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Recent experiments on coating flows and liquid drop impact both demonstratethat wetting failures caused by air entrainment can be suppressed by reducingthe ambient gas pressure. Here, it is shown that non-equilibrium effects in thegas can account for this behaviour, with ambient pressure reductions increasingthe gas' mean free path and hence the Knudsen number $Kn$. These effects firstmanifest themselves through Maxwell slip at the gas' boundaries so that forsufficiently small $Kn$ they can be incorporated into a continuum model fordynamic wetting flows. The resulting mathematical model contains flowstructures on the nano-, micro- and milli-metre scales and is implemented intoa computational platform developed specifically for such multiscale phenomena.The coating flow geometry is used to show that for a fixed gas-liquid-solidsystem (a) the increased Maxwell slip at reduced pressures can substantiallydelay air entrainment, i.e. increase the `maximum speed of wetting', (b)unbounded maximum speeds are obtained as the pressure is reduced only when slipat the gas-liquid interface is allowed for and (c) the observed behaviour canbe rationalised by studying the dynamics of the gas film in front of the movingcontact line. A direct comparison to experimental results obtained in thedip-coating process shows that the model recovers most trends but does notaccurately predict some of the high viscosity data at reduced pressures. Thisdiscrepancy occurs because the gas flow enters the `transition regime', so thatmore complex descriptions of its non-equilibrium nature are required. Finally,by collapsing onto a master curve experimental data obtained for drop impact ina reduced pressure gas, it is shown that the same physical mechanisms are alsolikely to govern splash suppression phenomena.

Air Entrainment in Dynamic Wetting: Knudsen Effects and the Influence of Ambient Air Pressure