Suppressing prompt splash with polymer additives

Splash suppression during drop impact continues to be a grand challenge. To date, only a few techniques for the complete suppression of splash exist. Reducing the ambient pressure and using complex surfaces (microstructured and/or soft) are two of the recently discovered ones which may not be very practical in many technological processes. The idea of using additives directly into the liquid used to produce the drops, to inhibit this undesirable phenomenon, is, therefore, desired. Prompt splash is a type of splashing that releases diminutive droplets at high speeds from the tip of the lamella at the spreading liquidsubstrate contact line immediately after the impact (within the first 10 mu m), without generating the typical thin-sheet or corona. Prompt splash remained hidden for many years until high-speed imaging allowed for its visualisation. Here, we demonstrate that by adding very low amounts of polymer (around 0.01 wt%) into normally splashing water droplets a reduction and even a complete suppression of the prompt splash is observed. In this work, a systematic experimental study of the impact of viscoelastic drops, by varying size, impact velocity, and the "degree" of viscoelasticity, is conducted. When capillary forces are insufficient to maintain the integrity of the drop, elastic forces seem to pull the attached small droplets/fingers back to the lamella preventing their ejection and, therefore, inhibiting prompt splash. However, surprisingly, larger quantities of the polymer additive lead to a secondary transition, in which another, more common, type of splash is induced: corona splash.