Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G36: Drops: Surface Wetting |
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Chair: Paul Steen, Cornell University Room: Portland Ballroom 251 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G36.00001: Measuring contact-line mobility during inertial spreading Paul Steen, Susan Daniel, Yi Xia During "inertial spreading", when inertia drives a partially wetting liquid across a solid, the role of bulk viscosity may be neglected. For such inertial-capillary motions, behavior of the moving contact-line (CL) can be understood within the context of ideal (or nearly ideal) fluid motion, provided an alternate to the Voinov-Hocking-Cox model of mobility is adopted. The alternate we adopt is the so-called Hocking condition. In this talk, we report experiments with Resonantly-Driven Droplets (RDD) whereby the bulk resonance of the drop amplifies the small and fast CL motion sufficiently to be measurable. The RDD approach enables us to measure a CL mobility and to infer a CL dissipation for droplets on a number of hydrophobic surfaces, surfaces with varying contact-angle hysteresis. Our results are compared to prior results in the literature, measured with alternative approaches. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G36.00002: Effects of elasticity and surface tension on the spreading dynamics of a thin film under the influence of intermolecular forces. Yuan-Nan Young, Howard Stone The spreading dynamics of a thin layer of viscous Newtonian fluid between an elastic sheet and a wetting solid substrate is examined using the lubrication theory. On the wetting substrate an ultra thin film (precursor film) develops as a result of the intermolecular force between the fluid and the wetting solid substrate. Such a precursor film prevents the stress singularity associated with a moving contact line. Following the methodology by $\backslash $citet\textbraceleft Glasner2003\textunderscore PoF\textbraceright , the effects of elasticity on the macroscopic contact line structure in the quasistatic limit are elucidated by an ordinary differential equation derived from an analysis of the energy and its dissipation. Similar to the case of a regular fluid interface with surface tension (capillary spreading), the elasto-capillary thin film profile also consists of a core at the center, an ultra thin film in the far field, and a contact line region where the core film profile connects smoothly to the precursor film. For capillary spreading, the precursor film transitions monotonically to the core film. Due to the interfacial elasticity, a spatial oscillation of film height in the contact line region is found. In addition, it is found that elasticity causes the sliding motion of the thin film: the contact angle close to zero as [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G36.00003: How a Nanodroplet Diffuses on Smooth Surfaces. Chu Li, Jizu Huang, Zhigang Li In this study, we investigate how nanodroplets diffuse on smooth surfaces through molecular dynamics (MD) simulations and theoretical analyses. The simulations results show that the surface diffusion of nanodroplet is different from that of single molecules and solid nanoparticles. The dependence of nanodroplet diffusion coefficient on temperature is surface wettability dependent, which undergoes a transition from linear to nonlinear as the surface wettability is weakened due to the coupling of temperature and surface energy. We also develop a simple relation for the diffusion coefficient by using the contact angle and contact radius of the droplet. It works well for different surface wettabilities and sized nanodroplets, as confirmed by MD simulations. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G36.00004: Liquid drop spreading on surfaces: Initial regimes revisited Surjyasish Mitra, Sushanta Mitra Liquid drop spreading on a given surface is fundamental towards technological processes like coating and paints, inkjet printing, surface characterization, etc. Though, the underlying dynamics is well understood, we have revisited this problem through experiments conducted on surfaces kept in air as well as immersed in water. It was found that the two key parameters that dictated the spreading process were drop-surrounding medium viscosity ratio and the characteristic viscous length scale. It was observed that irrespective of the drop liquid and surrounding liquid medium (air and water in this case), spreading always began in a regime dominated by drop viscosity, where the spreading radius scales as r$\sim$t . However, the prefactor of the scaling observed was different for air (of the order of unity) and under-water (much less than unity). Following this initial regime, a second intermediate regime dominated by drop inertia (typically found for water drops spreading in air) was observed only when the characteristic viscous length scale favored such a transition. In this regime as well, a non-universal prefactor was noted for the scaling law, i.e., r$\sim$t$^{\mathrm{1/2}}$. In all cases considered, the spreading process terminated in the Tanner's regime where the spreading radius scaled as r$\sim$t$^{\mathrm{1/10}}$. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G36.00005: Thickness effect in the statics and dynamics of wetting on soft materials Menghua Zhao, Matthieu Roche, Julien Dervaux, Laurent Royon, Tetsuharu Narita, Fran\c{c}ois Lequeux, Laurent Limat The wetting of liquids on soft materials such as elastomers has received a great deal of attention in the past decades. Many experiments were performed to gain insight into both the statics and dynamics of wetting in such systems, but most neglected the effect of finite thickness of the gel. Here we report results of a study of the thickness effect on both the statics and dynamics of wetting. We vary the thickness of soft silicone elastomers from 10$^{\mathrm{-2}}$ to a few mm. First, we develop a quantitative Schlieren optics enabling us to observe the surface deformation after the deposition of droplets. We measure the vertical deformation outside droplets as a function of droplet size, gel thickness and elasticity. We identify a submicrometer-deep dimple, that extends over mm away from the contact line. Second, we characterize the receding dynamics and we show that the dynamic contact angle, hence dissipation, depends on the thickness of the sample. We rationalize our experiments, with an analytical model accounting for the linear elastic response of the gel bulk and its surface tension. We find excellent agreement with experiments. