Experimental Study of the Dynamic Behavior of a Low Surface Tension Droplet Impacting on a Super-Hydrophobic Surface

The dynamic behavior of droplets with various surface tensions impacting on a super-hydrophobic surface is experimentally investigated. Two possible impact behaviors, named complete rebound and fragmentation-splashing, are observed on water and sodium dodecyl sulfate (SDS) droplet. Besides these two types of behavior, another behavior named partial rebound is only observed on sodium dodecyl sulfate droplet. A phase diagram is provided to quantitatively elucidate the dependence of droplet impact behaviors on the Reynolds number Re and sodium dodecyl sulfate concentration. The increased SDS concentration (i.e., the decreased droplet surface tension) can prevent complete rebound behavior and make gradual decrease in the critical Reynolds number for complete rebound and partial rebound and that for partial rebound and fragmentation-splashing. As for detailed behaviors of impact, the droplet retraction time t_r is more significantly affected by the droplet impact velocity and surface tension, while the spreading time t_s varies in a small range. As compared with t_s, the droplet retraction time t_r is much longer and possesses different evolving features with various droplets as Re varies. For the water droplet, t_r gradually decreases with increase in Re; and the trend shows to be opposite for the SDS droplet. Based on energy conservation principles, an analytic model for maximum spreading factor (β_max) of droplets with various surface tensions impacting on a super-hydrophobic surface is proposed, where the parameter for maximum spreading time is defined as t_s = D_m/2 \({{{v}}_{0}}\) . As compared with the experimental data, the predicted results are highly acceptable with the errors less than 10% and most errors less than 7%.