Predicting the maximum spreading of a liquid drop impacting on a solid surface: Effect of surface tension and entrapped air layer

The spreading of liquid droplets impacting a surface at high speed is well understood by now. However, when a droplet impacts a surface at relatively low impact velocities (<1 m/s), the wetting properties of the fluid become important, and the entrapped air layer between the impacting drop and the solid surface prevents the immediate wetting of the surface. To determine the influence of both wetting and the entrapped air, we perform experiments by systematically varying the surface tension of the liquid and the air pressure. Drop impact measurements at reduced air pressures show that the spreading is independent of the pressure; dynamic contact-angle measurements indicate that this happens because the air film breaks rapidly. By varying the surface tension and surface wettability, we show that droplet spreading at low velocities can be predicted from the wetting properties of the surface and the known energy balance for the impact.