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.

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.

Acoustic radiation force-driven particle manipulation in viscous microfluidic environments is governed by the interplay between the spatial structure of the acoustic pressure field and viscous dissipation. In this study, a three-dimensional fully coupled electro-solid–acoustic-fluid model is developed to investigate the formation mechanism of standing wave fields and the resulting particle migration dynamics in oil-based suspensions. The system is driven by piezoelectric transducers operating under single, in-phase dual, and quadrature-phase dual excitation conditions, enabling systematic analysis of the transition from one-dimensional to two-dimensional acoustic pressure node configurations. Results show that the acoustic radiation force distribution is strongly determined by the topology of the pressure field, and quadrature-phase excitation generates a stable point-like pressure node at the channel center, leading to symmetric two-dimensional force convergence and significantly enhanced particle trapping efficiency. Parametric analysis further reveals that the particle migration behavior is governed by the competition between acoustic radiation force magnitude, viscous drag, and residence time, while voltage and flow rate primarily modulate the effective strength and duration of acoustic interaction rather than altering the fundamental force topology. These findings elucidate the acoustofluidic mechanism underlying particle manipulation and provide a theoretical foundation for parameter optimization of particle enrichment systems in oil.

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.