Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D33: Drops: Wetting and Spreading I |
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Chair: Omar Matar, Imperial College London Room: Ballroom A |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D33.00001: Low-order modelling of droplets on hydrophobic surfaces Omar Matar, Alex Wray, Lyes Kahouadji, Stephen Davis We consider the behaviour of a droplet deposited onto a hydrophobic substrate. This and associated problems have garnered a wide degree of attention due to their significance in industrial and experimental settings, such as the post-rupture dewetting problem. These problems have generally defied low-order analysis due to the multi-valued nature of the interface, but we show here how to overcome this in this instance. We first discuss the static problem: when the droplet is stationary, its shape is prescribed by an ordinary differential equation (ODE) given by balancing gravitational and capillary stresses at the interface. This is dependent on the contact angle, the Bond number and the volume of the drop. In the high Bond number limit, we derive several low-order models of varying complexity to predict the shape of such drops. These are compared against numerical calculations of the ODE. We then approach the dynamic problem: in this case, the full Stokes equations throughout the drop must be considered. A low-order approach is used by solving the biharmonic equation in a coordinate system naturally mapping to the droplet shape. The results are compared against direct numerical simulations. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D33.00002: Optical Imaging of Water Condensation on Lubricant Impregnated Micropillar Arrays Tadashi Kajiya, Frank Schellenberger, Periklis Papadopoulos, Doris Vollmer, Hans-Jürgen Butt We explored the condensation of water drops on a lubricant-impregnated surface, i.e., a micropillar patterned surface impregnated with a ionic liquid. Growing drops were imaged in 3D using a laser scanning confocal microscope equipped with a temperature and humidity control. On a lubricant-impregnated hydrophobic micropillar array, different stages of condensation can be discriminated: - Nucleation on a lubricant surface. - Regular alignement between micropillars and formation of a three-phase contact line on a bottom of the substrate. - Deformation and bridging by coalescence, leading to a detachment of the drops from the bottom substrate to pillars'top faces. However, on a lubricant-impregnated hydrophilic micropillar array, the condensed water covers the micropillars by dewetting the lubricant. As a result, the surface loses its slippery property. Our results provide fundamental concepts how these solid/liquid hybrid surfaces can be applied for facile removal of condensed water, as well as necessity of the appropriate surface treatment. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D33.00003: Dynamics of Spreading on Micro-Textured Surfaces Alireza Mohammad Karim, Jonathan Rothstein, Pirouz Kavehpour Ultrahydrophobic surfaces, due to their large water-repellency characteristic, have a vast variety of applications in technology and nature, such as de-icing of airplane wings, efficiency increase of power plants, and efficiency of pesticides on plants, etc. The significance of ultrahydrophobic surfaces requires enhancing the knowledge on the spreading dynamics on such surfaces. The best way to produce an ultrahydrophobic surface is by patterning of smooth hydrophobic surfaces with micron sized posts. In this research, the micro-textured surfaces have been fabricated by patterning several different sizes of micro-textured posts on Teflon plates. The experimental study has been performed using forced spreading with Tensiometer to obtain the dependencw 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. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D33.00004: The spreading of a viscoplastic droplet by capillary action Maziyar Jalaal, Neil Balmforth, Boris Stoeber The spreading of yield stress liquid droplets on a dry surface occurs in a number of applications such as 3D printing. In the current study, the surface-tension-driven spreading of a yield-stress (Bingham) droplet on a solid wetting surface is studied. Neglecting gravity and using lubrication theory for viscoplastic fluids, we derived the thin film equation in 2D. Equations were solved numerically, where to avoid the moving contact line singularity, we used a pre-wetted film. Numerical solutions show the decelerating spreading of the droplet and its arrest due to the yield stress. Additionally, the final shape of the droplets was constructed, using an asymptotic method. Results were compared with the numerical solutions, where agreements were observed. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D33.00005: Fast and Slow Wetting Dynamics on nanostructured surfaces Dhiraj Nandyala, Amir Rahmani, Thomas Cubaud, Carlos Colosqui This talk will present force-displacement and spontaneous drop spreading measurements on diverse nanostructured surfaces (e.g., mesoporous titania thin films, nanoscale pillared structures, on silica or glass substrates). Experimental measurements are performed for water-air and water-oil systems. The dynamics of wetting observed in these experiments can present remarkable crossovers from fast to slow or arrested dynamics. The emergence of a slow wetting regime is attributed to a multiplicity of metastable equilibrium states induced by nanoscale surface features. The crossover point can be dramatically advanced or delayed by adjusting specific physical parameters (e.g., viscosity of the wetting phases) and geometric properties of the surface nanostructure (e.g., nanopore/pillar radius and separation). Controlling the crossover point to arrested dynamics can effectively modify the degree of contact angle hysteresis and magnitude of liquid adhesion forces observed on surfaces of different materials. