arXiv subjects

William D. Ristenpart

Publications and source records attributed to William D. Ristenpart.

5 recordsLinked to original sources

arXiv2026

Pumping and Steady Streaming driven by Two-Frequency Oscillations of a Cylinder

The classical problem of steady streaming induced by an oscillating object has been studied extensively, but prior work has focused almost exclusively on single-frequency oscillations, which result in symmetric, quadrupole-like flows. Here we demonstrate that dual-frequency oscillations induce asymmetric steady streaming with a non-zero net flux in a direction determined by the polarity of the oscillation \ -- the oscillator serves as a pump. We use numerical simulations and asymptotic analysis at low Reynolds number to examine 2D steady streaming around a cylinder, first focusing on frequency ratio two. The computational experiments show asymmetrical streaming and pumping, i.e., net flux downstream. It is well known from asymptotic analysis that steady streaming is second order in amplitude, and we show pumping occurs at third order. We then extend the analysis to general frequency ratios, where we give necessary conditions for pumping and predict the order in amplitude at which pumping occurs. Finally, we corroborate the theoretical results with computational simulations for different frequency ratios, and we discuss the implications for using dual-mode vibrations to pump fluids in lab-on-a-chip and other applications.

physics.flu-dyn↗

arXiv2022

Net motion induced by nonantiperiodic vibratory or electrophoretic excitations with zero time average

It is well established that application of an oscillatory excitation with zero time-average but temporal asymmetry can yield net drift. To date this temporal symmetry breaking and net drift has been explored primarily in the context of point particles, nonlinear optics, and quantum systems. Here, we present two new experimental systems where the impact of temporally asymmetric force excitations can be readily observed with mechanical motion of macroscopic objects: (1) solid centimeter-scale objects placed on a uniform flat surface made to vibrate laterally, and (2) charged colloidal particles in water placed between parallel electrodes with an applied oscillatory electric potential. In both cases, net motion is observed both experimentally and numerically with nonantiperiodic, two-mode, sinusoids where the frequency modes are the ratio of odd and even numbers (e.g., 2 Hz and 3 Hz). The observed direction of motion is always the same for the same applied waveform, and is readily reversed by changing the sign of the applied waveform, for example, by swapping which electrode is powered and grounded. We extend these results to other nonlinear mechanical systems, and we discuss the implications for facile control of object motion using tunable periodic driving forces.

physics.app-ph↗

arXiv2022

Controlling the direction of steady electric fields in liquid using non-antiperiodic potentials

When applying an oscillatory electric potential to an electrolyte solution, it is commonly assumed that the choice of which electrode is grounded or powered does not matter because the time-average of the electric potential is zero. Recent theoretical, numerical, and experimental work, however, has established that certain types of multimodal oscillatory potentials that are "non-antiperodic" can induce a net steady field toward either the grounded or powered electrode [Hashemi et al., Phys. Rev. E 105, 065001 (2022)]. Here, we elaborate on the nature of these steady fields through numerical and theoretical analyses of the asymmetric rectified electric field (AREF) that occurs in electrolytes where the cations and anions have different mobilities. We demonstrate that AREFs induced by a non-antiperiodic electric potential, e.g., by a two-mode waveform with modes at 2 and 3 Hz, invariably yields a steady field that is spatially dissymmetric between two parallel electrodes, such that swapping which electrode is powered changes the direction of the field. Additionally, using a perturbation expansion, we demonstrate that the dissymmetric AREF occurs due to odd nonlinear orders of the applied potential. We further generalize the theory by demonstrating that the dissymmetric field occurs for all classes of zero-time-average (no dc bias) periodic potentials, including triangular and rectangular pulses, and we discuss how these steady fields can tremendously change the interpretation, design, and applications of electrochemical and electrokinetic systems.

physics.app-ph↗

arXiv2020

The Impact of Vocalization Loudness on COVID-19 Transmission in Indoor Spaces

There have been several documented outbreaks of COVID-19 associated with vocalization, either by speech or by singing, in indoor confined spaces. Here, we model the risk of in-room airborne disease transmission via expiratory particle emission versus the average loudness of vocalization and for variable room ventilation rates. The model indicates that a 6-decibel reduction in average vocalization intensity yields a reduction in aerosol transmission probability equivalent to doubling the room ventilation rate. The results suggest that public health authorities should consider implementing "quiet zones" in high-risk indoor environments, such as hospital waiting rooms or dining facilities, to mitigate transmission of COVID-19 and other airborne respiratory diseases.

q-bio.QM↗

arXiv2010

Exploding Taylor Cones

Application of a sufficiently strong electric field to an aqueous solution induces a phenomenon known as `electrohydraulic discharge'. The electric field causes the water to break down, generating either a corona (at lower field strengths) or a pulsed arc (at higher field strengths). The discharge typically results in a complex combination of physical processes (e.g., cavitation and light emission) and chemical reactions (e.g., generation of free radicals and nonthermal plasmas). The combination of physical and chemical processes tends to destroy any organic molecules present, and accordingly electrohydraulic discharges are currently being investigated as a potentially inexpensive and environmentally friendly means for purifying drinking water and removing contaminants from wastewater. Two types of electrode configurations have been the main focus of research to date: (i) a `one-phase' system with both electrodes immersed in water, or (ii) a `two-phase' system with one electrode in air and the other submerged in water. In this fluid dynamics video, we demonstrate the striking consequences of triggering an electrohydraulic discharge in a two-phase system comprised of water and a viscous, insulating oil. An air/water interface typically remains stationary until the discharge occurs; in contrast, the oil/water interface deforms into a conical shape (i.e., a Taylor cone) stretching from the water phase toward the oil-immersed electrode. The behavior after the cone contacts the electrode depends sensitively on the properties of the water and oil, and we demonstrate that, under appropriate conditions, destructive explosions occur.

physics.flu-dyn↗