Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M34: Drops XIV: Shape Dynamics and Confinement |
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Chair: Petia Vlahovska, Brown University Room: 405 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M34.00001: Nonlinear Resonance of Mechanically Excited Sessile Drops Chun-Ti Chang, Susan Daniel, Paul Steen The spectrum of frequencies and mode shapes for an inviscid drop on a planar substrate have recently been documented. For vertical excitation, zonal modes respond to the driving frequency harmonically and non-zonal modes subharmonically, consistent with the prior literature. In this study, we report observations from the regime of nonlinear response. Here, zonals can respond non-harmonically, both sub- and super-harmonic responses are reported. The principal challenge to generating and observing superharmonic resonances of higher zonal modes is a mode-mixing behavior. However, using a simple visual simulation based on the ray-tracing technique, the individual contributions to the mixed resonance behavior can be extracted. In summary, results from experiment and theory show that the zonal modes, which respond harmonically and can mix with non-zonal modes without interfering with one another in the linear regime, tend to respond sub- or superharmonically and compete with non-zonal modes in the nonlinear regime. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M34.00002: Experiments on the harmonic response of coupled droplets to pressure forcing Christopher Tilger, Joseph Olles, Amir Hirsa The dynamic response of a coupled droplet system to sinusoidal forcing pressure was studied. The system consists of two droplets, each with pinned contact lines, coupled through a conduit. Applications of the coupled droplet system include fast focusing liquid lenses and liquid adhesion devices. The millimeter-scale pinned-contact lines associated with the coupled droplets minimize viscous dissipation and allow capillarity to maintain spherical caps. Phase lag, averaged over the period, between the motion of the droplets and the pressure signal was measured using a high speed imaging system. The instantaneous phase behavior of the system was also determined, describing the droplet configuration with respect to the input forcing. Finally, oscillation lag times associated with starting and stopping transients were determined along with damping coefficients for coupled droplets of different volumes. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M34.00003: Shape deformation dynamics of acoustically pulsed functional pendant droplet undergoing burning Saptarshi Basu, Ankur Miglani, Ranganathan Kumar Understanding surface dynamics at the droplet scale is a problem of fundamental significance and general utility. We show that the preferential entrapment driven homogenous boiling in burning functional droplet can induce severe bulk shape oscillations in the droplet. Internal pressure upsurge resulting from ebullition activity force ejects bubble from the droplet domain causing surface undulations and oscillations in bulk, thus driving the droplet into a nonperiodic swell-shrink cycle. The extent of droplet deformation depends on the frequency and intensity of these bubble expulsion events. Besides, the bubble ejections result in localized droplet fragmentation with the subsequent formation of pinched-off satellite droplets that aids secondary atomization. In a unique regime of single major bubble residing within the droplet the pre-ejection transient time is characterized by an interfacial DL instability, where volumetric bubble-shape oscillations drive the droplet oscillations. However, in the presence of longitudinal acoustic forcing at 100 Hz this instability appears to be suppressed since external pulsing modulates the droplet flame into an oscillatory heat source, resulting in delayed bubble incipience and reduced bubble growth rates. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M34.00004: Manipulating the breakup dynamics of a droplet by lading nanoparticles in the liquid phase Saptarshi Basu, Deepu P, Shubham Chowdhuri The deformation and breakup characteristics of a 5 $\mu $l sessile droplet excited via support motion are studied by employing high speed imaging. The support was actuated in a sinusoidal fashion using electromagnetic means at different frequencies and amplitudes. It is observed that under resonant conditions, the droplet shows vigorous oscillations and eventually disintegrates. Introducing nanoparticles into the liquid phase is seen to suppress the breakup of the droplet. By studying the oscillation behavior of glycerol-water mixtures at different concentrations, the effect of the presence of nanoparticles is established to be tantamount to increasing the fluid viscosity. Frequency spectra of the response (quantified in terms of the droplet height) of the different droplets revealed that higher modal excitation of the droplet is suppressed with increasing viscosity. This non-linear interaction among the higher harmonics explains the increasing trend of phase lag between the driving force and the response as the viscosity increases. A theoretical model based on proper orthogonal decomposition was developed which corroborates all the experimental trends. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M34.00005: Water Drop Shedding under Icing Conditions from Surfaces with Different Wettabilities Deepak Kumar Mandal, Antonio Criscione, Alidad Amirfazli A sessile water drop on a substrate exposed to airflow will shed if the adhesion force is overcome by external forces on the drop. There are a number of theoretical and experimental studies examining the shedding of drops as described in the normal laboratory conditions. Drop shedding under icing conditions is not well understood; this is not only important from fundamental perspective, but also for technological applications such as icing of aerodynamic surfaces (e.g. wings of an aircraft). An icing wind tunnel was designed where both airflow and surface temperature (system temperature) can be lowered up to -10 $^{\circ}$C. Drop shedding on surfaces with different wetting characteristics (from hydrophilic to superhydrophobic) were investigated to determine critical air velocity at which the drop starts to shed. Water drops of different volumes (5 - 100 $\mu $l) were used to analyze the influence of the drop volume on the critical air velocity for shedding on cold surfaces. Results show that the system temperature and wetting properties have a major influence on drop shedding under icing conditions. The critical velocity for drop shedding decreases as the volume of the droplet increases for a particular surface. The influence of different surface and ambient temperature on the shedding of the droplet will be presented. