Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J34: Interfacial Fluid Mechanics II: Wetting Phenomena |
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Sponsoring Units: DFD Chair: Edmund Webb III, Lehigh University Room: 210A |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J34.00001: Wetting on surfaces with tailored nano-scale defects Kristina Davitt, Romain Lhermerout, Etienne Rolley Surface heterogeneity is acknowledged as a cause of contact angle hysteresis. More recently, it has also been recognized as having an important effect on contact line dynamics [1]. However, it has proven difficult to design quantitative experiments [2]. There are two fundamental difficulties: (i) to control the heterogeneity and (ii) to fabricate a true reference surface. In many methods, additional well-controlled defects may be added to a surface, however, the untreated substrate itself already presents some chemical or topographical disorder (typically nano-scale). This is illustrated by the fact that even on a purportedly defect-free surface the hysteresis is non-negligible (\textgreater a few degrees). We report on the use of adsorbed short-chain polymer surfaces as nearly ideal reference surfaces (H\textless 0.01$^{\circ}$). Topographical defects of controlled size, shape and density are then added using nanosphere lithography with dilute colloidal suspensions, and the dependence of the hysteresis and low-velocity dynamics on the defect parameters are determined.\\[4pt] [1] K. Davitt \textit{et al.}, \textit{Langmuir }\textbf{29}, 6884 (2013).\\[0pt] [2] S.M.M. Ramos \textit{et al.}, \textit{Phys. Rev. E }\textbf{67}, 031604 (2003). [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J34.00002: Contact line dynamics on a pseudo-brush Romain Lhermerout, Kristina Davitt, Etienne Rolley, Hugo Perrin, Bruno Andreotti Polymer~ brushes are nanometric layers of polymers that are attached to a solid surface. They are well-known for strongly modifying the mechanical properties of the surface. Although friction and slippage experiments have been performed on such systems [1], the impact of brushes on wetting dynamics has not yet been investigated. We report measurements of contact line dynamics of simple liquids over so-called pseudo-brushes adsorbed on silicon. We will show that this system exhibits (i) a surprisingly low hysteresis, a feature of great utility when studying the impact of added defects on the contact angle hysteresis, and (ii) a specific contribution to the dissipation, which is localized at the contact line, in addition to the viscous dissipation in the liquid wedge. The pseudo-brush contribution can be isolated from the total dissipation [2] and a simple model is used to explain the role of the pseudo-brush.\\[4pt] [1] C. Cohen, F. Restagno, C. Poulard, L. L\'{e}ger, Soft Matter 7, 8535 (2011)\\[0pt] [2] G. Delon, M. Fermigier, J.H. Snoeijer, B. Andreotti, J. Fluid Mech. 604, 55-75 (2008) [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J34.00003: Deformation of Nanoscale Elastomeric Free-Standing Films by Sessile Liquid Droplets Rafael Schulman, Kari Dalnoki-Veress The study of liquid droplets on soft, deformable substrates has recently garnered a great deal of attention. In particular, it has been found that droplets deform elastic surfaces at the contact line, and that this deformation can yield contact angles that do not obey Young's law. Rather, the microscopic contact line geometry is dictated by a force balance between the three surface stresses, akin to the Neumann construction for droplets on liquid substrates. In our experiment, we place liquid droplets atop elastomeric free-standing films with thicknesses of hundreds of nanometers. Using optical microscopy, as well as atomic force microscopy, we directly measure the contact line geometry and induced deformation of the free-standing film. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J34.00004: A thermodynamic model for the wetting characteristics of hierarchical physically-patterned surfaces Michael Bell, Azar Shahraz, Kristen Fichthorn, Ali Borhan An understanding of wetting is important for many applications, including superhydrophobic self-cleaning and low-drag surfaces. Many natural examples of such surfaces exist, including insect legs, bird feathers, and plant leaves. The mechanism of superhydrophobicity on these surfaces is known to be related to their hierarchical roughness (i.e., roughness on micro and nano length scales), though the precise role of hierarchical roughness is not yet well understood. We present a two-dimensional thermodynamic model of the wetting of a hierarchically-grooved surface for droplets with variable Bond number. By investigating wetting phase diagrams over the microscale parameter space, we find that for negligible Bond number, surfaces with single-scale roughness are superhydrophobic only in small regions of the phase space--most notably in regions where the droplet is much larger than the size of the surface features--while the addition of nanoscale roughness greatly extends the range of drop sizes for which high contact angles are attained. We also investigate the case of non-negligible Bond number, for which we observe significant changes in the wetting phase diagram arising from gravity-induced interface deformations. