Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R4: Drops X: Interaction with Heterogeneous Surfaces |
Hide Abstracts |
Chair: Devaraj van der Meer, University of Twente Room: 307 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R4.00001: Lattice Boltzmann Simulations of Drop Impact on Heterogeneous Surfaces Kevin Connington, Taehun Lee, Jeff Morris, Joel Koplik The dynamics of drop impact on heterogeneous substrates is important to understand from a materials research-application perspective. The phenomenon occurs in many situations, from ink-jet printing to fuel injection processes. Since drop impact is a physically complicated process, difficult to understand through experiments and theory alone, it is important to develop methods to study such a process numerically. The impact process poses many computational difficulties as well. Some of these difficulties include the tracking of a liquid-gas interface that undergoes extreme deformation in a short time period, accurately including the effects of surface tension, and realistically resolving the dynamic liquid-gas-solid contact line. Due to its kinetic nature, the Lattice Boltzmann Method(LBM) can incorporate mesoscopic physics into its formulation to resolve these difficulties, which otherwise pose significant problems for traditional continuum solvers. In our simulations, the interface is captured, instead of tracked, by keeping account of an order parameter in the fluid as in the phase field method. Here we discuss our numerical method and report results describing impact on a superhydrophobic surface for drops of large density ratio. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R4.00002: Droplet Dynamics on Heterogeneous Substrates Daniel Herde, Uwe Thiele, Stephan Herminghaus, Martin Brinkmann Describing the dynamics of contact lines on heterogeneous substrates is important in fields ranging from coating technologies over microfluidics to enhanced oil recovery. The vast majority of studies on contact line dynamics considers the case of ideal, homogeneous substrates. Modelling chemically heterogeneous surfaces by introducing periodic variations in the wettability, we investigate the depinning and subsequent motion of 2D droplets driven by a body force. To this end, we study the time evolution of a sharp interface using Stokes dynamics together with Navier slip condition at the substrate, employing a boundary element formalism. The heterogeneity is represented by a position dependent microscopic contact angle. This allows us to quantify the modified droplet mobility, the spectrum of pinned and depinned solutions, and their dependence on periodicity and strength of the heterogeneity. Understanding the emergence of dynamic contact angles on heterogeneous substrates may open a new perspective on the relation of molecular and hydrodynamic models of contact line motion. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R4.00003: Modelling of contact angle hysteresis on rough, non-uniform and superhydrophobic surfaces with lattice Boltzmann method K.J. Kubiak, M.C.T. Wilson, J.R. Castrej\'on-Pita, I.M. Hutchings Contact Angle Hysteresis (CAH) is usually attributed to surface heterogeneity, contact line pinning, adsorption or interdiffusion. A model of CAH developed recently by Kubiak \& Wilson is demonstrated using the lattice Boltzmann method. The model is based on the dynamic surface heterogeneity, reorientation of surface molecules under wetting liquid, physical roughness, chemical heterogeneity and liquid adhesion and evaporation. Once the surface is wetted, the local static contact angle (CA) changes from its advancing value to match the receding static CA over time Ta. When the contact line retracts, the surface recovers its initial properties corresponding to the advancing static CA over time period Te, which corresponds to the physical evaporation. Further development of the model to include surface roughness and chemical heterogeneity is presented. The model shows good agreement with experimental results for several practical configurations i.e. droplet impact and coalescence, drops on tilted surface, and drops on superhydrophobic and non-uniform surfaces etc. The extended model exhibits great potential for predictive modelling using the lattice Boltzmann method, but can be also implemented in other schemes. