Effect of wettability on the impact force of water drops falling on flat solid surfaces

Drops falling on substrates with varying wettability exhibit distinct morphologies. However, the relation between the impact force exerted by a water drop and the substrate wettability has not been thoroughly explored. In this paper, we investigate the effect of contact angle (ranging from 40° to 180°) on the impact force of water drops, along with the spreading diameter, rim height, and retracting velocity. Our attention is focused to the inertial regime with the Weber number ranging from 30 to 100, which enables us to rationalize the dynamic relations and to correlate the kinematics of the drop with the impact force through scaling analysis. We find that substrate wettability has insignificant effect on the first force peak, which arises mainly from the momentum change during the initial impact. However, it does influence the second force peak, which originates from the momentum change in the flow redirecting from the radial inward direction to the vertical direction, accompanied by a column-shape Worthington jet. The second peak force gradually diminishes as the contact angle decreases, until it becomes indistinguishable below 40°, while the time at which the second peak force emerges is delayed.