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G36.00006: Wetting on a deformable substrate with finite deformations and asymmetrical substrate surface energies. Laurent Limat, Riccardo De Pascalis, Julien Dervaux, Ioan Ionescu, Benoit Perthame Wetting on soft compounds is still imperfectly understood, especially when the dry and wetted parts of the substrate have two different values of surface energies (contact angle different than 90 degrees). The problem is made very complex by geometrical non-linearities arising from finite slope of the substrate and finite deformations, that must be absolutely considered, to distinguish at second order between Young law and Neuman equilibrium of surface tensions. We have developed a numerical, finite element, code that allows one to minimize surface and bulk energies, with finite deformations and asymmetry of the surface energies. The results are compared to a linear theory based on Green function theory [1,2] and Fredholm integrals, and with recent experiments using X-ray visualization [3]. The non-linear numerics reproduce very well the observed profiles, while the linear approach gives helpful analytical approximates. [1] L. Limat, EPJ-E Soft Matter, 35, 134 (2012). [2] J. Dervaux {\&} L. Limat, Proc. Roy. Soc. A 471, 20140813 (2015). [3] S. J. Park, B. M. Weon, J. S. Lee, J. Lee, J. Kim {\&} J. H. Je, Nature Com. 5, 4369 (2014). [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G36.00007: Low-order models of the motion of sessile droplets on highly hydrophobic surfaces Alex Wray, Lyes Kahouadji, Omar Matar, Stephen Davis We consider the behaviour of a droplet deposited onto a hydroophobic substrate. This and associated problems have received attention due to their significance in a wide array of experimental and industrial contexts, such as the post-rupture wetting problem is of importance to coating flow applications. Such systems have typically defied low-order analysis due to the multi-valued nature of the interface, but we demonstrate how to resolve this issue in this instance. We begin by analysing the static case. We find that the system is governed by the Young-Laplace equation with the equilbrium shape depending on the Bond number, the contact angle and the volume of the droplet. We solve the system numerically, and use these results to validate a variety of low-order models. We then solve the dynamic problem using both direct numerical simulations and a low-order model based on conservation of energy. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G36.00008: Drop Spreading with Random Viscosity Feng Xu, Oliver Jensen Airway mucus acts as a barrier to protect the lung. However as a biological material, its physical properties are known imperfectly and can be spatially heterogeneous. In this study we assess the impact of these uncertainties on the rate of spreading of a drop (representing an inhaled aerosol) over a mucus film. We model the film as Newtonian, having a viscosity that depends linearly on the concentration of a passive solute (a crude proxy for mucin proteins). Given an initial random solute (and hence viscosity) distribution, described as a Gaussian random field with a given correlation structure, we seek to quantify the uncertainties in outcomes as the drop spreads. Using lubrication theory, we describe the spreading of the drop in terms of a system of coupled nonlinear PDEs governing the evolution of film height and the vertically-averaged solute concentration. We perform Monte Carlo simulations to predict the variability in the drop centre location and width (1D) or area (2D). We show how simulation results are well described (at much lower computational cost) by a low-order model using a weak disorder expansion. Our results show for example how variability in the drop location is a non-monotonic function of the solute correlation length increases. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G36.00009: Droplet wetting transitions on inclined substrates in the presence of external shear and substrate permeability Leonardo Espin, Satish Kumar Understanding the gravity-driven motion of droplets on inclined substrates in the presence of external shear and substrate permeability is important for applications such as spray coating and filtration. In this work, we use a lubrication-theory-based model to study how external shear and substrate permeability affect droplet wetting transitions. A nonlinear evolution equation for the droplet height as a function of time and two spatial variables is derived and numerically solved. The contact-line region is described using a precursor film and disjoining pressure. Depending on its direction, external shear can either suppress or drive wetting transitions, but does not appear to significantly change the critical droplet speeds associated with these transitions. Substrate permeability generally suppresses wetting transitions due to liquid absorption and does appear to significantly affect these critical droplet speeds. The strong influence of substrate permeability and external shear on droplet wetting transitions indicates that it will be important to account for these effects when developing accurate models for industrial applications. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G36.00010: Sinking, wedging, spreading -- viscous spreading on a layer of fluid Nico Bergemann, Anne Juel, Matthias Heil We study the axisymmetric spreading of a sessile drop on a pre-existing layer of the same fluid in a regime where the drop is sufficiently large so that the spreading is driven by gravity while capillary and inertial effects are negligible. Experiments performed with 5 ml drops and layer thicknesses in the range 0.1 mm $\le h \le 1$ mm show that at long times the radius of the drop evolves as $R \propto t^n$, where the spreading exponent $n$ increases with the layer thickness $h$. Numerical simulations, based on the axisymmetric free-surface Navier-Stokes equations, reveal three distinct spreading regimes depending on the layer thickness. For thick layers the drop sinks into the layer, accompanied by significant flow in the layer. By contrast, for thin layers the layer ahead of the propagating front is at rest and the spreading behaviour resembles that of a gravity-driven drop spreading on a dry substrate. In the intermediate regime the spreading is characterised by an advancing wedge, which is sustained by fluid flow from the drop into the layer. [Preview Abstract] |
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