Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M12: Drops: Spreading on Soft Surfaces |
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Chair: Karen Daniels, North Carolina State University Room: Georgia World Congress Center B217 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M12.00001: Droplet stick-slip on soft surfaces is caused by dynamic solid surface tension Mathijs Van Gorcum, Bruno Andreotti, Jacco Snoeijer, Stefan A. Karpitschka The contact line of a liquid drop on a solid exerts a nanometrically sharp surface traction. This provides an unprecedented tool to study highly localised and dynamic surface deformations of soft polymer networks. One of the outstanding problems in this context is the stick-slip instability, observed above a critical velocity, during which the contact line periodically depins from its own wetting ridge. Time-resolved measurements of the solid deformation are challenging, and the mechanism of dynamical depinning has remained elusive. Here we present direct visualisations of the dynamic wetting ridge formed by water spreading on a PDMS gel. Unexpectedly, it is found that the opening angle of the wetting ridge increases with speed, which cannot be attributed to bulk rheology, but points to a dynamical increase of the solid's surface tensions. From this we derive the criterion for depinning that is confirmed experimentally. Our findings reveal a deep connection between stick-slip processes and newly identified dynamical surface effects. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M12.00002: Gradient induced droplet motion over soft solids Aaron Bardall, Michael Shearer, Karen Elizabeth Daniels On a sufficiently-soft substrate, a resting fluid droplet will cause significant deformation of the substrate. For such solids, it has been found experimentally and theoretically that the elastic energy resulting from this deformation causes the fluid contact angle to deviate from that given by Young's Law for rigid surfaces. By introducing a bulk or surface gradient in stiffness or surface energy respectively, the contact angle becomes asymmetric causing a force imbalance that may cause the droplet to migrate to a region of smaller total energy, i.e. a stiffer region of the substrate. In contrast to droplet migration over rigid surfaces, it is known that the dominant resistance to motion over soft solids is viscoelastic dissipation within the deformed solid as opposed to fluid dissipation. In this talk we will present our analytic model to describe the droplet response to a substrate heterogeneity. Finally, we will discuss the consequences for droplet motion over soft solids. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M12.00003: Spreading and sliding of viscous droplets on soft substrates under external body forces Md Ehsanul Alam, Inmaculada Ríos-López, Thodoris D. Karapantsios, Tatiana Gambaryan-Roisman The deformability of substrates is known to significantly affect the wetting phenomena. In this experimental work, we investigate the influence of the substrate mechanical properties on the deformation and onset of sliding of a sessile droplet under increasing centrifugal force. The dependence of contact angle at the advancing and the receding edge of the droplet in the direction of the applied force, the drop length and the contour shape on the applied force is analysed. Theoretical models are validated against the obtained results to understand the underlying physical phenomena. The experimental setup used for this study is a custom made state of the art device equipped with a rotating arm, tilting head and an environmental chamber. Three cameras placed at right angles to each other give the side, top and back view of the drop. A custom made Matlab script facilitates the post processing of the video images. Polydimethylsiloxane (PDMS) is used as the substrate and water and 50 wt% glycerine solution as the wetting liquids. Drops on softer surfaces showed large hysteresis as compared to drops on the harder substrates. It is also shown that, by increasing the viscosity of the drop the force required to slide the drop on the softer surfaces is reduced. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M12.00004: Is Young’s equation valid for under-liquid wetting? Kumari Trinavee, Naga Siva Gunda, Sushanta Mitra The aim of the present work is to understand the wetting of two under-liquid systems i.e., oil (drop) in water medium and water (drop) in oil medium on two substrates- glass and Poly (methyl methacrylate) (PMMA). Theoretical contact angles calculated by Young’s equation and Owens-Wendt approach were compared with the experimental contact angle. Four different oils were used – laser oil, two type of silicone oils and DBP. It was found that both the theories could not correctly predict under-liquid contact angles on glass. However, with a modified formulation of Owens-Wendt approach by considering a thin water film beneath the oil drop on a glass substrate, close agreement with the experimental data was found. Furthermore, for water (drop) in oil medium, we also applied the modified Young’s equation with thin oil film beneath the drop and the experimental results show close agreement with the model. In contrast, such a variance was not observed for oil (drop) in water medium on PMMA substrate. But we found that measured contact angles vary with the theoretical contact angles for water (drop) in oil on PMMA substrate. Therefore, similar to the glass substrate, we conjecture here also that a thin oil film is sandwiched between the water drop and the substrate. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M12.00005: Oil droplet wetting dynamics on immiscible liquid surfaces Varun Kulkarni, Venkata Yashasvi Lolla, Sushant Anand The wetting dynamics of an immiscible oil with water has been long studied owing to its ubiquitous nature and applicability in disparate scenarios such as oil spills, agriculture and cosmetics. Although plenty of literature exists on spreading of spreading oils on water, or droplet impacts on alike substrates, comparatively little is known about the spreading dynamics of an oil that is inherently non-spreading on water. In the present work we analyze the scenario which involves the interaction of an oil droplet with a bulk liquid using high speed visualization. The topological transitions observed are systematically characterized in terms of the governing non-dimensional parameters and associated dominant force balances are established. A physical explanation and criteria for the occurrence of various transitions with appropriate scaling arguments is provided. Our results aim to further current understanding on interactions in oil/water systems and elucidate the underpinning physical mechanisms. