Using entropic lattice Boltzmann modeling to understanding impacting and evaporating droplets

Entropic lattice Boltzmann method for two-phase flow can be effectively used to predict and understand the behavior of droplets impacting or drying on different surfaces. We first look at drop impact on chemically heterogeneous flat surfaces consisting of a regular checkboard pattern with alternating hydrophilic and hydrophobic patches.  An equivalent contact angle representing Cassie’s apparent contact angle is determined for the distorted impacting droplets. During impact, the equivalent contact angle value increases until the maximum spreading is reached, after the equivalent contact angle remains constant until final state. Second, we analyze the contact line dynamics of evaporating droplets deposited on a set of parallel micro-ribs. The evaporating droplet undergoes a series of pinning-depinning events, showing alternatively constant contact radius and constant contact angle modes. LBM allows us to compute the Gibbs free energy taking into account the interfacial energy, pressure terms as well as viscous dissipation due to drop internal flow. The mechanism that causes the unpinning of the contact line results from an excess in Gibbs free energy. The energy barrier required for the contact line to depin can be enlarged by increasing the spacing or the rib height.