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J34.00005: Wetting ridge growth and contact line pinning on viscoelastic solid Su Ji Park, Joshua B. Bostwick, Jung Ho Je Dynamic wetting behaviors on soft viscoelastic solids are potentially important to interpret complex biological processes resulted from cell-substrate interactions. When a droplet sits on a soft surface, its surface tension deforms the contact line, creating a ``wetting ridge,'' which causes characteristic spreading behaviors. The key to understand the underlying mechanisms is to investigate wetting ridge dynamics during spreading. However, it is challenging to explore wetting ridge dynamics, mostly due to limitations in observation. Here, we directly visualize wetting ridges in real-time during spreading using x-ray microscopy with a high spatio-temporal resolution. We reveal that the growth of wetting ridges is dominated by their broadening in early stage and by their heightening in later stage. The two growth mechanisms control the ridge-geometry and determine the spreading behaviors. Most importantly, we find that the contact line pinning is enhanced by increased flexibility of the ridge cusp. Finally, we clarify two different mechanisms of pinning/depinning transitions: ``stick-slipping'' and ``stick-breaking.'' [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J34.00006: Self-pinning of a Nanosuspension Drop: MD Simulations Baiou Shi, Edmund Webb The behavior of nano-fluids, or fluid suspensions containing nano-particles, has garnered tremendous attention recently for applications in advanced manufacturing. Contact line pinning by the particles or self-pinning has been extensively considered during contact line retreat due to solvent evaporation. Here we will present our results from MD simulations on self-pinning of an advancing contact line. For a wetting system of identical liquid, solid and particle chemistry yet significant difference in advancing contact angles, self-pinning is observed for low $\theta_{adv}$ whereas it is not for high $\theta_{adv}$. The role of contact angle in determining likelihood for self-pinning is investigated on fundamental time and length scales. Meanwhile, relations between contact line velocity and advancing contact angle will be discussed from atomic scale computation results. The precursor film continues to advance across the surface even when the droplet is pinned. However a single layer of liquid on the outer facet of the particle surfaces is observed which manifests a rate limiting step for the precursor film advance. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J34.00007: Inertial Wetting Kinetics for Nanometer Scale Droplets Edmund Webb III, Baiou Shi Inertial spreading occurs immediately following contact between a droplet and solid surface. For low viscosity liquids with high wettability, high contact line velocities are observed during this stage. A counterintuitive result from atomic scale simulations is that even nanometer size metallic drops exhibit a regime of wetting that is governed by inertial effects. Using a Tolman length corrected surface tension to account for liquid/vapor interface curvature effects that manifest in small drops, inertial spreading data from molecular dynamics simulations for varying drop size (down to a few nm diameter) can be collapsed onto a single curve using otherwise continuum scale inertial capillary flow theory. In addition, for inertial spreading on a low advancing contact angle surface, a second nanoscale effect is observed, which is related to curvature gradients that manifest along a significant portion of the liquid/vapor interface in the smallest drops. This is caused by rapid advancement of a precursor wetting film. The duration of the inertial regime is computed and shown to scale with the inertial/capillary time scale. Evidence is presented that capillary waves play a role in determining the duration of the inertial wetting regime for low viscosity, highly wetting drops. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J34.00008: Inertia-driven droplet depinning on textured surfaces Sungyon Lee, Benjamin Wilcox, Alireza Hooshanginejad, Alex Berger, Feng Xu, Edward White The stability of drops on surfaces subject to forcing by wind and gravity is relevant to heat exchangers, fuel cells, and aircraft icing, and it lacks understanding in a high Reynolds number regime. To experimentally investigate this phenomenon, water drops are placed on the rough aluminum floor of a tiltable wind tunnel and brought to critical conditions for varying drop sizes, inclination angles, and flow speeds. In particular, the evolving 3D droplet shapes under flow are reconstructed based on a laser-speckle interface measurement tool, while the critical flow rates of droplet depinning are also noted. By accounting for the contact angle hysteresis and the pressure build-up in a nearly turbulent boundary layer, the critical depinning flow rate is theoretically predicted and is compared to the experimental results. We also observe and explain the transition of the drop depinning behavior from inertia-dominated to gravity-dominated regimes at non-zero inclination angles. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J34.00009: Electrowetting on Semiconductors Cesar Palma, Robert Deegan In traditional electrowetting-on-dielectric (EWOD) a sessile drop rests on a thin dielectric separating it from a conductor. A voltage applied between the droplet and the conductor causes an increase in the solid-liquid interface area and a concomitant reduction of the contact angle. The change in the contact angle is well modeled by the Young-Lippmann equation. Here we report experiments where the conductor is replaced by lightly-doped, single-crystal silicon. We observe contact angle changes that are polarity-dependent as well as a non-reversible light-induced wetting transition. As suggested previously we assume that the charge distributions in the system have a direct analogue with the charges states of a metal-oxide-semiconductor capacitor. We calculate the free energy of the system taking into account both capillary and electrostatic contributions. By minimizing this result we derive a modified form of the Young-Lippmann equation. We further enhance our model to include the effect of pinning and well known semiconductor surface effects including interface charges and work function differences. We find that this model works well with experimental results. