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 D38: Focus Session: Modeling, Computations and Applications of Wetting/Dewetting Problems IFSI
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Chair: Stephane Zaleski, Institut d'Alembert, CNRS & UPMC, Paris Room: 304 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D38.00001: Kinetic effects in dynamic wetting James Sprittles The maximum speed at which a liquid can wet a solid is limited by the need to displace gas lubrication films in front of the moving contact line. The characteristic height of these films is often comparable to the mean free path in the gas so that hydrodynamic models do not adequately describe the flow physics. In this talk, I will develop a model which incorporates kinetic effects in the gas, via the Boltzmann equation, and can predict experimentally-observed increases in the maximum speed of wetting when (a) the liquid's viscosity is varied, (b) the ambient gas pressure is reduced or (c) the meniscus is confined. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D38.00002: Laws of spreading: When hydrodynamic equations are not enough Pirouz Kavehpour, Alireza Mohammad Karim, Jonathan Rothstein, Stephen Davis For nearly 50 years, most of the researchers in the area of wetting and spreading have used a relationship between the dynamics contact angle and velocity and the equilibrium contact angle. Different forms of this relationship are known as Tanner's law, Hoffman-Voinov-Tanner law or Cox model, all of them are derived based on hydrodynamics assumptions. In this talk, we will discuss several common situations that this relationship is not valid and we propose a new way to look at spreading problem and its underlying physics. Our experimental result agrees with this interpretation of spreading dynamics. In addition, the experimental study has been performed using forced spreading with tensiometer to obtain the dependence of dynamic contact angle to the contact line velocity to describe the spreading dynamics of Newtonian liquids on the micro-textured surfaces. The effect of the geometrical descriptions of the micro-posts along with the physical properties of liquids on the spreading dynamics on micro-textured Teflon plates have been also studied. It was shown that hydrodynamic results are not valid for certain combination of fluid/solid systems. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D38.00003: Rapidly moving contact lines and damping contributions Yi Xia, Susan Daniel, Paul Steen Contact angle varies dynamically with contact line (CL) speed when a liquid moves across a solid support, as when a liquid spreads rapidly. For sufficiently rapid spreading, inertia competes with capillarity to influence the interface shape near the support. We use resonant-mode plane-normal support oscillations of droplets to drive lateral contact-line motion. Reynolds numbers based on CL speeds are high and capillary numbers are low. These are inertial-capillary motions. By scanning the driving frequency, we locate the frequency at peak amplification (resonance), obtain the scaled peak height (amplification factor) and a measure of band-width (damping ratio). We report how a parameter for CL mobility depends on these scanning metrics, with the goal of distinguishing contributions from the bulk- and CL-dissipation to overall damping. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D38.00004: Mechanisms of dynamic wetting failure in the presence of soluble surfactants Satish Kumar, Chen-Yu Liu, Marcio S. Carvalho A hydrodynamic model and flow visualization experiments are used to understand the mechanisms through which soluble surfactants can influence the onset of dynamic wetting failure. In the model, a Newtonian liquid displaces air in a rectangular channel in the absence of inertia. A Navier-slip boundary condition and constant contact angle are used to describe the dynamic contact line, and surfactants are allowed to adsorb to the interface and moving channel wall (substrate). The Galerkin finite element method is used to calculate steady states and identify the critical capillary number $Ca^{crit}$ at which wetting failure occurs. It is found that surfactant solubility weakens the influence of Marangoni stresses, which tend to promote the onset of wetting failure. The experiments indicate that $Ca^{crit}$ increases with surfactant concentration. For the more viscous solutions used, this behaviour can largely be explained by accounting for changes to the mean surface tension and static contact angle produced by surfactants. For the lowest-viscosity solution used, comparison between the model predictions and experimental observations suggests that other surfactant-induced phenomena such as Marangoni stresses may play a more important role. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D38.00005: Vorticity dipoles and a theoretical model of a finite force at the moving contact line singularity Peter Zhang, Adam DeVoria, Kamran Mohseni In the well known works of Moffatt (1964) and Huh \& Scriven (1971), an infinite force was reported at the moving contact line (MCL) and attributed to a non-integrable stress along the fluid-solid boundary. In our recent investigation of the boundary driven wedge, a model of the MCL, we find that the classical solution theoretically predicts a {\it finite} force at the contact line if the forces applied by the {\it two} boundaries that make up the corner are taken into consideration. Mathematically, this force can be obtained by the complex contour integral of the holomorphic vorticity-pressure function given by $G = \mu \omega + ip$. Alternatively, this force can also be found using a carefully defined real integral that incorporates the two boundaries. Motivated by this discovery, we have found that the rate of change in circulation, viscous energy dissipation, and viscous energy flux is also finite per unit contact line length. The analysis presented demonstrates that despite a singular stress and a relatively simple geometry, the no-slip semi-infinite wedge is capable of capturing some physical quantities of interest. Furthermore, this result provides a foundation for other challenging topics such as dynamic contact angle. