Experimental study on two water drops successively impinging on a solid surface

The present study pertains to the experimental impingement of two successive water drops on the solid surface. The drops were generated by a free-falling drop technique and a drop severance device with a velocity of 1.9 m/s at low Weber number. The initial drop diameter was fixed to either 930 μm or 1025 µm. Different from previous works, this study focused on four cases of drop impacts on stainless steel plates classified by the impingement timing of the leading drop deformation: case 1—single drop impingement for reference, case 2—the moment of the leading drop starting its spreading after impacting the surface, case 3—the instant of the leading drop reaching its maximum spreading, and case 4—the timing of the leading drop staying its maximum height. It is observed that the deformation behavior of two successive drops impacting on the surface is highly dependent on the impingement timing, which has minor effects on the change of the maximum drop spreading diameter, but affects significantly the change of the maximum height. The results reveal that the maximum spreading diameter of two drop impact for various cases is around 1.3 times larger than that of the single drop impingement. It is first observed that the maximum drop height for case 3 occurs at second recoil and is greater than those for case 2 and case 4 since the case 3 momentum effect at impingement timing acts in the inward direction, but the other two are in the outward direction.