Soft gels, bouncing jets

Most strategies for achieving liquid repellency rely on surface structure, for example the micro/nano-texturing used in superhydrophobic surfaces. Here, we show that liquid repellency can instead be achieved by tuning material softness alone, without any structuring or chemical treatment. We study the impact dynamics of a liquid jet striking a soft gel, and find that jet rebound occurs independent of the liquid's wetting properties: even soapy water, which strips conventional superhydrophobic surfaces of their repellency, bounces readily off a sufficiently soft substrate. The underlying mechanism is purely geometric, driven by the pressure of the impacting jet pressing a dimple into the soft, deformable gel; the jet then follows the contour of this dimple and bounces off at an angle. Together, these results establish substrate elasticity, independent of surface texture or chemical treatment, as a controllable design parameter for liquid-repellent surfaces, with potential relevance to anti-icing and anti-fouling applications.