transfer   doi: 10.1115/1.4037081
Estimation of thermal properties or diffusion properties from transient data requires that a model is available that is physically meaningful and suitably precise. The model must also produce numerical values rapidly enough to accommodate iterative regression, inverse methods, or other estimation procedures during which the model is evaluated again and again. Bodies of infinite extent are a particular challenge from this perspective. Even for exact analytical solutions, because the solution often has the form of an improper integral that must be evaluated numerically, lengthy computer-evaluation time is a challenge. The subject of this paper is improving the computer evaluation time for exact solutions for infinite and semi-infinite bodies in the cylindrical coordinate system. The motivating applications for the present work include the line-source method for obtaining thermal properties, the estimation of thermal properties by the laser-flash method, and the estimation of aquifer properties or petroleum-field properties from well-test measurements. In this paper the computer evaluation time is improved by replacing the integral-containing solution by a suitable finite-body series solution. Although the series solution is approximate, the precision of the series solution may be controlled to a high level and the required computer time may be minimized, by a suitable choice of the extent of the finite body. An easy-to-use relationship is developed for the finite-body size needed as a function of desired precision and as a function of time.
TOPICS: Diffusion (Physics), Lasers, Heat conduction, Transients (Dynamics), Thermal properties, Computers, Petroleum
Pakeeza Hafeez, Sanjeev Chandra and Javad Mostaghimi
J. Heat Transfer   doi: 10.1115/1.4037082
An experimental study of heat transfer in metal foam heat exchangers fabricated from 10 and 40 pores per inch (PPI) was conducted. Heat exchangers were made by either brazing Inconel sheets to foam or plasma spraying Inconel skins on the foam. A burner test rig was built to produce high temperature combustion gases at either 550 °C or 750 °C that were passed over the exposed surface of heat exchangers that were cooled by passing air through them at rates of up to 200 SLPM. Both pressure drop and temperature rise of the air were measured. Friction factors and volumetric heat transfer coefficients were calculated for air velocities varying from 0.1 to 5 m/s and dimensionless correlations to predict these derived. Heat exchangers with 40 PPI foam had higher heat transfer rates than those with 10 PPI foam, but also larger pressure drops. Thermal sprayed heat exchangers were found to perform better than those that were brazed since they had lower thermal contact resistance between the external shell and foam struts. An analytical model was developed assuming local thermal non-equilibrium (LTNE) and predictions from model were found to be in good agreement with experimental results.
TOPICS: Heat transfer, Gas flow, Heat exchangers, Metal foams, High temperature, Pressure drop, Shells, Contact resistance, Heat transfer coefficients, Equilibrium (Physics), Friction, Temperature, Combustion gases, Plasma spraying, Brazing
Weihong Li, Wei Shi, Xueying Li, Jing Ren and Hongde Jiang
J. Heat Transfer   doi: 10.1115/1.4037085
The effects of hole length to diameter ratio and compound angle on flat plate film cooling effectiveness are investigated from an experimental and numerical view. Film cooling effectiveness measurements are performed for seven blowing ratios (M) ranging from 0.3 to 2, five hole length to diameter ratios (L/D) from 0.5 to 5 and two compound angle (ß: 0°, 45°) using pressure sensitive paint (PSP) technique. Results indicate that discrete holes with L=0.5 and 1 show highest film cooling effectiveness regardless of compound angle. Round hole generally shows an increasing trend as L increases from 2 to 5, while compound angle hole shows a complex trend concerning with blowing ratios and length to diameter ratios. Compound angle enhances film cooling effectiveness with high blowing ratios and length to diameter ratios. In a parallel effort, LES approach is employed to solve the flow field and visualize vortex structures of in-tube and mainstream regions. It is demonstrated that the counter rotating vortex pair (CRVP) which is observed in the time-averaged flow field is originated in different vortex structures with varying blowing ratios and length to diameter ratios. Scalar field transportation features are also investigated to clarify how different vortex structures affect the temperature distribution and the film cooling effectiveness.
TOPICS: Pressure, Flow (Dynamics), Paints, Large eddy simulation, Film cooling, Vortices, Flat plates, Temperature distribution, Scalar field theory, Transportation systems
Fraj Echouchene and Hafedh Belmabrouk
J. Heat Transfer   doi: 10.1115/1.4037061
This paper investigates the effect of temperature-jump boundary condition on nonequilibrium entropy production under the effect of the dual-phase-lag heat conduction model in a two-dimensional sub-100 nm metal-oxide-semiconductor field-effect transistor (MOSFET). The transient Dual-phase lag (DPL) model is solved using finite-element method. Also, the influences of the governing parameters on global entropy generation for the following cases: (I) constant applied temperature and (II) temperature-jump boundary condition are discussed in detail and depicted graphically. The analysis of our results indicates that entropy generation minimization inside a MOSFET can be achieved by using temperature-jump boundary condition and for low values of Knudsen number. A significant reduction of the order of 85% of total entropy production is observed when a temperature-jump boundary condition is adopted.
