Enhanced diffusion and magnetophoresis of paramagnetic colloidal particles in rotating magnetic fields

Abstract

Dispersions of paramagnetic colloids can be manipulated with external magnetic fields to assemble structures via dipolar assembly and control transport via magnetophoresis. For fields held steady in time, the dispersion structure and dynamic properties are coupled. This coupling can be problematic when designing processes involving field-induced forces, as particle aggregation competes against and hinders particle transport. Time-varying fields drive dispersions out-of-equilibrium, allowing the structure and dynamics to be tuned independently. Rotating the magnetic field direction using two biaxial fields is a particularly effective mode of time-variation and has been used experimentally to enhance particle transport. Fundamental transport properties, like the diffusivity and magnetophoretic mobility, dictate dispersions' out-of-equilibrium responses to such time-varying fields, and are therefore crucial to understand to effectively design processes utilizing rotating fields. However, a systematic study of these dynamic quantities in rotating fields has not been performed. Here, we investigate the transport properties of dispersions of paramagnetic colloids in rotating magnetic fields using dynamic simulations. We find that self-diffusion of particles is enhanced in rotating fields compared to steady fields, and that the self-diffusivity in the plane of rotation reaches a maximum value at intermediate rotation frequencies that is larger than the Stokes-Einstein diffusivity of an isolated particle. We also show that, while the magnetophoretic velocity of particles through the bulk in a field gradient decreases with increasing rotation frequency, the enhanced in-plane diffusion allows for faster magnetophoretic transport through porous materials in rotating fields. We examine the effect of porous confinement on the transport properties in rotating fields and find enhanced diffusion at all pore sizes. The confined and bulk values of the transport properties are leveraged in simple models of magnetophoresis through tortuous porous media.

Removal of COD in wastewater by magnetic coagulant prepared from modified fly ash

“…Extensive research has shown that the magnetic ζ potential on the surface of magnetic powder is negatively charged ( Amiralian et al 2020; Scardigli et al 2021), the positive ions generated by the aqueous solution of coagulant gather around the negatively charged colloidal particles and magnetic powder under the effect of electric neutralization. As the static electromagnetic force disappears, the colloidal particles and magnetic powder particles are aggregated under the effect of van der Waals force, and then the ocs are further agglomerated and enlarged by the adsorption bridging effect of occulant, which nally makes the pollutants and water departed from wastewater (Lin et al 2021; Hamoud, Finqueneisel and Azambre 2017; Sherman et al 2019).…”

Clustering of self-thermophilic asymmetric dimers: the relevance of hydrodynamics

“…[1][2][3][4][5][6] A subset of active matter systems are synthetic microswimmers, which have in the recent years attracted large attention due to their potential in mimicking biological microorganisms, 7 in practical applications in fields such as microfluidics or microsurgery, 4,8 or in the development of new bio-mimetic materials. 6 Artificial microswimmers with a locomotion based on phoretic effects behave as passive colloids unless activated via thermal, [9][10][11][12][13] electric, [14][15][16][17][18] chemical, 19,20 or magnetic [21][22][23] gradients. Thermally activated phoretic microswimmers, or thermophoretic swimmers, are colloids whose surfaces are composed of or simply coated by materials with different absorption coefficients, such that local temperature gradients are created around the colloid by environmental heating.…”

Fast Ewald summation for Stokes flow with arbitrary periodicity

“…Stokes flow, also known as creeping flow or viscous flow, is a model of fluid flow in which inertial forces are assumed to be negligible in comparison to viscous forces (i.e., the Reynolds number is very small). This is often a valid assumption for phenomena involving suspension flows on the micro-and nanoscales (often in combination with Brownian motion), such as swimming microorganisms [1], cell dynamics [2], microfluidic devices [3], gels [4,5], dynamics of nanoparticles and nanofibrils [6,7,8,9], electrolytes [10], and antibodies [11]. For Stokes flow, the Navier-Stokes equations reduce to the Stokes equations, which for an incompressible Newtonian fluid are given by −∇p(x) + µ∇ 2 u(x) + g(x) = 0,…”

“…To facilitate the inclusion of Brownian motion, [27] proposed the Positively Split Ewald (PSE) method for the Rotne-Prager-Yamakawa (RPY) tensor of Stokesian dynamics, thus ensuring that both the short-range and long-range parts are symmetric positive definite. The PSE method has found much use in Brownian dynamics [28,29,30,4,7,5,11,31,2,8,9].…”