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J34.00010: How cats and dogs drink differently? Sunghwan Jung, Sean Gart, Jake Socha, Pavlos Vlachos Drinking is defined as the animal action of taking water into the mouth, but to fluid mechanists, it is simply one kind of fluid transport phenomena. Classical fluid mechanics show that fluid transport can be achieved by either pressure-driven or inertia-driven processes. In a similar fashion, animals drink water using pressure-driven or inertia-driven mechanisms. For example, domestic cats and dogs lap water by moving the tongue fast, thereby developing the inertia-driven mechanism. We will investigate how cats and dogs drink water differently and discuss the underlying fluid mechanics. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J34.00011: How droplets nucleate and grow on liquids and liquid impregnated surfaces Sushant Anand, Konrad Rykaczewski, Srinivas Prasad Bengaluru Subramanyam, Daniel Beysens, Kripa Varanasi Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and self-cleaning surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid--air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J34.00012: Phase-Field Modeling of the Buoyancy-Driven Detachment of a Wall-Bound Pendant Drop Andrea Lamorgese, Roberto Mauri We investigate numerically the critical conditions for detachment of an isolated, wall-bound emulsion droplet acted upon by surface tension and wall-normal buoyancy forces alone using a simple extension of a diffuse interface model for partially miscible binary mixtures that was previously employed for simulating several two-phase flow phenomena far and near the critical point [``Phase-Field Approach to Multiphase Flow Modeling," Milan J.~Math.~{\bf 79}, 597 (2011)] to allow for static contact angles other than $90^\circ$. We use the same formulation of the Cahn boundary condition as first proposed by Jacqmin [``Contact-line dynamics of a diffuse fluid interface," J.~Fluid Mech.~{\bf 402}, 57 (2000)], which accommodates a cubic (Hermite) interpolation of surface tensions between the wall and each phase at equilibrium. We show that this model can be successfully employed for simulating three-phase contact line problems in stable emulsions with nearly immiscible components. We also show the first numerical determination of critical Bond numbers as a function of static contact angle by phase-field simulation. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J34.00013: Asymptotic model for three-dimensional coating flow of nematic liquid crystal on an inclined substrate Michael Lam, Lin Te-Sheng, Linda Cummings, Lou Kondic We consider a coating flow of nematic liquid crystal (NLC) film on an inclined substrate. Exploiting the large aspect ratio in the geometry of interest, an asymptotic approach is utilized to derive a fourth order nonlinear partial differential equation governing the evolution of the free surface. Previous results have shown that there exist two-dimensional traveling wave solutions that translate down the substrate. In contrast to the analogous Newtonian flow, such solutions may be unstable to streamwise perturbations. Extending well-known results for Newtonian flow, we analyze the stability of the front with respect to transverse perturbations. Particular attention is paid to the interplay between the bulk elasticity and the anchoring conditions at the substrate and free surface. Using full numerical simulations, we validate the linear stability theory and present examples of downslope flow of NLC in the presence of both transverse and streamwise instabilities. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J34.00014: Contact Angle: Consequence of minimized Casimir Energy Suddarsun Shivakumar In 1805, T. Young, in his classic work, expressed the cosine of the angle subtended by the surface of a liquid droplet on a solid surface in terms of the surface energies of the respective mediums-solid, liquid and gas. More recently, London derived the van der Waals interaction energy using the then recent advent of quantum mechanics. Later, in 1937, H. C. Hamaker attempted to derive the interaction energies between two interacting mediums in contact. But, the van der Waals interaction energies for two bodies diverges as the bodies come in contact. To circumvent this undesired divergence, Hamaker introduced a cut-off distance parameter in his analysis, which typically is argued to be of atomic length. All future work on contact angles, since Hamaker, to our knowledge, has never been discussed without relying on this cut-off parameter. We here show that the contact angle is independent of the cut-off parameter, and free of divergence. Thus, contact angle is a measurable physical quantity. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J34.00015: Sub-nanometric substrate structural changes enhance the solid/liquid slip boundary condition Joshua McGraw, Antoine Bridet, Samuel Grandthyll, Hendrik H\"ahl, Frank M\"uller, Karin Jacobs Alkylsilane self-assembled monolayers (SAMs) have long been used as model substrates for their ease of preparation and hydrophobic properties. We have long observed that these monolayers also provide a slip boundary condition for dewetting polymer films, and that the slip condition is switchable if the alkyl chain length is changed (from 12 to 18 backbone carbons, for example). Typically, this change is affected in a quantized way, using one or the other chain length, thus obtaining one or the other slip condition. It has been suggested that the specific structure of the resulting SAM controls the slip condition. Here, we present results in which this structure is changed in two continuous ways. First, we prepare SAMs containing bidisperse mixtures of alkyl silanes, with the composition as a control parameter. Second, we thermally anneal the SAMs, resulting in an irreversible loss of carbon from the monolayer. In both cases, we find an enhanced slip condition which is tuneable over a certain range. [Preview Abstract] |
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