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 (R_max), 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 R_d varies nonmonotonically with γ. At fixed γ, R_d increases with ε, while the dimensionless encapsulation time τ_d 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.