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R4.00004: A computational study of the impact of molten drops onto textured surfaces Mehdi Raessi, Rajkamal Sendha We used an in-house, three-dimensional computational tool to study the impact and spreading of molten drops onto substrates with various surface patterns. The computational tool is GPU-accelerated and solves the mass, momentum and energy equations in the liquid phase and the conduction equation in the substrate. The drop, 40 $\mu m$ in diameter, is molten alumina, which is widely used in the thermal spray coatings. The surface patterns are created by positioning cuboid obstacles that their side dimension is at least 3 $\mu m$. We investigated the effects of obstacle height, aspect ratio and spacing as well the impact velocity on the spreading dynamics and the final geometry of the drop. In our study, the impact velocity was varied from 40 to 90 $m/s$, the obstacle height from 1 to 5 $\mu m$, and the obstacle spacing from 2 to 26 $\mu m$. The results show that the flattened drop has a disk-shape geometry when the obstacle spacing is either smallest or largest, and that significant deformations and fingering occur at the intermediate spacings. A quantitative relation was developed between the obstacle spacing and the final spread diameter of the drop. The results show the collapse of the final spread diameter normalized by the obstacle spacing when plotted against the spacing at different impact velocity and obstacle height. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R4.00005: Two dimensional droplet spreading over chemically heterogeneous substrates Rajagopal Vellingiri, Nikos Savva, Serafim Kalliadasis We investigate the spreading dynamics of a partially wetting two-dimensional droplet over chemically heterogeneous substrates. The contact line singularity is removed by assuming slip at the liquid-solid interface. Assuming small contact angles and strong surface tension effects, a long-wave expansion of the Stokes equations yields a single evolution equation for the droplet thickness. The chemical nature of the substrate is incorporated through local variations in the microscopic contact angle, which appear as boundary conditions in the governing equation. By asymptotically matching the flow in the bulk of the droplet with the flow in the vicinity of the contact lines, we obtain a set of coupled ordinary differential equations for the locations of the two droplet fronts. We verify the validity of our matching procedure by comparing the solutions of the ordinary differential equations with solutions of the full governing equation. A number of interesting features that are not present in chemically homogeneous substrates are found, such as the existence of multiple equilibria, the pinning of the droplet fronts at localized chemical features and the possibility for the droplet fronts to exhibit a stick-slip-like behavior. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R4.00006: Three-dimensional contact line dynamics on heterogeneous substrates Nikos Savva, Serafim Kalliadasis Three-dimensional contact line dynamics on heterogeneous substrates is examined theoretically by using the motion of a viscous, partially wetting droplet over a horizontal and chemically heterogeneous substrate as a model system. We utilize a long-wave model for the evolution of the droplet thickness, whereby inertial and gravitational effects are neglected and the contact angles are assumed to be everywhere small. Noteworthy is that the present formalism does not depend on an imposed, Cox-Voinov-type relation between the apparent contact angle and the contact line speed. Instead, the contact line speed is found as part of the solution, facilitated by mapping the free-boundary problem to a fixed, circular domain. Analytical progress can be made by considering perturbations from a circular contact line and asymptotically matching the flow in the two-dimensional boundary layer around the contact line with the flow in the bulk of the droplet. This matching procedure eventually leads to a set of differential equations for the Fourier coefficients of the contact line. The derived equations are solved for a few representative substrate configurations. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R4.00007: Destiny of a drop on a fiber: from barrel to clamshell and back Burak Eral, J. de Ruiter, R. de Ruiter, J.M. Oh, C. Semprebon, M. Brinkman, F. Mugele Drops on cylindrical fibers are a familiar sight, for instance in the form of dew drops on spider webs. They can exist in two competing morphologies, a cylindrically symmetric barrel state completely engulfing the fiber and an asymmetric clamshell state, in which the drop sits on the side of the fiber. Despite their omnipresence and their practical relevance the physical mechanisms governing the stability of the two morphologies remained elusive. Using electrowetting-functionalized fibers we determined of the stability limits of both morphologies as a function of the two relevant control parameters, the contact angle and the liquid volume. While clamshells are found to prevail for large contact angles and small volumes, and barrels prevail for small angles and large volumes, there is also a wide range of intermediate parameter values, for which both morphologies are mechanically stable. Mapping out the energy landscape of the system by numerical minimization of the free energy we find that the barrel state is easily deformed by non-axisymmetric perturbations. From a general perspective, the demonstration of electrowetting-based reversible switching of liquid morphologies on fibers opens up opportunities for designing functional textiles and porous materials. