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 A31: Drops: Surface Interactions |
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Chair: Detlef Lohse, University of Twente Room: 312 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A31.00001: Formation of surface nanodroplets under controlled flow conditions Detlef Lohse, Xuehua Zhang, Ziyang Lu, Huanshu Tan, Lei Bao, Yinghe He, Chao Sun Nanodroplets on a solid surface (i.e. surface nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, lab-on-chip devices, and near-field imaging techniques. Oil nanodroplets can be produced on a solid-liquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. In this work, we experimentally and theoretically investigate the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We find significant effects from the flow rate and the flow geometry on the droplet size. We develop a theoretical framework to account for these effects. The main idea is that the droplet nuclei are exposed to an oil oversaturation pulse during the exchange process. The analysis gives that the volume of the nanodroplets increases with the Peclet number $Pe$ of the flow as $\propto Pe^{3/4}$, which is in good agreement with our experimental results. In addition, at fixed flow rate and thus fixed Peclet number, larger and less homogeneously distributed droplets formed at less narrow channels, due to convection effects originating from the density difference between the two solutions of the solvent exchange. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A31.00002: Spontaneous Pattern Formation of Surface Nanodroplets from Competitive Growth Shuhua Peng, Detlef Lohse, Xuehua Zhang Nanoscale droplets on a substrate are of great interest because of their relevance for droplet-based technologies for light manipulation, lab-on-chip devices, miniaturised reactors, encapsulation and many others. In this work, we establish a basic principle for the symmetrical arrangement of surface nanodroplets during their growth under simple flow conditions. In our model system, nanodroplets nucleate at the rim of spherical cap microstructures on a substrate, as a pulse of oversaturation is supplied by a solvent exchange process. We find that, while growing, the nanodroplets self-organise into highly symmetric arrangements, with respect to position, size, and mutual distance. The angle between the neighbouring droplets is four times the ratio between the base radii of the droplets and the spherical caps. We show and explain how the nanodroplets acquire the symmetrical spatial arrangement during their competitive growth and why and how the competition enhances the overall growth rate of the nucleated nanodroplets. This mechanism behind the nanodroplet self-organisation promises a simple approach for the location control of droplets with a volume down to attoliters. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A31.00003: Thin viscous films on rotating spheres: statics, dynamics and instability Di Kang, Marina Chugunova, Ali Nadim We examine the behavior of a thin viscous liquid film on a rotating solid sphere under the influence of gravity, centrifugal force and surface tension. The model is based on the lubrication approximation in axisymmetric spherical coordinates, with no-slip at the liquid-solid interface and with normal and tangential stress balances, including Marangoni effects, at the liquid-air interface. The rotation axis is assumed to be aligned with the direction of gravity and the Coriolis force is neglected, identifying parameter regimes when the latter is justified. We show that for constant surface tension, the energy-minimizing steady states are of three different types: uniformly positive film thickness, or states with one or two dry zones on the sphere. The transient dynamics in approaching those states are also described. A stability analysis when Marangoni effects are present but in the absence of gravity and rotation identifies the parameter regimes for instability to occur and the corresponding unstable modes. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A31.00004: How surface nanodroplets sit on a microlens Ivan Devic, Shuhua Peng, Huanshu Tan, Detlef Lohse, Xuehua Zhang Wetting of micro-patterned surfaces is of the great interest in the fundamental research and many practical applications such as open microfluidics, metal corrosion, pesticide spray and water collection. In this work, we investigate nanodroplets, partially wetting a flat surface and partially wetting a spherical surface (spherical microlens) which has a small contact angle with the flat surface. We have developed a theoretical approach for minimising the free surface energy of the nanodroplet at the rim of microlens and have also connected to data from experiments. Since the diffusion length scale is long enough for nanodropets to obtain quasi-static shape in our experiments, with our approach we are also able to obtain growing or shrinking dynamics of the nanodroplet in this system. Of particular interest is behaviour of the contact angle of nanodroplet on the spherical surface of microlens. We find that contact angle of the minimum free surface energy shape deviates more from Young's angle as the nanodroplet gets smaller compared to the spherical microlens, while contact angle of larger drops asymptotically approaches Young's angle. Theoretical results partially agree with our experimental data, due to the surface heterogeneity of both substrates in our experiments. