Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P22: Drops: Multiple Drop Interactions |
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Chair: Alexander Wray, University of Strathclyde, Glasgow Room: 604 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P22.00001: Hydrodynamic Spin Lattices Pedro Saenz, Giuseppe Pucci, Sam Turton, Alexis Goujon, Ruben Rosales, Jorn Dunkel, John Bush In this talk, we will introduce a hydrodynamic analog system that allows us to investigate simultaneously the wave-mediated self-propulsion and interactions of effective spin degrees of freedom in inertial and rotating frames. Millimetric liquid droplets can walk across the surface of a vibrating fluid bath, self-propelled through a resonant interaction with their own guiding wave fields. A walking droplet, or `walker’, may be trapped by a submerged circular well at the bottom of the fluid bath, leading to a clockwise or counter-clockwise angular motion centered at the well. When a collection of such wells is arranged in a 1D or 2D lattice geometry, a thin fluid layer between wells enables wave-mediated interactions between neighboring walkers. Through experiments and mathematical modeling, we demonstrate the spontaneous emergence of coherent droplet rotation dynamics for different types of lattices. For sufficiently strong pair-coupling, wave interactions between neighboring droplets may induce local spin flips leading to ferromagnetic or antiferromagnetic order. Transitions between these two forms of order can be controlled by tuning the lattice parameters or by imposing a Coriolis force mimicking an external magnetic field. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P22.00002: The dipole approximation for droplets in Hele-Shaw flows Yoav Green For the past decade, the interaction between two droplets flowing in a Hele-Shaw cell has been modelled as a dipolar force. On one hand, the correspondence between experiments and the dipole model is remarkable. On the other hand, the dipole model doesn't satisfy the required no-flux boundary condition at the droplet interfaces. Despite of this contradiction, the dipole model remained popular. In an attempt to bridge this gap, Sarig et al[1] derived an exact analytical solution, for a two-droplet system satisfying zero-flux at both droplet interfaces, that showed remarkable correspondence to experiments. However, their solution was given in terms of infinite Fourier series, which didn't allow for the desired resolution of the contradiction. By deriving approximations for their Fourier series, for the cases of small and large droplet spacing, we resolved the contradiction [2]. We showed that the large spacing approximation reduces to the expected dipole-like solution. We extended our solution to arbitrary droplet numbers. Our infinite lattice solution includes a corrections term relative to the dipole model. Finally, our small spacing solution also provides a novel lower limit. [1] Sarig et al, J. Fluid Mech. 800, 264 (2016) [2] Y. Green, J. Fluid Mech. 853, 253 (2018) [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P22.00003: The Stability and Dynamics of Bouncing Droplet Rings Miles Couchman, John Bush Millimetric droplets bouncing on the surface of a vibrating fluid bath may interact through their shared wavefield to form bound states. In this talk, we present the results of a combined experimental and theoretical investigation of the stability of droplet rings. As the bath's vibrational acceleration is increased progressively, droplet rings are observed to destabilize into a variety of dynamical states including steady rotational motion, radial oscillations, azimuthal oscillations, and azimuthally traveling waves. The instability observed is dependent on the ring's initial radius and drop number, and whether the drop's are bouncing in- or out-of-phase relative to their neighbors. As the vibrational acceleration is further increased, more exotic forms emerge including pentagonal and square structures. A linear stability analysis based on the trajectory equation of Couchman \textit{et al}. (JFM, 2019) rationalizes the observed instabilities and provides insight into the dynamics of droplet lattices and other aggregates. Connections with vortex arrays in superfluid helium and Bose Einstein condensates are discussed. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P22.00004: Gradient sensing on a lattice: frustrated droplets and ways to drive them Anton Molina, Stefan Karpitschka, Manu Prakash Soft matter systems exhibiting lifelike properties such as motility and gradient sensing challenge us to develop new ways to engineer their emergent, collective behavior. Two-component Marangoni-contracted droplets display a dynamic behavior resembling chemotaxis and a capacity to self-organize due to long-ranged vapor mediated interactions. Here, we present results describing droplet ensembles constrained to organize on a honeycomb lattice. We observe experimentally and confirm by simulation that this is a frustrated system with properties that scale with system size. An external, rotating gravitational field is used to drive the system. High driving amplitudes yield field driven dynamics while low amplitudes produce static configurations characterized by the formation of local vertex states. This transition is characterized by dynamic, non-local structures. We discuss both experimental and simulation results and explore the role of disorder in droplet size. The macroscopic nature of this system gives access to microstate configurations enabling mechanistic connections between global driving and the dynamics of a finite-sized, frustrated system. