Electrohydrodynamic interaction, deformation, and coalescence of suspended drop pairs at varied angle of incidence

Abstract

The electrohydrodynamic interaction of liquid drop pairs suspended in another immiscible liquid and subjected to uniform electric field is examined using the leaky dielectric model and the explicit forcing lattice Boltzmann method, by taking into account the nonlinear inertia effects. This facilitates explorations of wider parametric contrast, precise electrohydrodynamic interaction, and postcoalescence breakup phenomena that remained unknown. The influence of dielectrophoretic and electrohydrodynamic forces in prolate- and oblate-shaped deformations, coalescences, and repulsive motion of leaky drop pairs appearing at widely varied incidence angle (alpha >= 0 degrees) to the applied electric field is studied. The electrically driven flow at alpha = 0 degrees evolves in the form of decisively important outflow- and inflow-natured counterrotating vortex pairs in and around the drops. With suitably tuned conductivity (sigma(in)/sigma(out)) and permittivity (epsilon(in)/epsilon(out)) p ratios of drop fluid to surrounding outer medium, the relative impacts of attractive electric force versus outflow- or inflow-natured vortex pair-induced hydrodynamic force was optimized to distinctly facilitate the prolate- and the oblate-type deformations of a drop pair, their coalescence, departure, and the postcoalescence breakup. For varied epsilon(in)/epsilon(out) over a range 0.25 <= epsilon(in)/epsilon(out )<= 20.0, and using fixed conductivity ratio sigma(in)/sigma(out) = 5.0 and electric capillary number Ca-E = 0.46 (the ratio of electric force and surface tension), the dipolar electric force is appropriately set with respect to the surrounding four outflow-type outer vortex pair-induced hydrodynamic force to enforce two prolately deformed drops move apart for epsilon(in)/epsilon(out) < (epsilon(in)/epsilon(out))(crit1) approximate to 2.4 and coalesce for 2.4 < epsilon(in)/epsilon(out) < 9. 57 ,The low pressure that grew at the neck of near-contact or coalescing drops facilitated the transport of inner fluid into neck region and helped the drop pair's coalescence. For epsilon(in)/epsilon(out) > (epsilon(in)/epsilon(out))(crit3) approximate to 9. 57, exceeding exceeding a critical value, the deformed oblate drop pair moved closer and coalesced; and depending on epsilon(in)/epsilon(out) the coalesced drop subsequently broke into a number of satellite or daughter drops spread in a direction perpendicular to the electric field; and such a phenomenon is newly identified. For the nonaligned drop pairs with alpha < 54.7 degrees or alpha > 125.3 degrees, the attractive radial (F-r) component of dielectrophoretic force drove two drops closer, while the torque produced by tangential (F-t) components of the electric force made them rotate and align to the electric field upon coalescence. For 54.7 degrees < alpha < 125.3 degrees the dipolar radial force (F-r) appeared repulsive and drop pair moved apart. Importantly for nonaligned drop pairs both oblate and prolate deformations are noticed, for suitably selected dielectric properties.