TOPICS: Entropy, Temperature, MOSFET transistors, Boundary-value problems, Transistors, Heat conduction, Metal oxide semiconductors, Transients (Dynamics), Finite element methods, Knudsen number
Mahdi Nabil and Alexander S Rattner
J. Heat Transfer   doi: 10.1115/1.4037062
Characterization of wavy film heat and mass transfer is essential for numerous energy-intensive chemical and industrial applications. While surface tension is the underlying cause of film waviness, widely used correlations for falling-film heat transfer do not account for surface tension magnitude as a governing parameter. Furthermore, although the effect of Prandtl number on wavy falling film heat transfer has been highlighted in many studies, it is not included in most published Nusselt number correlations. Contradictory trends for Nusselt number variation with Prandtl number are found in correlations that do account for such effects. A systematic simulation-based parametric study is performed here to determine the individual effects of Reynolds, Prandtl, and Capillary numbers on heat transfer in laminar-wavy falling films. First-principles based volume-of-fluid (VOF) simulations are performed for wavy falling condensation with varying fluid properties and flow rates. A sharp surface tension volumetric force model is employed to evaluate interface dynamics accurately. The numerical model is first validated for smooth falling-film condensation heat transfer and wavy falling film thickness. The simulation approach is applied to isolate and quantify Nusselt number variations with Reynolds, Prandtl, and Capillary numbers. Finally, based on the collected simulation data, a new Nusselt number correlation for laminar-wavy falling film condensation is proposed.
TOPICS: Surface tension, Film condensation, Prandtl number, Heat transfer, Simulation, Fluids, Condensation, Mass transfer, Flow (Dynamics), Heat, Computer simulation, Dynamics (Mechanics), Film thickness
Yue-Ting Zhou and Tae-Won Kim
J. Heat Transfer   doi: 10.1115/1.4036981
A dynamic, partially permeable crack model for orthotropic materials is established with the crack full of thermal medium. Besides external thermal and elastic loadings, the heat flux of the crack interior depending on the crack opening displacement is applied on the crack surfaces. Thus, the heat conduction is dependent on elastic field. First, the heat conduction equation is solved exactly in terms of unknown heat flux of the crack interior. Then, the elastic field is presented for real or complex eigenvalues cases on the basis of the operator theory. Finally, the thermal and elastic fields are presented analytically, and the heat flux of the crack interior is determined explicitly. Numerical results are offered to show the influences of the thermal conductivity coefficient, normal and shear loadings and crack velocity on the distributions of the heat flux, temperature difference across the crack surfaces and thermal stress intensity factor. Figures illustrate that increasing the crack velocity leads to a more thermally impermeable crack and produces a bigger temperature difference across the crack surfaces.
TOPICS: Heat conduction, Stress, Fracture (Materials), Eigenvalues, Thermoelasticity, Heat flux, Temperature, Thermal conductivity, Displacement, Shear (Mechanics), Thermal stresses
Guohui Zhou, Ji Li, Lucang LV and G.P. Peterson
J. Heat Transfer   doi: 10.1115/1.4036982
Presented here are the results of an experimental investigation of two ultra-thin miniature loop heat pipes (mLHPs) with different internal wicking structures: one with a primary wicking structure in the evaporator and a secondary wicking structure in the liquid line and the other only with the same primary wicking structure in the evaporator, but no secondary wick. The systematic experimental investigation was conducted using natural convection as the cooling mechanism in order to study the heat transfer performance of the two mLHPs and fully examine the effects of the secondary wick. The results indicated that both of the test articles could effectively dissipate 12 W at all test orientations with a minimum total thermal resistances of 6.38 oC/W and 6.39 oC/W, respectively. However, the results indicated that the presence of the secondary wicking structure in the liquid line at low power loads, resulted in more stable startup characteristics and a weaker dependence on the different orientations. Moreover, it was demonstrated that the steady-state evaporator temperatures of the test article with the secondary wicking structure in the liquid line were much lower than those observed for a 1 mm thick copper plate with the same geometric dimensions for all heat loads in the horizontal orientation, showing a higher thermal performance.
