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

Abstract: A study on the initial wetting velocity (V w0) 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, V w(t), was investigated at five different impact velocities (V i). The value of V w0 was found to be a few meters per second; V w then decreased dramatically in a few ms to approximately 1/10 of V w0. Moreover, the V w for water on parafilm became negative (i.e., retraction started) at approximately 2–3ms (dimensionless time t*∼1). A general phenomenon was observed: (i) a higher V i results in a larger V w0; (ii) at the first millisecond, the wetting velocity was dominated primarily by V i and was nearly independent of the surface hydrophobicity; (iii) for t >2ms, the V w depends mainly on the surface hydrophobicity; and (iv) for t =1–3ms, the V w for the droplet with higher V i became smaller for both solid substrates while the droplet continued spreading. [Copyright &y& Elsevier]