Size Effects on Droplet Displacing Process in Micropores by Multiscale Modeling

Transport mechanisms of small droplets on walls in micropores become significant for applications in energy, resource and biomedical engineering, however, a suitable numerical tool remains challenging. Macroscopic approach is ideal both in computing cost and simplicity but its applicability is doubted for nanoscale droplet, yet no clear evaluation on when exactly does it become invalid has been made. This work evaluates the applicability of macroscopic approach for the displacing process of droplet in a micropore and investigates relevant size effects, by comparing the simulation results of multiscale modeling and macroscopic method. Three types of size effects affecting the displacement results are identified: Laplace pressure, low interfacial density, and breakdown of macroscopic description. For the system studied, the Laplace pressure dominates for relatively big droplet, then low density region becomes significant for drop diameter smaller than 18 times molecule diameter, and finally macroscopic description gradually fails for drop diameter smaller than 13 times molecule diameter. We further investigate the influences of system scale and fluid type on these size effects and discuss the relative importance of each size effect under different conditions. Results indicate that traditional macroscopic approach may be invalid even when continuum assumption still holds due to other size effects, and corrections for those effects can be made to extend the applicability of macroscopic method.