TOPICS: Heat pipes, Stress, Natural convection, Steady state, Heat, Temperature, Heat transfer, Cooling, Copper, Dimensions
Shuo Yang, Ruquan Liang, Song Xiao, Jicheng He and Shuo Zhang
J. Heat Transfer   doi: 10.1115/1.4036871
The influence of shear airflow on the free surface deformation and the flow structure for large Prandtl number fluid (Pr=111.67) has been analyzed numerically as the parallel airflow is induced into the surrounding of liquid bridge from the lower disk or the upper disk. Against the former studies, an improved level set method is adopted to track any tiny deformation of free surface, where the area compensation is carried out to compensate the non-conservation of mass. Present results indicate that the airflow can excite flow cells in the isothermal liquid bridge. The airflow induced from the upper disk impulses the convex region of free interface as the airflow intensity is increased, which may exceed the breaking limit of liquid bridge. The free surface is transformed from the "S"-shape into the "M"-shape as the airflow is induced from the lower disk. For the non-isothermal liquid bridge, the flow cell is dominated by the thermocapillary convection at the hot corner if the airflow comes from the hot disk, and another reversed cell flow near the cold disk appears. While the shape of free surface depends on the competition between the thermocapillary force and the shear force when the airflow is induced from the cold disk.
TOPICS: Fluids, Bridges (Structures), Air flow, Surface deformation, Gravity (Force), Flow (Dynamics), Prandtl number, Disks, Shapes, Shear (Mechanics), Corners (Structural elements), Impulse (Physics), Convection, Deformation
Tingting Liu, Liancun Zheng, Yiming Ding and Lin Liu
J. Heat Transfer   doi: 10.1115/1.4036872
This paper investigates the effects of thermo-capillarity on the flow and heat transfer in power-law liquid film over an unsteady stretching sheet. The surface tension is assumed to vary linearly with temperature and the thermal conductivity of the fluid is assumed power-law-dependent on the velocity gradient with modified Fourier’s law. The local similarity solutions are obtained numerically and some interesting new phenomena are found. Results indicate that the thermally-induced surface tension provides an opposite force in the direction of the stretching sheet which may cause the fluid adjacent to the free surface flows in the opposite directions. The effect of thermo-capillarity tends to decrease the thin film thickness and results in a smaller temperature distribution. With increasing unsteadiness parameter, the thin film thickness has a local maximum and thermal boundary layer is confined to the lower part of the thin film for bigger Prandtl number, while the temperature in the thin film remains equal to the slit temperature with Prandtl number close to 0.
TOPICS: Capillarity, Thin films, Temperature, Flow (Dynamics), Fluids, Prandtl number, Surface tension, Temperature distribution, Thermal boundary layers, Thermal conductivity, Liquid films, Heat transfer
Guanchen Li and Michael R. von Spakovsky
J. Heat Transfer   doi: 10.1115/1.4036735
Conventional first-principle approaches for studying nonequilibrium processes depend on the mechanics of individual particles or quantum states and as a result, require many details of the mechanical features of the system to arrive at a macroscopic property. In contrast, thermodynamics, which has been successful in the stable equilibrium realm, provides an approach for determining macroscopic properties without the mechanical details. Nonetheless, this phenomenological approach is not generally applicable to a nonequilibrium process except in the near-equilibrium realm and under the local equilibrium and continuum assumptions, both of which limit its ability to describe nonequilibrium phenomena. Furthermore, predicting the thermodynamic features of a nonequilibrium process (of entropy generation) across all scales is difficult. To address these drawbacks, steepest-entropy-ascent quantum thermodynamics (SEAQT) can be used. It provides a first-principle thermodynamic-ensemble based approach applicable to the entire nonequilibrium realm even that far-from-equilibrium and does so with a single kinematics and dynamics, which crosses all temporal and spatial scales. Based on prior developments by the authors, SEAQT is used here to study the heat and mass diffusion of indistinguishable particles. The study focuses on the thermodynamic features of far-from-equilibrium state evolution, which is separated from the specific mechanics of individual particle interactions. Results for nonequilibrium size (volume) and concentration effects on the evolutionary state trajectory are presented for the case of high temperature and low particle concentration, which, however, do not impact the generality of the theory and will in future studies be relaxed.
TOPICS: Thermodynamics, Heat, Diffusion (Physics), Entropy, Identical particles (Quantum mechanics), Equilibrium (Physics), Particulate matter, Kinematics, Dynamics (Mechanics), Trajectories (Physics), High temperature
Md Mahamudur Rahman and Matthew McCarthy
J. Heat Transfer   doi: 10.1115/1.4036693
While significant enhancements in pool boiling critical heat flux (CHF) and heat transfer coefficient (HTC) have been demonstrated using structured surfaces, fundamental questions remain about the nature of the enhancements and the role of structure length scale. This work presents a systematic investigation of structures from 100’s of nanometers to several millimeters . Specifically, copper substrates were fabricated with five different microchannel geometries and four different copper oxide nanostructured coatings. Additionally, twenty different multiscale structures were fabricated coinciding with each permutation of the various microchannels and nanostructures. The nanostructured coatings were observed to increase CHF via surface wicking, but decrease HTC due to the suppression of nucleation. The microchannels were observed to increase both CHF and HTC, generally outperforming the nanostructured coatings. The multiscale surfaces exhibited superior performance, with CHF and HTC values as high as 313 W/cm2 and 461 W/m2K, respectively. Most importantly, multiscale surfaces were observed to exhibit the individual enhancement mechanisms seen from each length scale, namely increased nucleation and bubble dynamics from the microchannels, and wicking- enhanced CHF form the nanostructures. Additionally, two of the surfaces tested here exhibited uncharacteristically high HTC values due to a decreasing wall superheat at increasing heat fluxes. While the potential mechanisms producing this counterintuitive behavior are discussed, further research is needed to definitively determine its cause.
