Search for: All records

Traditional surface cleaning methods often suffer from drawbacks such as chemical harshness, potential for surface damage, and high‐energy consumption. This study investigates an alternative approach: acoustic‐driven surface cleaning using millimeter‐sized bubbles excited at low, sub‐cavitation frequencies. We identify and characterize a distinct translational resonance of these bubbles, occurring at significantly lower frequencies (e.g., 50 Hz for 1.3 mm diameter bubbles) than the Minnaert resonance for a bubble of the same size. At this translational resonance, stationary bubbles exhibit amplified lateral swaying, while bubbles sliding on an inclined surface display pronounced “stop‐and‐go” dynamics. The theoretical model treats the bubble as a forced, damped harmonic oscillator. In this framework, surface tension supplies the restoring force, while the inertia is governed primarily by the hydrodynamic added mass of the surrounding fluid. It accurately predicts the observed resonant frequency scaling with bubble equilibrium radius (). Cleaning efficacy, assessed using protein‐based artificial soil on glass slides, was significantly improved when bubbles were driven at their translational resonant frequency compared to off‐resonant frequencies or nonacoustic conditions. These findings demonstrate that leveraging translational resonance enhances bubble‐induced shear and agitation, offering an effective and sustainable mechanism for surface cleaning. 

Hand clapping, a ubiquitous human behavior, serves diverse daily-life purposes. Despite prior research, a comprehensive understanding of its physical mechanisms remains elusive. To bridge this gap, we integrate human data, parametric experiments, finite-element simulations, and theoretical frameworks to investigate the acoustic properties of clapping sound and their connections with the fluid flow and soft matter collision. Motion-audio synchronization reveals the flow-excitation nature of the hand cavity resonance. The classical Helmholtz resonator model, incorporating occasional pipe standing wave contributions for finger grooves, reliably predicts clapping sound frequencies across various real and engineered hand configurations. Material elasticity, coupled with the dynamic collision process, has minor effects on the sound frequency but a major impact on the temporal evolution of the sound signals, as reflected by the quality factors of resonance. Both spatial and dynamic factors for sound intensity are examined. We establish a quadratic scaling relationship between hand cavity gauge pressure and clapping speed, elucidating the positive correlation between faster claps and louder sounds. Our work advances the knowledge of hand-clapping acoustics and offers insights into sound signal synthesis, processing, and recognition. Furthermore, these findings may facilitate low-cost acoustical diagnostics in architecture and enhance rhythmic sound patterns in music and language education. 

Free surface flows driven by boundary undulations are observed in many biological phenomena, including the feeding and locomotion of water snails. To simulate the feeding strategy of apple snails, we develop a centimetric robotic undulator that drives a thin viscous film of liquid with the wave speed$$V_w$$. Our experimental results demonstrate that the behaviour of the net fluid flux$$Q$$strongly depends on the Reynolds number$$Re$$. Specifically, in the limit of vanishing$$Re$$, we observe that$$Q$$varies non-monotonically with$$V_w$$, which has been successfully rationalised by Pandeyet al.(Nat. Commun., vol. 14, no. 1, 2023, p. 7735) with the lubrication model. By contrast, in the regime of finite inertia ($${Re} \sim O(1)$$), the fluid flux continues to increase with$$V_w$$and completely deviates from the prediction of lubrication theory. To explain the inertia-enhanced pumping rate, we build a thin-film, two-dimensional model via the asymptotic expansion in which we linearise the effects of inertia. Our model results match the experimental data with no fitting parameters and also show the connection to the corresponding free surface shapes$$h_2$$. Going beyond the experimental data, we derive analytical expressions of$$Q$$and$$h_2$$, which allow us to decouple the effects of inertia, gravity, viscosity and surface tension on free surface pumping over a wide range of parameter space. 

Abstract This study explores the impact of feathers on the hydrodynamic drag experienced by diving birds, which is critical to their foraging efficiency and survival. Employing a novel experimental approach, we analyzed the kinematics of both feathered and nonfeathered projectiles during their transition from air to water using high‐speed imaging and an onboard accelerometer. The drag coefficients were determined through two methods: a direct calculation from the acceleration data and a theoretical approach fitted to the observed velocity profiles. Our results indicate that feathers significantly increase the drag force during water entry, with feathered projectiles exhibiting approximately double the drag coefficient of their smooth counterparts. These findings provide new insights into the role of avian feather morphology in diving mechanics and have potential implications for the design of bioinspired aquatic vehicles in engineering. The study also discusses the biological implications of increased drag due to feathers and suggests that factors such as body shape might play a more critical role in the diving capabilities of birds than previously understood. 

Transition from steady unidirectional motion to oscillatory motion revealed for a Marangoni swimmer interacting with passive particle raft under 1D confinement and a cargo transport potential observed in the steady regime. 

Abstract Fluid-mechanics research has focused primarily on droplets/aerosols being expelled from infected individuals and transmission of well-mixed aerosols indoors. However, aerosol collisions with susceptible hosts earlier in the spread, as well as aerosol deposition in the nasal cavity, have been relatively overlooked. In this paper, two simple fluid models are presented to gain a better understanding of the collision and deposition between a human and aerosols. The first model is based on the impact of turbulent diffusion coefficients and air flow in a room on the collisions between aerosols and humans. Infection rates can be determined based on factors such as air circulation and geometry as an infection zone expands from an infected host. The second model clarifies how aerosols of different sizes adhere to different parts of the respiratory tract. Based on the inhalation rate and the nasal cavity shape, the critical particle size and the deposition location can be determined. Our study offers simple fluid models to understand the effects of geometric factors and air flows on the aerosol transmission and deposition.