Initial wetting velocity of droplet impact and spreading: Water on glass and parafilm

A study on the initial wetting velocity (Vw0) for a water droplet impinging on glass and parafilm surfaces was performed using a fast-speed video camera system. The silhouette images were recorded sequentially from the moment the droplet made contact with the solid surface. The profile of the droplet during spreading revealed the droplet-wetting diameter (especially in the first millisecond), then the wetting velocity was determined. The relaxation of the wetting velocity, Vw(t), was investigated at five different impact velocities (Vi). The value of Vw0 was found to be a few meters per second; Vw then decreased dramatically in a few ms to approximately 1/10 of Vw0. Moreover, the Vw for water on parafilm became negative (i.e., retraction started) at approximately 2–3 ms (dimensionless time t* ∼ 1). A general phenomenon was observed: (i) a higher Vi results in a larger Vw0; (ii) at the first millisecond, the wetting velocity was dominated primarily by Vi and was nearly independent of the surface hydrophobicity; (iii) for t > 2 ms, the Vw depends mainly on the surface hydrophobicity; and (iv) for t = 1–3 ms, the Vw for the droplet with higher Vi became smaller for both solid substrates while the droplet continued spreading.