Xiaolei Hu

Hollow-droplet impact on superhydrophobic corrugated surfaces plays a critical role in spray cooling, coating, anti-icing, and additive manufacturing, yet the interplay between internal cavity deformation and surface topography remains poorly understood. To investigate impact dynamics of hollow droplets on periodically corrugated superhydrophobic surfaces at low Weber numbers, we employ an improved embedded-boundary method combined with the volume-of-fluid method. By systematically varying the Weber number (We) and corrugation density (ηe), we identify four distinct wetting regimes: Wenzel state mode, Cassie–Wenzel transition mode, sticky Cassie state (SC) mode, and retracting Cassie state(RC) mode. A phase diagram in the We–ηe plane is constructed to classify these regimes and illustrate their transition pathways. Sparse corrugations promote liquid impregnation and Wenzel-type pinning, whereas denser textures suppress deep penetration and favor Cassie-type wetting. Within the Cassie regime, insufficient retraction inertia results in temporary adhesion (SC mode), whereas higher We allows the droplet to overcome localized suction and rebound in the RC mode. The competition between adhesive and retractive capillary pressures provides a unified interpretation of the observed wetting transitions. These results reveal how corrugation geometry and impact inertia jointly govern hollow-droplet adhesion, retraction, and rebound, providing design guidelines for textured superhydrophobic surfaces with tailored liquid retention or repellency.

Microparticle separation has become a powerful label-free detection technique. This study presents a microfluidic particle separation method based on acoustic standing waves. A multiphysics coupled model integrating acoustic fields, laminar flow, and particle tracing was developed to characterize particle migration and separation behaviors under the combined action of acoustic and hydrodynamic fields. We systematically investigate the effects of acoustic field parameters, hydrodynamic conditions, and particle material properties on particle separation. The results reveal that particle migration is governed by acoustic radiation forces and hydrodynamic transport. Under the optimized operating conditions, the numerical model achieved complete size-selective separation of 1, 5, and 10 μm particles, corresponding to 100% separation efficiency and 100% separation purity. Enhanced acoustic excitation promotes transverse displacement, whereas flow regulation alters streamline distributions and outlet collection regions, thereby influencing particle redistribution. Distinct separation behaviors are observed under different acoustofluidic conditions, resulting in under-migration, stable separation, or excessive lateral displacement. In addition, particle material properties significantly affect separation trajectories and outlet distributions because of differences in the acoustic contrast factor. Particles with moderate acoustic contrast achieve optimal confinement, while weak or strong responses compromise separation performance. This study demonstrates that effective separation can be achieved through coordinated control of acoustic and hydrodynamic effects. These findings provide mechanistic insights into standing-wave acoustofluidic separation and offer guidance for the design and optimization of acoustofluidic devices.

Off-center pursuit collisions of high-viscosity molten glass microbeads are ubiquitous in industrial manufacturing, playing a critical role in determining product sphericity and ultimate performance. We developed a three-dimensional numerical framework using the open-source solver Basilisk, integrating the volume of fluid method with adaptive mesh refinement, and investigated the effects of Weber number (We) and impact parameter (B) on morphological evolution, internal flow topology, and energy conversion during off-center pursuit collisions. Six distinct collision regimes are identified: (I) bouncing, (II) coalescence, (III) reflexive separation with satellite droplets, (IV) reflexive separation without satellites, (V) stretching separation with a single satellite, and (VI) stretching separation with multiple satellites. The results indicate that in head-on collisions (B = 0), the reflexive separation observed at high We is driven primarily by end-pinching mechanism. For off-center collisions (B > 0), impact-induced angular momentum significantly alters the flow topology, triggering non-axisymmetric shear flows. As B increases to 0.5, the regime transitions to stretching separation, governed by the interplay of shear forces and centrifugal effects, accompanied by multiple satellite formation. A coupled analysis of velocity and pressure fields reveals that vortical motion induced by moderate B mitigates violent collision focusing at the geometric center, thereby promoting coalescence. Furthermore, energy budget analysis demonstrates that rotational kinetic energy sustains high surface energy during the elongation phase, facilitating stretching separation. Through comparison with the results of other researchers, we observe that the critical We required for separation increases with increasing Oh. This study provides theoretical guidelines for optimizing spheroidization parameters to suppress agglomeration and splashing defects while enhancing product sphericity.

