Droplet Encapsulation in Superhydrophobic Surface-Attached Bubbles Driven by Laser-Induced Cavitation.

As a novel phenomenon recently unveiled in immersion lithography research, bubble-encapsulated droplets vanish in mere fractions of a second. Despite their subsecond lifetimes, bubble-encapsulated droplets hold significant potential for applications including cell incubation, droplet microreactors, and in situ sampling in environmental fluids. Here, we couple patterned superhydrophobic surfaces (PSHSs) with laser-induced cavitation to achieve controllable droplet encapsulation inside a surface-attached bubble. High-speed shadowgraphy shows that collapse near an attached bubble generates a microjet directed toward the PSHS. With the dimensionless separation distance γ and radius ratio ε scaled by the maximum cavitation bubble radius (Rmax), the internal jet transitions between conical, concave, and crown-like morphologies. During jet impaction, the liquid thread undergoes necking and pinch-off under the coupled action of inertia, surface tension, and gravity, producing droplets that remain confined within the surface-attached bubble for days. At fixed ε, the microjet velocity decreases with increasing γ, whereas the encapsulated droplet radius Rd varies nonmonotonically with γ. At fixed γ, Rd increases with ε, while the dimensionless encapsulation time τd decreases with ε. These results establish a repeatable, noncontact route to long-lived bubble-encapsulated droplets and quantitatively demonstrate the tunability of this encapsulation technique while deepening mechanistic insights into its encapsulation formation dynamics.