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D33.00006: Dynamic Wetting on Graphene-Coated Surface: Molecular Dynamics Investigation Shih-Wei Hung, Junichiro Shiomi Wettability of graphene-coated surface gained significant attention recently due to discussion on the ``transparency'' (whether the wetting characteristics follow that of graphene or the underlying surface) and practical applications of graphene. In terms of static contact angle, the wettability of graphene-coated surfaces have been widely studied by experiments, simulations, and theory in recent years. However, the studies of dynamic wetting on graphene-coated surfaces are limited. In the present study, molecular dynamics simulation was performed to study the dynamic wetting of water droplet on graphene-coated surfaces from a microscopic point of view. The results show that the degree of similarity between the spreading behavior on graphene-coated surface and that on pure graphene (or that on the underlying surface) depends on time, i.e. how nonequilibrium the interface dynamics is. We also found that this feature can be altered by introducing defects into graphene. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D33.00007: Wenzel Wetting on Slippery Rough Surfaces Birgitt Stogin, Xianming Dai, Tak-Sing Wong Liquid repellency is an important surface property used in a wide range of applications including self-cleaning, anti-icing, anti-biofouling, and condensation heat transfer, and is characterized by apparent contact angle ($\theta^*$) and contact angle hysteresis ($\Delta \theta^*$). The Wenzel equation (1936) predicts $\theta^*$ of liquids in the Wenzel state [1], and is one of the most fundamental equations in the wetting field. However, droplets in the Wenzel state on conventional rough surfaces exhibit large $\Delta \theta^*$, making it difficult to experimentally verify the model with precision. As a result, precise verification of the Wenzel wetting model has remained an open scientific question for the past 79 years. Here we introduce a new class of liquid-infused surfaces [2] called slippery rough surfaces --- surfaces with significantly reduced $\Delta \theta^*$ compared to conventional rough surfaces---and use them to experimentally assess the Wenzel equation with the highest precision to date. \newline [1] R.N. Wenzel, Ind. Eng. Chem. 28: 988-994 (1936). \newline [2] T.S. Wong et al., Nature 477: 443 – 447 (2011). [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D33.00008: Droplet spreading and absorption on rough, permeable substrates Leonardo Espin, Satish Kumar Wetting of permeable substrates by liquids is an important phenomenon in many natural and industrial processes. Substrate heterogeneities may significantly alter liquid spreading and interface shapes, which in turn may alter liquid imbibition. A new lubrication-theory-based model for droplet spreading on permeable substrates that incorporates surface roughness is developed in this work. The substrate is assumed to be saturated with liquid, and the contact-line region is described by including a precursor film and disjoining pressure. A novel boundary condition for liquid imbibition is applied that eliminates the need for a droplet-thickness-dependent substrate permeability that has been employed in previous models. A nonlinear evolution equation describing droplet height as a function of time and the radial coordinate is derived and then numerically solved to characterize the influence of substrate permeability and roughness on axisymmetric droplet spreading. Because it incorporates surface roughness, the new model is able to describe the contact-line pinning that has been observed in experiments but not captured by previous models. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D33.00009: A boundary condition for fluid/fluid flow at the solid interface Shahriar Afkhami, Stephane Zaleski, Alexandre Guion, Jacopo Buongiorno When using classical hydrodynamics, the imposition of no-slip boundary condition on a solid surface results in a logarithmically singular shear force at the fluid/fluid/solid line of contact. Boundary conditions that admit some form of slip at the solid interface, such as the linear slip-shear relation also known as Navier slip boundary condition, have been used to regularize this singularity. The widely used Cox's theoretical work is also derived under the assumption of slip at the solid interface. Here we present a boundary condition, based on Cox's analysis, that resolves the contact line singularity in the numerical simulation of contact line flows using a volume of fluid method. This boundary condition provides a universal formula that determines the contact angle as a function of the gird size. The model can be thought of as a subgrid-scale model for computations, where the resolution is not sufficient to accurately describe the contact line inner region frictional drag. We provide numerical examples for a wide range of steady flow problems. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D33.00010: Phase-field simulations of contact-line dynamics on rough surfaces Fengchao Yang, Pengtao Yue, Xiaopeng Chen Wetting of solid surfaces is ubiquitous in nature, and in most cases the surfaces are not smooth. In this work, we will investigate how surface roughness influences contact-line dynamics by simulating the forced wetting in a capillary tube with microposts or microgrooves on its wall. A phase-field method is used to capture fluid interfaces as well as moving contact lines. The governing equations are solved by an implicit finite-element method on an adaptive triangular mesh, which conforms to the topological patterns on the tube wall and also refines at the fluid interface. With our computational setup, we can capture the advancing and receding contact angles. As the contact line moves, it jumps from one pillar to the next; consequently, the apparent contact angle exhibits a periodic behavior. By comparing with the result on a smooth surface, we will explain how surface roughness affects contact-line dynamics by modifying the effective contact angle and slip length. Numerical results on different arrangements and shapes of microposts will be presented. [Preview Abstract] |
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