Dynamics of drop disintegration and coalescence with and without electric fields
Ranging from raindrops in thunderclouds, to electrosprays in mass spectrometry to printing and coating processes of 5 microns or thinner, the scenario of liquid drops subject to strong electric fields is more common than it is perceived to be. Such drops develop sharp conical tips from which thin jets emanate which subsequently disintegrate into a fine spray of charged droplets. Despite being one of the oldest and most celebrated problems in science, there exist conflicting theories and measurement on the size and charge of these small electrospray droplets. In this work, dynamics of uncharged liquid drops subjected to an electric field is simulated by solving the Navier-Stokes equations augmented with Maxwells equations using the Galerkin finite element method. Theory and simulations are used here to show that liquid conductivity can be tuned to obtain three distinct scaling regimes for the size and charge of droplets thus formed, a finding that has been missed by previous studies and that bridges the gap between experiments and theory. It is further shown that these charged droplets are Coulombically stable, i.e., they do not explode into finer droplets, irrespective of the size and physical properties of the parent drop, making it the most fundamental law of semi-conducting liquids. Emulsion systems are common to a various industries ranging from food to pharmaceuticals to oil and gas to chemicals to cosmetics with some needing to be stabilized and others be broken. At the heart of emulsions lie liquid drops dispersed in a second liquid. Interactions and coalescence events of these individual drops is, not surprisingly, central to understanding the macro properties and behavior of emulsions. If and when these dispersed liquid drops interact and coalesce, they undergo a topological change involving finite time singularities. One of the important indicators for emulsion stability (or its instability) is the time it takes for two drops to drive away the second liquid in between and subsequently coalesce - used to estimate shelf life of bottled products, e.g., mayonnaise, or residence time of coalescer units for water-oil separations. Simulations are used here to calculate this coalescence time and, more generally, elucidate the approach and pre-contact dynamics of drops identifying conditions conducive for coalescence in terms of flow and fluid properties.
Basaran, Purdue University.
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