Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q11: Drops: Wetting and Spreading IIDrops FSI
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Chair: Uddalok Sen, University of Illinois at Chicago Room: 504 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q11.00001: Spreading of a liquid bridge Joachim Delannoy, Daniel Beilharz, Christophe Clanet, David Quere We observe the spreading of a liquid bridge under a horizontal surface. After being pulled up to twice its capillary length, a bridge is formed between a liquid bath and a flat horizontal surface. This bridge then spreads radially over a large range (several centimeters) at a constant speed: the radius of the bridge $r$ progress linearly with the time ($r \sim t$). We study experimentally the parameters impacting the spreading, and develop a theoretical analysis to model the dynamics. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q11.00002: Bridging of Liquid Droplets into a Porous Substrate Kevin Murphy, Jonathan Boreyko When the top of a sessile droplet is brought into contact with an opposing surface, the droplet can transfer to the new surface. Previous reports have characterized the extent and speed of droplet transfer as a function of the surface and droplet properties; however, the two surfaces have always been impermeable. What if the surface receiving the liquid was porous instead? Here, we use side-view high-speed imaging to capture the transfer of liquid from a solid substrate to an opposing porous surface. Variables to consider include the wettability of the donor surface, the porosity and pore size of the receiving surface, and the droplet's volume, viscosity, and surface tension. Generally, the transfer process is split into two regimes: the wetting transition, similar to the wetting of the receiving solid surface in the solid-to-solid transfer, and the wicking transition, where the liquid is pulled into the porous surface. The wetting transition scales with the capillary-inertial velocity for low viscosity fluids and the visco-capillary velocity for viscous fluids, while the wicking transition scales with Darcy's Law. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q11.00003: Fluorescence microscopy of precursor films in evaporating droplets. Sahar Andalib, Pirouz Kavehpour Precursor films are present near the contact line of wetting fluids during spreading process on a solid surface. Despite its importance in many industrial applications like coating technologies, microfluidics, heat pipes, etc., the underlying mechanism of precursor films is not yet fully understood. In the present study, fluorescence microscopy is used to visualize and study the behavior of an evaporating precursor film. In spite of the limitations of other techniques such as ellipsometry, interferometry, and atomic force microscopy in capturing the phenomena, fluorescence microscopy provides adequate spatial as well as temporal range and resolution and is a noninvasive method. This work will contribute to our understanding of the physics of evaporating contact lines where there exists a gap between numerical and theoretical models due to singularity in evaporation rate and viscous forces. Detailed experimental data will provide valuable insight into the mechanism of fluid spreading and interfacial phenomena. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q11.00004: Effect of surface roughness on contact line dynamics of a thin droplet Debanik Bhattacharjee, Babak Soltannia, Hadi Nazaripoor, Mohtada Sadrzadeh Any surface possesses inherent roughness. Droplet spreading on a surface is an example of a contact line problem. The tri-phase contact line is prone to stress singularity which can be relieved by using precursor film assumption and disjoining pressure. In this study, an axisymmetric, incompressible, Newtonian droplet spreading on a surface was investigated. An evolution equation which tracks the droplet height over time was obtained considering the lubrication approximation. The nonlinear PDE of evolution equation was solved using finite difference scheme. A simplified Gaussian model was used as a starting point to assess the role of roughness in the dynamics of contact line. The preliminary results revealed that, for both impermeable and permeable surfaces, the apparent contact angle increased in the presence of defects whereas the equilibrium stage remained unaffected. The apparent contact angle, however, was more strongly dependent on the nature and density of defects for impermeable surfaces due to the longer droplet lifetime. Furthermore, random self-affine and non-Gaussian models are employed. The mathematical model results are finally compared with theoretical models like the Cassie-Baxter, Wenzel, and Penetration modes. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q11.00005: The role of characteristic parameters in the numerical prediction of droplet radius and contact angle Debanik Bhattacharjee, Hadi Nazaripoor, Mohtada Sadrzadeh Identifying proper characteristic parameters (radius and height of droplet) is the first step toward numerical prediction of droplet base radius and contact angle. In this study, a developed model based on lubrication approximation was applied to examine the effect of characteristic parameters on the droplets spreading over permeable and impermeable substrates. Characteristic radius and height were first evaluated based on two main stages of spreading (initial and equilibrium). The model predictions were then compared with the experimental results available in the literature. The study provides evidence that the selection of characteristic length scales based on both stages provides accurate prediction of the droplet base radius and contact angle. In addition, a modified scaling relation was proposed which relates the theoretical value of the disjoining pressure parameter to its numerical counterpart through the lubrication ratio. This relation enabled accurate prediction of the numerical disjoining pressure (error of \textpm 5{\%}). The proposed method greatly simplified the initial guess for the disjoining pressure parameter in the numerical simulation as previously there was no possibility of ascertaining the value. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q11.00006: Droplet Depinning on Inclined Surfaces at High Reynolds Numbers Edward White, Natasha Singh, Sungyon Lee Contact angle hysteresis enables a sessile liquid drop to adhere to a solid surface when the surface is inclined, the drop is exposed to gas-phase flow, or the drop is exposed to both forcing modalities. Previous work by Schmucker and White (2012.DFD.M4.6) identified critical depinning Weber numbers for water drops subject to gravity- and wind-dominated forcing. This work extends the Schmucker and White data and finds the critical depinning Weber number obeys a two-slope linear model. Under pure wind forcing at Reynolds numbers above 1500 and with zero surface inclination, $We_{\mathrm{crit}} = 8.0$. For non-zero inclinations, $\alpha$, $We_{\mathrm{crit}}$ decreases proportionally to $A\,Bo\,\sin\alpha$ where $A$ is the drop aspect ratio and $Bo$ is its Bond number. The same relationship holds for $\alpha < 0$ when gravity resists depinning by wind. Above $We \approx 4$, depinning is dominated by wind forcing; at $We < 4$, depinning is gravity dominated. While $We_{\mathrm{crit}}$ depends linearly on $A\,Bo\,\sin\alpha$ in both forcing regimes, the slopes of the the limit lines depend on the forcing regime. The difference is attributed to different drop shapes and contact angle distributions that arise depending on whether wind or gravity dominates the depinning behavior. