Criterion of wetting failure in a Couette flow

Wetting failure occurs when the speed of the moving substrate exceeds a threshold, characterized by a critical Capillary number, above which the stationary contact line cannot be sustained. In most experimental and theoretical studies, it is found that the onset of wetting failure is accompanied by a geometry constraint that the apparent contact angle vanishes. In a Couette device, however, it is reported that wetting failure occurs at nonzero apparent contact angles. Using a lubrication theory, we investigate the contact dynamics in a Couette flow. The critical Capillary number is predicted. It is suggested that the criterion of vanishing apparent contact angle still holds if it is well defined.