Precisely Timed On‐Demand Droplet Ejection and Collision in Free Space for Controlled Reaction Initiation

ABSTRACT

Precise initiation and synchronization of chemical reactions in free space remain major challenges for time‐resolved studies in structural biology, chemistry, and materials science. Although modern probes provide femtosecond resolution, experimental access to early reaction dynamics is limited by the ability to trigger, mix, and deliver nano‐to‐picoliter liquid samples with deterministic spatial and temporal control. Existing liquid jets and drop‐on‐demand systems cannot reliably define reaction start times in flight or synchronize them to external pulse patterns. We present an acousto‐pneumatic droplet ejection platform that delivers single sub‐nanoliter droplets on‐demand with microsecond timing accuracy and reproducible trajectories, enabling deterministic one‐drop‐per‐trigger operation synchronized to external signals at repetition rates up to 100 Hz. Deterministic mid‐air droplet collisions define reaction initiation directly at the probe and are compatible with demanding pulse structures such as X‐ray free‐electron lasers. The programmable platform offers high‐resolution, time‐resolved experimentation, spanning ultrafast X‐ray studies and precision nanoliter dispensing.