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q11.00007: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q11.00008: Moving contact lines on vibrating surfaces Zlatko Solomenko, Peter Spelt, Julian Scott Large-scale simulations of flows with moving contact lines for realistic conditions generally requires a subgrid scale model (analyses based on matched asymptotics) to account for the unresolved part of the flow, given the large range of length scales involved near contact lines (Sui et al., Annu. Rev. Fluid Mech. 2014). Existing models for the interface shape in the contact-line region are primarily for steady flows on homogeneous substrates, with encouraging results in 3D simulations (Solomenko et al., J. Comput. Phys. 2017). Introduction of complexities would require further investigation of the contact-line region, however. Here we study flows with moving contact lines on planar substrates subject to vibrations, with applications in controlling wetting/dewetting. The challenge here is to determine the change in interface shape near contact lines due to vibrations. To develop further insight, 2D direct numerical simulations (wherein the flow is resolved down to an imposed slip length) have been performed to enable comparison with asymptotic theory, which is also developed further. Perspectives will also be presented on the final objective of the work, which is to develop a subgrid scale model that can be utilized in large-scale simulations. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q11.00009: Influence of viscosity coefficients during spreading and coalescence of droplets in liquids Thomas Cubaud, Bibin M. Jose We experimentally characterize the role of absolute viscosities on the dynamics of droplet spreading on solids and droplet-droplet coalescence in liquid/liquid systems for a broad range of fluid parameters. In particular, we show the existence of a viscous function based on both inner and outer fluid viscosities that allows for the determination of the critical wetting velocity and the evolution of the contact diameter during immersed spreading and coalescence of droplets. Our approach demonstrates the reduced influence of fluid viscosity from initial wetting to spreading and coalescence of droplets and provides insights into the influence of wetting contact lines on spontaneous capillary phenomena [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q11.00010: Low-order modelling of a drop on a highly-hydrophobic substrate: statics and dynamics Alexander W. Wray, Omar K. Matar, Stephen H. Davis We analyse the behaviour of droplets resting on highly-hydrophobic substrates. This problem is of practical interest due to its appearance in many physical contexts involving the spreading, wetting, and dewetting of fluids on solid substrates. In mathematical terms, it exhibits an interesting challenge as the interface is multi-valued as a function of the natural Cartesian co-ordinates, presenting a stumbling block to typical low-order modelling techniques. Nonetheless, we show that in the static case, the interfacial shape is governed by the Young-Laplace equation, which may be solved explicitly in terms of elliptic functions. We present simple low-order expressions that faithfully reproduce the shapes. We then consider the dynamic case, showing that the predictions of our low-order model compare favourably with those obtained from direct numerical simulations. We also examine the characteristic flow regimes of interest. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q11.00011: Micro-scale dynamics of oil droplets at a permeable surface Guy Ramon, Gali Fux Microscopic imaging was used to quantify the deformation of droplets at the surface of a permeable membrane during separation of oil/water micro-emulsions. The shape of individual droplets was imaged in 3D, using confocal microscopy, as a function of the permeation rate through the membrane (V), droplet radius (R) and membrane permeance (k). These parameters, along with the water viscosity ($\mu )$ and the water-oil surface tension coefficient ($\sigma )$, were used to construct a modified capillary number, accounting for the proximity to the membrane surface. The results demonstrate a clear correlation between drop deformation from a sphere to an approximate hemisphere in response to increases in the capillary number. Furthermore, the reversibility of droplet deposition was assessed through image analysis of membrane surface coverage. The results demonstrate that droplets deposited at low permeation are easily removed by axial shear flow (applied in the absence of permeation), leaving a clean surface, whereas at a high permeation deposition is mostly irreversible. A wetting transition, dependent on the stability of a thin film separating the droplets from the membrane, is proposed as a mechanism for explaining the observed irreversible oil deposition. [Preview Abstract] |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q11.00012: Thermocapillary droplet actuation on structured solid surfaces George Karapetsas, Nikolaos T. Chamakos, Athanasios G. Papathanasiou The present work investigates, through 2D and 3D finite element simulations, the thermocapillary-driven flow inside a droplet which resides on a non-uniformly heated patterned surface. We employ a recently proposed sharp-interface scheme capable of efficiently modelling the flow over complicate surfaces and consider a wide range of substrate wettabilities, i.e. from hydrophilic to super-hydrophobic surfaces. Our simulations indicate that due to the presence of the solid structures and the induced effect of contact angle hysteresis, inherently predicted by our model, a critical thermal gradient arises beyond which droplet migration is possible, in line with previous experimental observations. The migration velocity as well as the direction of motion depends on the combined action of the net mechanical force along the contact line and the thermocapillary induced flow at the liquid-air interface. We also show that through a proper control and design of the substrate wettability, the contact angle hysteresis and the induced flow field it is possible to manipulate the droplet dynamics, e.g. controlling its motion along a predefined track or entrapping by a wetting defect a droplet based on its size as well as providing appropriate conditions for enhanced mixing inside the droplet. [Preview Abstract] |
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