This study presents a method for enriching oil-suspended particles within a rectangular microfluidic channel using acoustic standing waves. A modified Helmholtz equation is solved to establish the acoustic field model, and the equilibrium between acoustic radiation forces and viscous drag is described by combining Gor’kov potential theory with the Stokes drag model. Based on this force balance, the particle motion equation is derived, enabling the determination of the critical particle size necessary for efficient enrichment in oil-filled microchannels. A two-dimensional standing-wave microchannel model is subsequently developed, and the influences of acoustic, fluidic, and particle parameters on particle migration and aggregation are systematically investigated through theoretical analysis and numerical simulations. The results indicate that when the channel dimension and acoustic wavelength satisfy the half-wavelength resonance condition, a stable standing-wave field forms, effectively focusing suspended particles at the acoustic pressure nodes. Enrichment efficiency is found to be strongly dependent on inlet flow velocity, particle diameter, acoustic frequency, temperature, and particle density. Lower flow velocities and larger particle sizes result in higher enrichment efficiencies, with the most uniform and stable pressure distribution achieved when the acoustic frequency matches the resonant channel width. Increases in temperature and particle density enhance the acoustic radiation force, thereby accelerating the aggregation of particles. These findings offer theoretical foundations and practical insights for acoustically assisted online monitoring of wear particles in lubricating oils, contributing to advanced condition assessment and fault diagnosis in mechanical systems.

The texture of superhydrophobic surfaces can reduce solid–liquid contact time by altering the droplet impact dynamics, which is widely used in applications such as self-cleaning and anti-icing. However, the influence of the roughness of superhydrophobic surfaces on the behavior of droplet impact has not been fully elucidated, which limits the engineering applications of such surfaces. In this study, superhydrophobic surfaces with different roughnesses were prepared, and the kinetic behavior of droplets impacting these surfaces for different Weber numbers (We) was systematically analyzed by combining high-speed photographic experiments and hydrodynamic simulations. The results show that the droplets become more thermodynamically unstable due to the significant tensile deformation developed at the droplet retraction stage with increasing We, and secondary droplet splashing occurs due to the Plateau–Rayleigh instability. Moreover, the critical We for a droplet transitioning from complete rebound to breakage splashing decreases monotonically with increasing surface roughness. The physical mechanism of the droplet splashing phenomenon on superhydrophobic surfaces facilitated by surface roughness is elucidated by modeling the quantitative relationship between the air spillage velocity and the initial impact velocity of the droplets, combined with the synergistic analyses of the contact force distributions, the internal velocity field, and the pressure field distributions during the retraction phase of the droplets. This study not only deepens our understanding of the complex physical processes underlying the impact of droplets on superhydrophobic surfaces but also provides a theoretical basis for self-cleaning and anti-icing applications.

Wear debris is the direct byproduct of frictional interactions between mechanical components. The morphology, quantity, and material characteristics of the debris provide critical information on the wear condition of oil-lubricated systems. Timely detection and monitoring of the debris are critical to preventing further abrasive damage and ensuring reliable operation of the components. Various sensing technologies have been developed to address this challenge. Despite a lot of real-time monitoring research have been done over the past decade, a comprehensive review of online and inline monitoring methods remains lacking. To address this gap, this study presents a systematic bibliometric analysis of recent research on oil wear debris real-time monitoring. The results show that there is a growing interest and rapid development in this field. Key advances in these technologies are reviewed, including electrical sensors, magnetic sensors, acoustic sensors, energy-based sensors, and optical sensors. In addition to analyzing the current literature, this study also highlights the technical limitations and outlines the future research directions. The objective is to provide researchers with an in-depth understanding of the debris monitoring methods in this domain and offer guidance for future research endeavors.