Jiangtao Cheng

Droplet impingement on micropillar-arrayed viscoelastic substrates constitutes a tightly coupled fluid-solid-gas interaction problem spanning multiple length scales, in which impact-induced micropillar deformation and interstitial gas flow dynamically interact with droplet spreading. The viscoelastic rheology of the substrate further augments the modeling complexity. Here, we report a multiscale computational framework that can bridge the gap between the microscale structural deformation and the macroscale droplet dynamics via bidirectional coupling. This approach employs nested zoomed-in and zoomed-out procedures, wherein the macroscale velocity and pressure fields of the zoomed-out domain are imposed as the boundary conditions on the localized microscale zoomed-in domains in both the gas- and liquid-covered regions. Representative microscale simulations are implemented to resolve micropillar deformation, while the response in the non-simulated regions is reconstructed through a boundary-velocity scaling strategy. The microscale solutions are subsequently projected back to the macroscale model using the bilinear interpolation, enabling efficient two-way coupling across different scales. This framework effectively captures liquid spreading evolutions while resolving individual micropillar oscillations with reduced computational cost. Our numerical results demonstrate that interstitial gas dissipates droplet kinetic energy in both the normal and shear directions, thereby mitigating micropillar deformation, particularly for substrates with larger gap densities. Notably, maximum deformation is observed at the intermediate gap densities of 0.10 or 0.15, rather than at the highest density of 0.20, over a range of impact velocities. Overall, this multiscale and multiphase computational strategy provides an efficient way for modeling the coupled fluid-solid-gas interactions involving liquid microdroplet, viscoelastic microstructures and interstitial gas.

Droplet impingement on micropillar-arrayed viscoelastic substrates constitutes a tightly coupled fluid-solid-gas interaction problem spanning multiple length scales, in which impact-induced micropillar deformation and interstitial gas flow dynamically interact with droplet spreading. The viscoelastic rheology of the substrate further augments the modeling complexity. Here, we report a multiscale computational framework that can bridge the gap between the microscale structural deformation and the macroscale droplet dynamics via bidirectional coupling. This approach employs nested zoomed-in and zoomed-out procedures, wherein the macroscale velocity and pressure fields of the zoomed-out domain are imposed as the boundary conditions on the localized microscale zoomed-in domains in both the gas- and liquid-covered regions. Representative microscale simulations are implemented to resolve micropillar deformation, while the response in the non-simulated regions is reconstructed through a boundary-velocity scaling strategy. The microscale solutions are subsequently projected back to the macroscale model using the bilinear interpolation, enabling efficient two-way coupling across different scales. This framework effectively captures liquid spreading evolutions while resolving individual micropillar oscillations with reduced computational cost. Our numerical results demonstrate that interstitial gas dissipates droplet kinetic energy in both the normal and shear directions, thereby mitigating micropillar deformation, particularly for substrates with larger gap densities. Notably, maximum deformation is observed at the intermediate gap densities of 0.10 or 0.15, rather than at the highest density of 0.20, over a range of impact velocities. Overall, this multiscale and multiphase computational strategy provides an efficient way for modeling the coupled fluid-solid-gas interactions involving liquid microdroplet, viscoelastic microstructures and interstitial gas.

Self-propelled droplet jumping has widespread applications in surface cleaning, condensation heat transfer, hydrogen production, and triboelectric nanogenerator due to the passive yet effective cross-interface transfer of mass, momentum, energy and charge, whose rates generally increase with droplet size. However, as droplet size increases, gravity inevitably impedes droplet’s mobility, imposing a capillary length constraint of 2.7 mm for water droplet, beyond which self-propelled jumping remains a persistent challenge. Here, we report passive jumping of water puddle in the unprecedented centimeter scale from a superhydrophobic surface through the burst of an entrained bubble, breaking the capillary length limitation for droplet jumping. By virtue of direct and localized impact at droplet base, the bubble-burst-induced capillary waves play a paradigm-shifting role in shortening the impact duration, depressing droplet spreading, and facilitating momentum transfer. With >90% conversion to droplet jumping momentum, the impacting momentum of capillary waves scales linearly while droplet jumping height scales quadratically with bubble radius. Through studying the synergistic interplay between bubble bursting, fluidic jetting and droplet jumping, this work reveals a previously unexplored mechanism of capillary wave impact in fluid-structure interactions and offers a promising strategy for droplet actuations and the directional printing of particles in additive manufacturing.

Tweezers based on optical, electric, magnetic, and acoustic fields have shown great potential for contactless object manipulation. However, current tweezers designed for manipulating millimeter‐sized objects such as droplets, particles, and small animals exhibit limitations in translation resolution, range, and path complexity. Here, a novel acoustic vortex tweezers system is introduced, which leverages a unique airborne acoustic vortex end effector integrated with a three‐degree‐of‐freedom (DoF) linear motion stage, for enabling contactless, multi‐mode, programmable manipulation of millimeter‐sized objects. The acoustic vortex end effector utilizes a cascaded circular acoustic array, which is portable and battery‐powered, to generate an acoustic vortex with a ring‐shaped energy pattern. The vortex applies acoustic radiation forces to trap and spin an object at its center, simultaneously protecting this object by repelling other materials away with its high‐energy ring. Moreover, The vortex tweezers system facilitates contactless, multi‐mode, programmable object surfing, as demonstrated in experiments involving trapping, repelling, and spinning particles, translating particles along complex paths, guiding particles around barriers, translating and rotating droplets containing zebrafish larvae, and merging droplets. With these capabilities, It is anticipated that the tweezers system will become a valuable tool for the automated, contactless handling of droplets, particles, and bio‐samples in biomedical and biochemical research.

Robotic manipulation of small objects has shown great potential for engineering, biology, and chemistry research. However, existing robotic platforms have difficulty in achieving contactless, high-resolution, 4-degrees-of-freedom (4-DOF) manipulation of small objects, and noninvasive maneuvering of objects in regions shielded by tissue and bone barriers. Here, we present chirality-tunable acoustic vortex tweezers that can tune acoustic vortex chirality, transmit through biological barriers, trap single micro- to millimeter-sized objects, and control object rotation. Assisted by programmable robots, our acoustic systems further enable contactless, high-resolution translation of single objects. Our systems were demonstrated by tuning acoustic vortex chirality, controlling object rotation, and translating objects along arbitrary-shaped paths. Moreover, we used our systems to trap single objects in regions with tissue and skull barriers and translate an object inside a Y-shaped channel of a thick biomimetic phantom. In addition, we showed the function of ultrasound imaging–assisted acoustic manipulation by monitoring acoustic object manipulation via live ultrasound imaging.