Solidification dynamics of impinging droplets in distinct regimes due to intertwined influences of Weber and Stefan numbers: a unifying theoretical and experimental framework

In contact with subcooled substrates, droplets spread and solidify simultaneously, an interplay of fluid dynamics, heat transfer, and phase transition with its broad occurrence in nature and industry. Existing hypotheses attribute contact line pinning (CLP) mechanism to critical contact angle, volume, or temperate is reached. The extent of substrate subcooling determined by Stefan numbers, however, significantly alters physics of CLP and triggers various solidifying modes, rendering a possibility to unify seemingly contradicting hypotheses in their respective regimes. This work focuses on impacting droplet solidification at intermediate Weber numbers and its morphology at the onset of splash under intertwined effects of We and Ste. It aims to reveal the ultimate underlying physics behind CLP for impacting and spreading droplets with distinct solidification modes. Impacting hexadecane droplets on subcooled glass surfaces are experimentally studied with ranges of We from 1.0 to 800 and Ste from 0.01 to 3.0. The asymptotic spreading threshold is non-monotonic, where fingering and satellite droplets formation are “optimally” suppressed with certain Ste for moderately high We, suggesting fundamental influences of solidification modes on spreading dynamics.