A model for detachment of a partially wetting drop from a solid surface by shear flow

Liquid drop detachment from a solid surface by simple shear flow is modeled based on the experimental observations available in the literature, A liquid drop adhered to a solid surface deforms in the presence of a simple shear flow to form an advancing and a receding dynamic contact angle, The drop slides on the solid surface when the drag due to the shearing fluid overcomes the retentive force due to the contact angle hysteresis, A drop having an equilibrium contact angle, theta(c), approaching 180 degrees detaches from the solid surface at the onset of its sliding motion, However, a drop with theta(e) much lower than 180 degrees slides on the solid surface and will not detach, With further increase in the shear rate, the sliding drop detaches from the solid surface when the lift force equals the adhesive, gravitational, and buoyancy forces of the drop, Based on this premise, an approximate mathematical model for the detachment of a partially wetting drop is constructed, The experimental results available in the literature for Pristane and Squalane drop detachment are compared for slide and lift as the mode of detachment, The critical shear rate for the detachment of Pristane drops, having theta(e) of 175 degrees, is predicted well by the model where sliding as the mode of detachment is assumed, whereas the experimental data for Squalane drops, having theta(e) of 126 degrees, is well predicted by the model where lift is considered the mode of detachment. (C) 1997 Academic Press.