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D38.00006: The role of convective acceleration in determining the velocity and dynamic angle at the contact line Joseph Thalakkottor, Kamran Mohseni The major challenges associated with the moving contact line problem are in determining the velocity and the dynamic contact angle at the contact line. Our theoretical and numerical studies show that the key factor common to both is the role of convective acceleration in the vicinity of the moving contact line. We present our unified slip boundary condition, which demonstrates that slip velocity near a contact line is not just dependent on shear rate, but also on linear strain rate. Also, we present a microscopic dynamic contact angle model that shows dependency not only on surface tensions of respective interfaces, but also on their gradients. The presence of both the linear strain rate and surface tension gradient in the vicinity of the contact line is attributed to convective acceleration. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D38.00007: Large scale simulations of forced dewetting Shahriar Afkhami, Stephane Zaleski We report on the numerical simulations of moving contact lines, in a Volume-Of-Fluid context, for the forced dewetting problem. In our study, we use the Gerris flow solver and quantify the amount of numerical slip in the numerical method. Using the asymptotic hydrodynamic theory of the vicinity of the contact line and matching it to the static theory of menisci, we derive a theory for the effect of the mesh size and the imposed contact angle at that scale on the large scale regions of the simulation. The numerical slip along with an adaptation of the contact angle, which involves a numerical scaling factor, lead to an implicit dynamic contact angle model in our simple numerical method already used for static contact angles. We show that the scaling factor in our theory is related to a microscopic length scale that involves unknown coefficients which appear in the higher order terms in the asymptotic matching; we deduce the coefficients by direct comparison of the resolved numerical simulations with Cox's asymptotic theory. We show that our numerical procedure can be thought of as a subgrid model, that is imposing a dynamic contact angle consistent with the contact line velocity predicted by the asymptotic analysis. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D38.00008: Numerical studies of film formation in context of steel coating. Wojciech Aniszewski, Stephane Zaleski, Stephane Popinet In this work, we present a detailed example of numerical study of film formation in the context of metal coating. Liquid metal is drawn from a reservoir onto a retracting solid sheet, forming a coating film characterized by phenomena such as longitudinal thickness variation (in $3$D) or waves akin to that predicted by Kapitza and Kapitza (visible in two dimensions as well). While the industry standard configuration for Zinc coating is marked by coexistence of medium Capillary number (Ca$=0.03$) and film Reynolds number above $1000$, we present also parametric studies in order to establish more clearly to what degree does the numerical method influence film regimes obtained in the target configuration. The simulations have been performed using \mathtt{Basilisk}, a grid-adapting, strongly optimized code derived from \mathtt{Gerris}. Mesh adaptation allows for arbitrary precision in relevant regions such as the contact line or the meniscus, while a coarse grid is applied elsewhere. This adaptation strategy, as the results indicate, is the only realistic approach for numerical method to cover the wide range of necessary scales from the predicted film thickness (hundreds of microns) to the domain size (meters). [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D38.00009: Forced dewetting for robust scalable generation of Double Emulsions drops with thin shells Antoine Vian, Esther Amstad Double emulsions drops are small drops contained in larger drops. They can be used as picoliter-sized vessels to conduct chemical or biochemical reactions or to conduct high throughput screening assays. Key to a successful application of these drops is a good stability against coalescence and rupture. Previous studies have shown that the mechanical stability of double emulsion drops increases if their shell is reduced below the µm scale. Unfortunately, the fabrication of double emulsion drops with such thin shells at high throughputs is still challenging. We present here a new microfluidics device that reduces the thickness of double emulsion shells to values as low as 250 nm at very high throughputs. This is achieved by injecting double emulsion drops with thick shells into a microfluidic channel that is intersected by many shunt channels. These shunt channels remove a large volume fraction of the oil, contained in the shells of double emulsion drops, thereby reducing their shell thickness to values below 250 nm. We demonstrate that the reduction of the shell thickness of double emulsions improves their mechanical stability and lowers their permeability. [Preview Abstract] |
Sunday, November 19, 2017 4:12PM - 4:25PM |
D38.00010: Dynamic drying transition versus free-surface cusps Jens Eggers We study air entrainment by a solid plate plunging into a viscous liquid, theoretically and numerically. At dimensionless speeds Ca $=$ U$\eta $/$\gamma $ of order unity, a near-cusp forms due to the contact line. The radius of curvature of the cusp's tip scales by the slip length, multiplied by an exponential of Ca. The pressure of the air drawn inside the cusp leads to a bifurcation, at which air is entrained. [Preview Abstract] |
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