Contact Angles of Droplets during Spread and Recoil after Impinging on a Heated Surface

T he contact angle of a droplet impinging upon a hot surface undergoes changes as the drop spreads and recoils. The motion of the liquid and the effect of evaporation from the edges of the drop affect the contact angle. The changes in the contact angle during spreading affect the spreading characteristics of the droplet upon impact. However, the models available in literature for the maximum spreading ratio (maximum spread diameter divided by the initial droplet diameter) do not include this effect. In addition, the actual area of contact of a droplet is needed in the heat transfer studies. The present work reports an experimental study conducted to characterize the contact angle variation as a function of the heater surface ®nish, heater material, and heater surface temperature prior to impact. It is seen that the dynamic advancing contact angle extends beyond the equilibrium advancing and receding contact angles during the motion of the interface. Since the droplet spreading is in ̄uenced by the dynamic advancing contact angle, it is proposed to use the dynamic contact angle measurements in the available models for maximum spreading ratio. The experimental results obtained on the maximum spreading diameter indicate the validity of this approach.