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M34.00006: Oil droplet behavior at a pore entrance in the presence of crossflow: Implications for microfiltration of oil-water dispersions Tohid Darvishzadeh, Volodymyr Tarabara, Nikolai Priezjev The behavior of an oil droplet pinned at the entrance of a micropore and subject to clossflow-induced shear is investigated numerically by solving the Navier-Stokes equation. We found that in the absence of crossflow, the critical transmembrane pressure required to force the droplet into the pore is in excellent agreement with a theoretical prediction based on the Young-Laplace equation. With increasing shear rate, the critical pressure of permeation increases, and at sufficiently high shear rates the oil droplet breaks up into two segments. The results of numerical simulations indicate that droplet breakup at the pore entrance is facilitated at lower values of the surface tension coefficient, higher oil-to-water viscosity ratio and larger droplet size but is insensitive to the value of the contact angle. Using simple force and torque balance arguments, an estimate for the increase in critical pressure due to crossflow and the breakup capillary number is obtained and validated for different viscosity ratios, surface tension coefficients, contact angles, and drop-to-pore size ratios. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M34.00007: Oval track droplets racing to a circle: a generic behavior for confined droplets relaxation and a geometrical model Pierre-Thomas Brun, Mathias Nagel, Francois Gallaire Working in a Hele-Shaw cell, the ideal case of the relaxation of a flattened cylindrical droplet of apparent elliptical cross section is considered. Even though the typical Reynolds number imposed by the problem size is extremely low we found out that the investigated pancake droplet relaxes in a remarkable non-monotonous way. After a transient regime, where the droplet adopts a ``peanut'' shape, it relaxes among a novel family of ovals that to our knowledge has never been reported. These shapes, further referred to as $\kappa_2$ ovals, are recovered from geometrical constrains that arise from a linear stability analysis. Far from being limited to initially elliptical relaxing droplets the $\kappa_2$ ovals appear to be generic and are found in the relaxation of any initially symmetrical shape. This point is well understood thanks to the previously evoked linear stability analysis. A practical example of such relaxations is provided when considering the coalescence of two identical droplets. Experimental movies of FC40 oil droplets in a water continuous phase are provided. Coalescence is achieved in a PDMS micro-channel and the obtained images are used for comparison with our theoretical work. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M34.00008: Simulations of drop transport through obstacle arrays Rui Zhang, Joel Koplik Motivated by possible applications to the separation of deformable cells or drops, we use molecular dynamics simulations to investigate the transport properties of liquid drops in a periodic lattice of cylindrical posts at capillary numbers O(1) and Reynolds numbers O(10), and compare the results to previous studies for rigid or weakly deformable particles. A drop impacting a single obstacle is observed to deform, deflect, split and/or recombine, depending on the incident velocity and impact parameter, as well as the degree of mixing with the carrier fluid. We characterize the collision outcome and trajectory deflection as a function of these parameters. The calculations are extended to a periodic array of cylindrical obstacles, where we focus on the survival probability, drop size distribution and trajectory deflection. The results are compared to the directional locking effect observed for rigid particles in obstacle arrays. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M34.00009: New Large Length Scale Capillary Fluidics Investigations Using a Drop Tower Mark Weislogel, Andrew Wollman, Brentley Wiles Drop Towers provide brief terrestrial access to microgravity environments. When exploited for capillary fluidics research, the drop tower allows for unique control over an experiment's initial conditions which can enable, enhance, or otherwise improve methods to study capillary flows and phenomena at significantly larger length scales than can be achieved on the ground. In this work a new, highly accessible, 2.1 s tower is introduced for such research. Enabled in part by simple macro-fabrication methods, a variety of new demonstrative experiments are presented for purely capillarity-driven flows leading to droplet ejections, bubble ingestions, sinking flows, particle injections, and multiphase flows. Due to the repeatability of the passive flows, each experiment may be used in turn as a means to study other phenomena and forward-looking research themes are suggested that include large length scale passive phase separations, heat and mass transfer, droplet dynamics, combustion, and more. [Preview Abstract] |
Tuesday, November 26, 2013 9:57AM - 10:10AM |
M34.00010: Zero-gravity mean free surface curvature of a confined liquid in a radially-vaned container Yongkang Chen, Michael Callahan, Mark Weislogel A variety of increasingly intricate container geometries are under consideration for the passive manipulation of liquids aboard spacecraft where the impact of gravity may be neglected. In this study we examine the mean curvature of a liquid volume confined within a radial array of disconnected vanes of infinite extent. This particular geometry possesses a number of desirable characteristics relevant to waste water treatment aboard spacecraft for life support. It is observed that under certain conditions the slender shape of the free surface approaches an asymptote, which can be predicted analytically using new hybrid boundary conditions proposed herein. This contribution represents possibly the final extension of what has been referred to as the method of de Lazzer et al. (1996). The method enables the integration of the Young-Laplace equation over a domain bounded by the wetted portion of the solid boundaries, symmetry planes, and circular arcs representing free surfaces at the center plane of the liquid body. Asymptotic solutions at several limits are obtained and the analysis is confirmed with numerical computations. [Preview Abstract] |
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