TOPICS: Copper, Boiling, Critical heat flux, Microchannels, Coatings, Nanostructures , Nucleation (Physics), Heat transfer coefficients, Pool boiling, Dynamics (Mechanics), Heat, Flux (Metallurgy), Bubbles
Chukwudi Azih and Metin I. Yaras
J. Heat Transfer   doi: 10.1115/1.4036689
A key subject of interest for technologies that involve flows of fluids at the supercritical thermodynamic state is the development of prediction methods that capture the fluid dynamics and convection heat transfer at this state. Due to the elevated temperatures and pressures associated with certain working fluids at this thermodynamic state, surrogate fluids are often used as substitutes for performing experiments during the design stages of prototype development. The success of this approach depends on the development of similarity criteria or fluid-to-fluid models. Similarity criteria for mixed-convection heat transfer in supercritical fluids are proposed based on a set of non-dimensional dynamic similarity parameters and state-space parameters developed through our current understanding of the physical mechanisms that affect heat transfer in fluids at this state. The proposed similarity criteria are successfully validated using data from ducted flows of supercritical fluids with configurations having upstream, downstream, or wall-normal oriented gravitational acceleration.
TOPICS: Flow (Dynamics), Supercritical fluids, Heat transfer, Mixed convection, Modeling, Fluids, Engineering prototypes, Convection, Design, Temperature, Gravitational acceleration, Fluid dynamics
Mehrdad Mehrvand and Shawn A. Putnam
J. Heat Transfer   doi: 10.1115/1.4036691
The demands for increasingly smaller, more capable, and higher power density technologies have heightened the need for new methods to manage and characterize extreme heat fluxes. This work presents the use of an anisotropic-version of the Time-domain Thermoreflectance (TDTR) technique to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and sub-cooled channel-flow configuration. Studies focused on room temperature, single-phase, degassed water flowing at an average velocity of ≈3.5 m/s in a ≈480 µm hydraulic diameter microchannel (e.g., Re ≈ 1850), where the TDTR pump heating laser induces a local heat flux of ≈900 W/cm2 in the center of the microchannel with a hot-spot area of ≈250 µm2. By using a differential TDTR measurement approach, we show that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the single-phase convective heat transfer coefficient, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Our measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, our flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ≈1060±190 kW/m2·K, where the Nusselt number correlations predict ≈900±150 kW/m2·K.
TOPICS: Thermoreflectance, Water, Microchannels, Heat transfer coefficients, Heating, Forced convection, Natural convection, Pumps, Power density, Heat flux, Fluid dynamics, Flow (Dynamics), Heat, Temperature, Cooling, Fluids, Metals, Lasers, Heat conduction, Flux (Metallurgy), Coolants, Anisotropy, Channel flow, Convection
Collin T. Burkhart, Kara Maki and Michael J. Schertzer
J. Heat Transfer   doi: 10.1115/1.4036681
This investigation provides experimental evidence examining the role of interface capture on the transport and deposition of colloidal material in evaporating droplets. It finds that deposition patterns cannot be characterized by the ratio of the interface velocity to the particle diffusion rate alone when the two effects are of the same order. Instead, the ratio of radial velocity to the particle diffusion rate should also be considered. Ring depositions are formed when the ratio of radial velocity to the particle diffusion rate is greater than the ratio of interface velocity to diffusion. Conversely, uniform depositions occur when the ratio of radial velocity to diffusion is smaller than the ratio of interface velocity to diffusion. Transitional depositions with a ring structure and non-uniform central deposition are observed for cases where the characteristic ratios are similar in magnitude. Since both ratios are scaled by diffusion rate, it is possible to characterize the deposition patterns observed here using a ratio of interface velocity to radial velocity. Uniform patterns form when interface velocity is greater than radial velocity and ring patterns form when radial velocity is larger. However, Marangoni effects are small and DLVO forces repel particles from the surface in these cases. Further research is required to determine if the conclusions here can be extended or modified to describe deposition patterns when particles are subjected to appreciable Marangoni recirculation and attractive DLVO forces.
TOPICS: Diffusion (Physics), Drops, Evaporation, Particulate matter

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