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P22.00005: Competitive evaporation of multiple sessile droplets Stephen Wilson, Alexander Wray, Brian Duffy An asymptotic model is derived for the competitive diffusion-limited evaporation of multiple thin sessile droplets under the assumption that the droplets are well separated. Comparisons are made with numerical solutions of the full governing equations in order to verify the accuracy of the model; in particular, the model is found to perform well even outside its anticipated range of applicability, up to and including the limit of touching droplets. The shielding effect of other droplets is demonstrated, and the model is used to investigate the effect of this shielding on droplet evolutions and lifetimes, as well as on the coffee-ring effect. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P22.00006: On Splashing Dynamics of Diesel Drop Trains Under Engine-Relevant Impingement Conditions: a Computational Study David Markt Jr., Ashish Pathak, Mehdi Raessi, Roberto Torelli, Seong-Young Lee In this study, advanced 3D simulations of micron-sized diesel drop trains impinging on a solid substrate are presented. The droplet size and impact velocity are representative of the impingement conditions during fuel injection in internal combustion engines. Generally, diesel fuel injection is studied using Lagrangian-Eulerian solvers which rely on spray-wall interaction sub-models to predict the drop impingement outcomes. We have found such sub-models may predict inaccurate splashing outcomes under such extreme impingement conditions. Therefore, using drop trains as an idealized spray, highly-resolved simulations are presented and quantities such as the splashed mass ratio and secondary droplet distribution are compared to common spray-wall interaction sub-models. The effects of liquid film thickness, contact angle and ambient gas pressure on the splashing dynamics are also quantified. By varying the impingement conditions, a robust analysis is presented highlighting the dominant parameters which affect splashing. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P22.00007: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 6:47PM - 7:00PM |
P22.00008: Manipulation of drop-drop interactions in millifluidic device using inlet spacing design E. M. Arun Sankar, Raghunathan Rengaswamy Colloidal particles of different shapes and anisotropy are required for various applications. Researchers have reported synthesis of simple structures of colloidal particles in 1D microfluidic devices. Synthesis of more complicated 2D structures by contacting drops in a 2D device demands knowledge of their movement. When there are many drops in the system, each drop disturbs the flow field and interact with other drops. By appropriately manipulating these hydrodynamic interactions, one can arrange the drops in a desired shape. A simple model which captures the dominant interactions in 2D millichannel Hele-Shaw geometry, and can be used in an optimization framework, was proposed from our group. We noticed that, the hydrodynamic interactions between the drops and therefore the resulting droplet pattern formation are affected by the initial inter-droplet spacing. The spacing between the drops can be changed by infusing (withdrawing) the continuous phase fluid into (from) the space between drops. In this work, we demonstrate manipulation of hydrodynamic interactions to arrange the drops in a desired shape in a millifluidic device by sending them into the channel at optimum spacing. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P22.00009: Measurement of hydrodynamic forces on near-contact droplets Reece Kearney, Gregory P Bewley, Linda Bu Collisions between particles in turbulent flows are important in many natural and industrial processes, including rainfall and combustion. Though three-dimensional particle tracking is used routinely to measure the motions of fluid, detection of collisions between particles has been limited to non-automated methods due to the difficulty of differentiating between particles as they come close together. Near contact, particle pairs may experience forces that arise from interactions between the background flow and the motion of the two particles, and these forces may tend to promote or prevent collision. In our experiment, we observe water droplets ranging from 10 microns to 100s of microns in radius. We present measurements of the hydrodynamic interactions between droplets at separations of less than one diameter using an in-house particle tracking algorithm that resolves trajectories approaching and including contact. [Preview Abstract] |
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