Effects of dynamic wetting and liquid–solid slip on self-propelled nanodrops in tapered nanochannels

Abstract

Drops inside tapered microchannels exhibit self-propelled behavior, driven by the capillary pressure gradient within the drops. This driven force may be balanced by the viscous drag and the contact line drag to determine the drop displacement, in analogy to the way to predict capillary imbibition. However, how the drops move exactly with time at the nanoscale is unclear. This study employs molecular dynamics simulations to explore the dynamics of nanodrops within tapered channels with hydrophobic and hydrophilic coatings. The simulations reveal that in a hydrophobic tapered channel, drops migrate toward the wider side of the channel but may halt midway as the driving pressure approaches zero during their movements. Conversely, in hydrophilic tapered channels, drops move unlimitedly toward the channel's tip. Incorporating considerations for dynamic contact angles based on the molecular kinetic theory and liquid–solid slip, a theoretical model is derived that accurately predicts the drop displacement observed in molecular simulations without free parameters. In our simulations of drop motion in hydrophilic tapered channels, the drop displacement x is found linear with time x ∼t, as the viscous drag is dominant and the slip length is small. However, the theory further predicts that drop displacement may behave as x2 ∼t when slip length is large. Conversely, under dominant contact line drag, the theory predicts x3 ∼t for drop motion in tapered nanoslits. These findings underscore the critical influence of dynamic wetting and liquid–solid slip in precisely predicting drop motions on solid surfaces at the nanoscale.