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A31.00005: Driven drops with contact line damping Yi Xia, Chun-Ti Chang, Paul Steen A water droplet placed on a hydrophobic plate is driven by plate-normal oscillations. Resulting droplet motions are largely inviscid, having Reynolds number >100 (Ohnesorge ~0.002). We are interested in isolating the effective damping, sometimes called Davis dissipation, owing to a moving contact line that is not completely mobile. In this talk, we report energy budgets as influenced by contact angle -- contact line speed relationships for variously prepared surfaces. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A31.00006: Measuring the resonant vibration of a sessile droplet using MEMS based cantilevers Thanh-Vinh Nguyen, Kiyoshi Matsumoto, Isao Shimoyama We directly measure the normal force distribution on the contact area during the 1$^{\mathrm{st}}$ mode resonant vibration of a droplet using an array of MEMS based cantilever. The measurement result shows that the normal force change is the largest at the periphery of the contact area. The ratio between the amplitude of the normal force change at the periphery of the contact area over that at the center of the contact area was approximately 20 times, in the case of 1.8 $\mu $L water droplet whose equilibrium contact angle is 140 degrees. We also demonstrate a method to estimate viscosity based on the measurement of the droplet vibration using MEMS cantilevers. The proposed method is able to estimate viscosity using less than 3 $\mu $L sample and has a simple operating principle. We believe that this method is suitable for point-of-care testing and characterization of chemical and biological solutions. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A31.00007: The Stability of the Static Pendant Drop xin lin, Lewis Johns, Ranga Narayanan The instability of a pendant drop is explained. The liquid in the drop is heavier than the surrounding fluid. The scaled groups that describe the stability are the scaled volume and the Bond number. We show without computations that the volume of the drop has a maximum value beyond which it must break catastrophically. However this upper bound on volume is not the instability limit for the drop for all Bond numbers. There exists a critical Bond number, above which the drop breaks before the upper bound on volume can be reached. We discuss why this occurs and what it means for the physics of the break-up. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A31.00008: Numerical simulations of pendant droplets Carlos Pena, Lyes Kahouadji, Omar Matar, Jalel Chergui, Damir Juric, Seungwon Shin We simulate the evolution of a three-dimensional pendant droplet through pinch-off using a new parallel two-phase flow solver called BLUE. The parallelization of the code is based on the technique of algebraic domain decomposition where the velocity field is solved by a parallel GMRes method for the viscous terms and the pressure by a parallel multigrid/GMRes method. Communication is handled by MPI message passing procedures. The method for the treatment of the fluid interfaces uses a hybrid Front Tracking/Level Set technique which defines the interface both by a discontinuous density field as well as by a local triangular Lagrangian mesh. This structure allows the interface to undergo large deformations including the rupture and coalescence of fluid interfaces. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A31.00009: Statistical analysis of Contact Angle Hysteresis Nachiketa Janardan, Mahesh Panchagnula We present the results of a new statistical approach to determining Contact Angle Hysteresis (CAH) by studying the nature of the triple line. A statistical distribution of local contact angles on a random three-dimensional drop is used as the basis for this approach. Drops with randomly shaped triple lines but of fixed volumes were deposited on a substrate and their triple line shapes were extracted by imaging. Using a solution developed by Prabhala et al. (Langmuir, 2010), the complete three dimensional shape of the sessile drop was generated. A distribution of the local contact angles for several such drops but of the same liquid-substrate pairs is generated. This distribution is a result of several microscopic advancing and receding processes along the triple line. This distribution is used to yield an approximation of the CAH associated with the substrate. This is then compared with measurements of CAH by means of a liquid infusion-withdrawal experiment. Static measurements are shown to be sufficient to measure quasistatic contact angle hysteresis of a substrate. The approach also points towards the relationship between microscopic triple line contortions and CAH. [Preview Abstract] |
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