Numerical Investigation of Head-on Binary Drop Collisions in a Dynamically Inert Environment

The results of three-dimensional numerical simulations of drop collisions without the effect of a surrounding environment are presented. The numerical model is based on an Eulerian, finite-difference, Volume-of-Fluid method. Surface tension is included using the Continuum Surface Force method. Head-on collisions using equal size drops with three different fluid properties of water, mercury and tetradecane are presented. Various drop diameters ranging from 200 μm to 5 mm are considered. A separation criterion based upon deformation data is found. The lower critical Weber numbers are found for mercury and water drops while tetradecane drops did not result in separation for the range of Weber numbers considered. The effect of Reynolds number is investigated and regions of permanent coalescence and separation are plotted in the Weber-Reynolds number plane. The role of viscosity and its effect on dissipation is also investigated. Finally, the validity of the assumptions made in some of the collision models is assessed

Digital Microfluidics: Magnetic Transportation and Coalescence of Sessile Droplets on Hydrophobic Surfaces

Magnetic digital microfluidics is advantageous over other existing droplet manipulation methods, which exploits magnetic forces for actuation and offers the flexibility of implementation in resource-limited point-of-care applications. This article discusses the dynamic behavior of a pair of sessile droplets on a hydrophobic surface under the presence of a permanent magnetic field. A phase field method-based solver is employed in a two-dimensional computational domain to numerically capture the dynamic evolution of the droplet interfaces, which again simultaneously solves the magnetic and flow fields. On a superhydrophobic surface (i.e., θc = 150°), the nonuniform magnetic field forces the pair of sessile droplets to move toward each other, which eventually leads to a jumping off phenomenon of the merged droplet from the solid surface after coalescence. Also, there exists a critical magnetic Bond number Bomcr, beyond which no coalescence event between droplets is observed. Moreover, on a less hydrophobic surface (θc ≤ 120°), the droplets still coalesce under a magnetic field, although the merged droplet does not experience any upward flight after coalescence. Also, the merging phenomenon at lower contact angle values (i.e., θc = 90°) appears significantly different than at higher contact angle values (i.e., θc = 120°). Additionally, if the pair of sessile droplets is dispersed to a different surrounding medium, the viscosity ratio plays a significant role in the upward flight of the merged droplet, where the coalesced droplet exhibits increased vertical migration at higher viscosity ratios