Slip flow of concentrated emulsions in microchannels: Effects of surface wettability

• Droplets tend to adhere to the wall instead of forming a lubricated layer. • The volume fraction in the adsorbed layer differs from the bulk volume fraction. • Slip velocity is the most significant difference for various surface wettabilities . • Slip velocity cannot be determined solely by the intrinsic droplet contact angle. • Wall slippage is simultaneously influenced by wall-water and wall-oil interactions. Concentrated emulsions in microchannels exhibit complex flow characteristics, with wall slippage often being prominent. In this work, the slip flow of monodisperse concentrated emulsions, affected by the surface wettability of channel walls, is investigated using dissipative particle dynamics simulations. The effects of surface wettability, defined by the intrinsic contact angle (CA), on wall slippage, measured by the slip velocity, are systematically examined under identical external forces. Two scenarios modifying surface wettability are considered: (i) changing wall-continuous phase affinity and (ii) changing wall-dispersed phase affinity. It is found that for various wettabilities , the flow behavior varies significantly near the wall, while essentially remaining the same away from the wall. This result can be attributed to the microstructure near the wall, which generally consists of crowded, adhered droplets rather than forming a lubricated layer. In the adsorbed layer, the apparent CA of adhered droplets differs from the intrinsic CA, and their local volume fraction differs from the bulk volume fraction. Even the same intrinsic CA, resulting from various combinations of wall-liquid affinities, may yield different microstructures and, consequently, different slip velocities. Finally, a simple model based on the sum of both wall-water and wall-oil contributions is proposed to successfully depict the simulation results.