publications

2023

1. Transport of a passive scalar in wide channels with surface topography: An asymptotic theory

   J V Roggeveen , H A Stone , and C Kurzthaler

   Journal of Physics: Condensed Matter, Apr 2023

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   We generalize classical dispersion theory for a passive scalar to derive an asymptotic long-time convection-diffusion equation for a solute suspended in a wide, structured channel and subject to a steady low-Reynolds-number shear flow. Our asymptotic theory relies on a domain perturbation approach for small roughness amplitudes of the channel and holds for general surface shapes expandable as a Fourier series. We determine an anisotropic dispersion tensor, which depends on the characteristic wavelengths and amplitude of the surface structure. For surfaces whose corrugations are tilted with respect to the applied flow direction, we find that dispersion along the principal direction (i.e. the principal eigenvector of the dispersion tensor) is at an angle to the main flow direction and becomes enhanced relative to classical Taylor dispersion. In contrast, dispersion perpendicular to it can decrease compared to the short-time diffusivity of the particles. Furthermore, for an arbitrary surface shape represented in terms of a Fourier decomposition, we find that each Fourier mode contributes at leading order a linearly-independent correction to the classical Taylor dispersion diffusion tensor.

2. A calibration-free model of micropipette aspiration for measuring properties of protein condensates

   James V. Roggeveen , Huan Wang , Zheng Shi , and 1 more author

   Biophysical Journal, Oct 2023

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   There is growing evidence that biological condensates, which are also referred to as membraneless organelles, and liquid-liquid phase separation play critical roles regulating many important cellular processes. Understanding the roles these condensates play in biology is predicated on understanding the material properties of these complex substances. Recently, micropipette aspiration (MPA) has been proposed as a tool to assay the viscosity and surface tension of condensates. This tool allows the measurement of both material properties in one relatively simple experiment, in contrast to many other techniques that only provide one or a ratio of parameters. While this technique has been commonly used in the literature to determine the material properties of membrane-bound objects dating back decades, the model describing the dynamics of MPA for objects with an external membrane does not correctly capture the hydrodynamics of unbounded fluids, leading to a calibration parameter several orders of magnitude larger than predicted. In this work we derive a new model for MPA of biological condensates that does not require any calibration and is consistent with the hydrodynamics of the MPA geometry. We validate the predictions of this model by conducting MPA experiments on a standard silicone oil of known material properties and are able to predict the viscosity and surface tension using MPA. Finally, we reanalyze with this new model the MPA data presented in previous works for condensates formed from LAF-1 RGG domains.

2022

1. Motion of asymmetric bodies in two-dimensional shear flow

   James V. Roggeveen , and Howard A. Stone

   Journal of Fluid Mechanics, Mar 2022

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   At low Reynolds numbers, axisymmetric ellipsoidal particles immersed in a shear flow undergo periodic tumbling motions known as Jeffery orbits, with the orbit determined by the initial orientation. Understanding this motion is important for predicting the overall dynamics of a suspension. While slender fibres may follow Jeffery orbits, many such particles in nature are neither straight nor rigid. Recent work exploring the dynamics of curved or elastic fibres have found Jeffery-like behaviour along with chaotic orbits, decaying orbital constants and cross-streamline drift. Most work focuses on particles with reflectional symmetry; we instead consider the behaviour of a composite asymmetric slender body made of two straight rods, suspended in a two-dimensional shear flow, to understand the effects of the shape on the dynamics. We find that for certain geometries the particle does not rotate and undergoes persistent drift across streamlines, the magnitude of which is consistent with other previously identified forms of cross-streamline drift. For this class of particles, such geometry-driven cross-streamline motion may be important in giving rise to dispersion in channel flows, thereby potentially enhancing mixing.

2017

1. Analysis of alternative energy harvesting methods to power atmospheric robotic explorers on Jupiter

   James Roggeveen , Adrian Stoica , and Marco Dolci

   In 2017 IEEE Aerospace Conference , Mar 2017

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   WindBots is a NIAC funded concept for a longduration atmospheric robot explorer targeting the region between 0.3 bar and 10 bar on Jupiter. The WindBot would explore this region, which extends from 15km above to 125km below the reference surface at 1 bar. Proposed WindBot mission scenarios include a glider which would use updrafts to gain altitude before gliding to find another updraft, mimicking the action of soaring birds such as frigate birds. In another mission scenario, a balloon-like WindBot would use buoyancy to maintain altitude within its operational region. An important requirement of a WindBot is survival with energy obtained in-situ to provide power for the 30-50W avionic payload envisioned for the mission. The solar intensity below Jupiter’s clouds is small and a solar solution as well as a nuclear solution have been ruled out from this study. This paper analyzes methods of energy generation and harvesting in Jupiters atmosphere and offers a comparison of the relative power densities. Wind, thermal, and magnetic energy, as well as other modalities, are sources of energy for a WindBot to transform into mechanical or electrical energy. Mechanisms evaluated include vibration harvesting and turbine technologies to utilize smallscale wind velocity gradients. Natural atmospheric thermal and pressure gradients may be tapped to provide power to a WindBot. The possibility of inductive generation using Jupiters strong magnetic field is also considered. Finally, these methods are evaluated on their power density and applicability to proposed WindBot designs.