arXiv subjects

Brady Wu

Publications and source records attributed to Brady Wu.

5 recordsLinked to original sources

arXiv2025

Non-reciprocity and multibody interactions in acoustically levitated particle systems: A three body problem

In active fluids and active solids the constituents individually generate movement by each extracting energy from their environment or from their own source. Non-reciprocal interactions among these active constituents then enable novel collective behavior that often can be strikingly counterintuitive. However, non-reciprocity in these cases typically requires that the interacting bodies have different physical properties or it needs to be programmed explicitly into all pairwise interactions. Here we show that collective activity in a driven system can emerge spontaneously through multibody nonreciprocal forces, even if all bodies are individually non-active and have identical properties. We demonstrate this with as few as three identical spheres, acoustically levitated in air, which exhibit collective activity as they interact through non-pairwise forces: similar to the classic gravitational three-body problem, the interaction between two spheres depends sensitively on the relative position of the third sphere. Non-reciprocity arises naturally from both near-field sound scattering and microstreaming forces among the spheres. The underdamped dynamics in air furthermore make it possible to go beyond collective center-of-mass propulsion or rotation and observe internal, engine-like reconfigurations that follow limit cycles. These findings open up new possibilities for self-assembly, where now multibody interactions not only determine the resulting structure but also drive the spontaneously emerging dynamics.

cond-mat.soft↗

arXiv2022

Hydrodynamic Coupling Melts Acoustically Levitated Crystalline Rafts

The acoustic levitation of small particles provides a versatile platform to investigate the collective dynamical properties of self-assembled many-body systems in the presence of hydrodynamic coupling. However, acoustic scattering forces can only generate attractive interactions at close range in the levitation plane, limiting self-assembly to rafts where particles come into direct, dissipative, contact. Here, we overcome this limitation by using particles small enough that the viscosity of air establishes a repulsive streaming flow at close range. By tuning the size of particles relative to the characteristic length scale of the viscous flow, we control the interplay between attractive and repulsive forces. In this novel granular raft, particles form an open lattice with tunable spacing. Hydrodynamic coupling between particles gives rise to spontaneous excitations in the lattice, in turn driving intermittent particle rearrangements. Under the action of these fluctuations, the raft transitions from a predominantly quiescent, crystalline structure, to a two-dimensional liquid-like state. We show that this transition is characterized by dynamic heterogeneity and intermittency, as well as cooperative particle movements, that produce an effectively `cageless' crystal. These findings shed light on fluid-coupling driven excitations that are difficult to isolate and control in many other hydrodynamic systems.

cond-mat.soft↗

arXiv2021

Relaxation and Recovery in Hydrogel Friction on Smooth Surfaces

\textbf{Background} Hydrogels are crosslinked polymer networks that can absorb and retain a large fraction of liquid. Near a critical sliding velocity, hydrogels pressed against smooth surfaces exhibit time-dependent frictional behavior occurring over multiple timescales, yet the origin of these dynamics is unresolved. \textbf{Objective} Here, we characterize this time-dependent regime and show that it is consistent with two distinct molecular processes: sliding-induced relaxation and quiescent recovery. \textbf{Methods} Our experiments use a custom pin-on-disk tribometer to examine poly(acrylic acid) hydrogels on smooth poly(methyl methacrylate) surfaces over a variety of sliding conditions, from minutes to hours. \textbf{Results} We show that at a fixed sliding velocity, the friction coefficient decays exponentially and reaches a steady-state value. The time constant associated with this decay varies exponentially with the sliding velocity, and is sensitive to any precedent frictional shearing of the interface. This process is reversible; upon cessation of sliding, the friction coefficient recovers to its original state. We also show that the initial direction of shear can be imprinted as an observable "memory", and is visible after 24 hrs of repeated frictional shearing. \textbf{Conclusions} We attribute this behavior to nanoscale extension and relaxation dynamics of the near-surface polymer network, leading to a model of frictional relaxation and recovery with two parallel timescales.

cond-mat.soft↗

arXiv2020

The origin of large amplitude oscillations of dust particles in a plasma sheath

Micron-size charged particles can be easily levitated in low-density plasma environments. At low pressures, suspended particles have been observed to spontaneously oscillate around an equilibrium position. In systems of many particles, these oscillations can catalyze a variety of nonequilibrium, collective behaviors. Here, we report spontaneous oscillations of single particles that remain stable for minutes with striking regularity in amplitude and frequency. The oscillation amplitude can also exceed 1 cm, nearly an order of magnitude larger than previously observed. Using an integrated experimental and numerical approach, we show how the motion of an individual particle can be used to extract the electrostatic force and equilibrium charge variation in the plasma sheath. Additionally, using a delayed-charging model, we are able to accurately capture the nonlinear dynamics of the particle motion, and estimate the particle's equilibrium charging time in the plasma environment.

physics.plasm-ph↗

arXiv2020

Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces

Hydrogel consists of a crosslinked polymer matrix imbibed with a solvent such as water at volume fractions that can exceed 90\%. They are important in many scientific and engineering applications due to their tunable physiochemical properties, bio-compatibility, and ultra-low friction. Their multiphase structure leads to a complex interfacial rheology, yet a detailed, microscopic understanding of hydrogel friction is still emerging. Using a custom-built tribometer, here we identify three distinct regimes of frictional behavior for polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on smooth surfaces. We find that at low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network, and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms between the gel and surface that obeys elastohydrodynamic theory. Between these regimes, the frictional force decreases by an order of magnitude and displays slow relaxation over several minutes. Our results can be interpreted as an interfacial shear thinning of the polymers with an increasing relaxation time due to the confinement of entanglements. This transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.

cond-mat.soft↗