A novel kinetically-controlled de-pinning model for evaporating water microdroplets

A numerical investigation of neutrally hydrophobic water microdroplet evaporation on a flat, isothermal surface was conducted. The axisymmetric time-dependent governing equations of continuity, momentum, energy, and species were solved using FLUENT. The numerical model includes temperature- and species-dependent thermodynamic and transport properties. The explicit volume of fluid (VOF) model with dynamic meshing and variable-time stepping was utilized. The continuum surface force (CSF), the gravitational body force, and Schrage's molecular kinetic-based evaporation model were included in the governing equations. A novel approach was used to model de-pinning by using Blake's molecular kinetic-based contact line motion theory. Experimentally, droplet evaporation data was acquired with a standard dispensing/imaging system and high-speed photography. There is good agreement between the measured and predicted dimensionless droplet profile as characterized by the droplet volume (∀d/∀0), dynamic contact angle (θ/θ0), contact radius (R/R0), and apex height (H/H0) when the de-pinned microdroplet numerical model is used. The de-pinning time (td) and volume (∀d/∀0) are controlled by both the de-pinning parameters (Kw and λ = n− 2) and the accommodation coefficient (ε). On the other hand, the de-pinning contact angle (θd/θ0) and height (Hd/H0) are independent of ε.