Modulating Coalescence Timing of Liquid Marbles via Wettability Adaptation.

Droplet coalescence is ubiquitous in nature, and its regulation is significant in industrial processes and biomedical applications. While bare droplets suddenly coalesce in contact, the droplets covered with liquid-repellent particles to form "liquid marbles (LMs)" are not. Previously, the external stimuli-responsive breakage of the particle layer enables the regulation of the coalescence timing. However, preprogramming the coalescence timing of droplets without stimuli is challenging. In this work, LMs that break the particle layer in preprogrammed time are reported. The particles have a core wettable site and are tethered with a low-wettability flexible molecular chain, which gradually increases wettability with time. The time-dependent wettability variation is observed because of the differences in the adaptation of the molecular chain; thus, it is repeatedly available, and its speed is controllable by chain length. The formed LMs expose bare droplet surfaces in preprogrammed timing, which enables the modulation of coalescence timing from 2 to 45 min without relying on external stimuli. Moreover, the additivity of the particles enables the fine-tuning of the coalescence time with ≈1 min resolutions. Further, the contact of several LMs with different adaptation times enables cascade droplet coalescence, opening a new route for droplet manipulation.