The spreading of a water droplet impacting on a flat solid surface was studied using both numerical models and experiments. Surface tension was varied in experiments by adding traces of a surfactant to water. Droplet impact was photographed, and liquid-solid contact diameter and contact angle measured from photographs. A numerical solution of the Navier Stokes equation using a modified SOLA-VOF method was used to model droplet deformation during impact. Measured values of contact angle were used as a boundary condition for the numerical model. Under the condition of low We numbers (We<40) impacting droplets spread on the surface until surface tension forces overcame liquid inertia, after which they recoiled off the surface. Decreasing surface tension did not affect droplet spreading, but did reduce the height of recoil. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model correctly modelled droplet shape during deformation. Accurate predictions were obtained for droplet contact diameter during spreading, and at equilibrium. The model overpredicted droplet contact diameters during recoil. When the contact angle was assumed constant in the model, equal to the measured equilibrium value, predictions were less accurate.

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