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M12.00006: Enhanced voltage generation through electrolyte flow on liquid filled surfaces Bei Fan, Anindita Bhattacharya, Prabhakar R Bandaru The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, it is shown that enhanced electrical potential differences/streaming potential may be obtained through the flow of salt water on liquid filled surfaces infiltrated with lower dielectric constant liquid, such as oil, harnessing electrolyte slip and associated surface charge. A record of large figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV/Pa, is obtained through the use of the liquid filled surfaces, greater by a factor of 1.4, when compared to air filled surfaces. These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale with applications to new electrical power sources. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M12.00007: Pinning of Sliding Droplets on Microstructured Surfaces Henning Bonart, Jens-Uwe Repke, Christian Kahle Most often, the structured packing surfaces build into absorption columns contain a texture with dimensions in the same order of magnitude as the liquid film thickness. While letting a droplet flow down such structured surfaces, frequently it halts midstream on the plate. We perform numerical simulations to asses the conditions of surface structures which lead to the pinning of a sliding droplet. A thermodynamic consistent Cahn-Hilliard-Navier-Stokes model is adopted to calculate the two phase flow. Boundary conditions which allow for dynamic contact angle hysteresis and slip between the moving contact lines and the solid surface are applied. As the resulting model forms a very tightly coupled and nonlinear system of equations the discretization and solution strategy is carefully selected to allow efficient and accurate simulations. The model and the solution strategy are validated against analytical and experimental data of static and dynamic wetting of droplets. We present results of gravity-driven droplets on inclined surfaces with structures in the size of the droplets and discuss the observed requirements for pinning a sliding droplet to a surface. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M12.00008: Cahn-Hilliard Navier-Stokes simulations for design of superhydrophobic surfaces Matthew R Tranter, Benjamin Aymard, Serafim Kalliadasis, David N Sibley Self-cleaning is an important feature that is desirable in many applications, such as solar panels. The effect of varying the topography of a surface can aid the ability of droplets, of rainwater for instance, to bead-up, roll down, and pick up dust particles in the process. To this end we consider the motion of a two-dimensional droplet on an inclined substrate and study the effect of varying bottom topography on the motion. The model used to describe the motion is a Navier-Stokes model, incorporating gravity, coupled to a Cahn-Hilliard model for a phase-field. This model is chosen so that the droplet interface is described by an isoline of the phase-field, avoiding posing the problem with a moving boundary, although we still implement an adaptive meshing routine to resolve the interface accurately. The droplet propagation speed in the case of a smooth substrate is compared to the case when the substrate has a sawtooth or triangular pattern, amongst others. The frequency of the patterns is also varied. Results presented will allow provisional conclusions to be drawn on surface designs. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M12.00009: Wettability-Patterned Surfaces for Management of Low Surface Tension Liquids Jared Morrissette, Aritra Ghosh, Uddalok Sen, Joseph Mates, Constantine Megaridis Superhydrophobicity (extreme repellency to water) has been an extensively studied topic in recent years. Generally, it is easy to make a surface superhydrophobic as water has a relatively high surface tension (72 mN/m), but it can become difficult to prepare surfaces which can repel low surface tension liquids such as oils and fuels with surface tensions below 40 mN/m. Here, we present a robust, fluorinated material system which can be easily sprayed onto a substrate and is capable of repelling low surface tension liquids (down to 23.8 mN/m). Certain regions of these superoleophobic (extreme repellency to oils) coatings are made superoleophilic through laser processing. After laser processing, the wettability-patterned substrates are then used to passively manage and transport low surface tension liquids. This is achieved by harnessing surface tension forces which arise from the spatial contrast of surface energy between superoleophobic/superoleophilic domains of the substrate. In this work, we demonstrate and study the liquid transport dynamics (e.g. velocity, acceleration) of several oils (varying surface tension and viscosity) on open-air, wettability-patterned substrates through high-speed image analysis. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M12.00010: Dynamic wetting of drop spreading over asymmetric sawtooth surface structures Yaerim Lee, Naoto Matsushima, Susumu Yada, Satoshi Nita, Takashi Kodama, Gustav Amberg, Junichiro Shiomi A liquid drop spreading over a dry solid surface is ubiquitous in nature and the technological importance is increasing. While there is lack of understanding in the mechanism of early rapid spreading, it has been shown that a friction related to the contact line movement plays an important role in hindering the spreading. The hindrance by line friction can be magnified by micro-scale roughness as it increases the effective surface area. However, the role of actual geometry of the roughness has remained unclear. In this work, we have formulated a toy model that accounts for the roughness geometry and applied to the case of asymmetric sawtooth structures in which the surface area is the same, but the dynamic contact angle varies depending on the spreading direction. By series of spreading experiments, the imbalanced growth of the spreading radius between the two opposite directions perpendicular to the sawtooth alignment is found to be well quantified by the toy model. The results reveal that the engineered surface geometry can realize directional spreading. Furthermore, the toy model validates the mechanism of the early spreading as a time extension of the contact line movement caused by the local departure of the contact angle from its static value in terms of roughness shapes. |
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