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R4.00008: Modes of Contact Line motion on topographic substrates Ciro Semprebon, Stephan Herminghauss, Martin Brinkmann The link between the geometry of a regular topographic pattern, the material Contact Angle and the macroscopic Contact Angle Hysteresis is still poorly understood. Employing the apparent contact angle as a control parameter, we numerically determine the shape of a liquid interface on patterns consisting of square arrays of pillars with different cross section. Both hydrophilic and hydrophobic substrates are considered. The increase of the apparent contact angle leads to a sequence stable morphologies which may belong to different interface topologies. Eventually, the interface becomes unstable, defining the Advancing Angle. Abrupt changes of the Advancing Angle, while varying the spacing, aspect ratio and material Contact Angle of the pillars, correspond to transitions to different Advancing Modes. With the same procedure we calculate Receding Angles, and consequently the Contact Angle Hysteresis. Finally, we discuss similarities and differences among pillars with either circular and square cross sections. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R4.00009: Investigation of Contact Angle Behavior and Stability of Drops to Airflow Forcing on Rough Surfaces Jason Schmucker, Edward White A method for measuring full-field, instantaneous drop interface profiles on rough surfaces has been implemented to study contact angles and stability to wind forcing on metallic surfaces with micron-scale roughness. Wind tunnel experiments are conducted to produce criteria for runback of drops and set these thresholds for measured water drops spanning a range of Bond numbers from $Bo = 0.5$ to $5$ on roughness in the range of $R_A=0.8$ to $4.9$ with drop based Reynolds numbers spanning an order of magnitude. More importantly, these stability limits are tested with particular care taken to observe their relation to the behavior of both the contact line and contact angle distribution as the drop adjusts its configuration to find a stable condition until it is no longer able to do so and is blown downstream. Results such as critical shear rates and contact angles are discussed and compared with previous numerical studies in the literature such as Dimitrakopoulos [J.Fluid.Mech. 580, 2007] and Ding and Spelt [J.Coll.Sci. 599, 2008] along with experimental results such as Milne [Langmuir 25:24, 2009]. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R4.00010: Interplay of surface roughness and air pressure in splash suppression Andrzej Latka, Sidney Nagel Surface roughness has generally been found to increase the splashing of drops impacting on a solid surface [1]. However, we have recently found [2] that the effect of roughness is considerably more complex: it does promote the prompt splashing associated with rough surfaces, but also inhibits the splashing that occurs through the formation of a thin sheet. Consequently, as roughness is varied, the drop can splash through one or both of these mechanisms. Surprisingly, for sufficiently high viscosities there is a region where increasing roughness eliminates splashing entirely. We show that this result is robust and holds for a variety of liquids impinging on surfaces of different materials and geometries. The mechanism through which roughness causes splashing is intimately connected with the ambient gas. We find, as in the case of thin-sheet splashing [3], that the removal of the surrounding air suppresses prompt splashing. As the gas pressure decreases, fewer droplets are ejected until the drop completely ceases to splash. This threshold pressure increases with liquid viscosity and decreases with surface roughness and impact velocity. [1] K. Range, F. Feuillebois, \textit{J. Colloid Interface Sci}. \textbf{203}, 16 (1998); [2] A. Strandburg-Peshkin, M. M. Driscoll, S. R. Nagel, \textit{BAPS}. DFD.AH.7 (2009). [3] L. Xu, W. W. Zhang, S. R. Nagel,\textit{ Phys. Rev. Lett. }\textbf{94}, 184505 (2005); L. Xu, \textit{Phys. Rev. E} ~\textbf{75}, 056316 (2007); M. M. Driscoll, C. S. Stevens, S. R. Nagel, \textit{Phys. Rev. E} \textbf{82}, 036302 (2010). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700