Paramagnetic particles and mixing in micro-scale flows

Abstract

Mixing in microscale flows with rotating chains of paramagnetic particles can be enhanced by adjusting the ratio of viscous to magnetic forces so that chains dynamically break and reform. Lattice Boltzmann (LB) simulations were used to calculate the interaction between the fluid and suspended paramagnetic particles under the influence of a rotating magnetic field. Fluid velocities obtained from the LB simulations are used to solve the advection diffusion equation for massless tracer particles. At relatively high Mason numbers, small chains result in low edge velocities, and hence mixing is slower than at other Mason numbers. At low Mason numbers, long, stable chains form and produce little mixing toward the center of the chains. A peak in mixing rate is observed when chains break and reform. The uniformity of mixing is greater at higher Mason numbers because more small chains result in a larger number of small mixing areas.

Magnetic microsphere-based mixers for microdroplets

“…5͑b͒ illustrate the competing effects of and and follow the general relation that is obtained through a rational curve fit, The mixing half-life initially decreases with increasing Ma to assume a minimum near Ma= 0.1, and then increases as the value Ma increases. Similar results have been reported by Calhoun et al 35 who determined a critical Ma= 0.12 through LB simulations, and Kang et al 34 who, through a direct numerical method, also established an optimum Ma= 0.002 ͑equivalent to Ma= 0.06 as per our definition͒, which they correlated with the occurrence of rotating and corotating flows caused by alternating chain breakup and reformations that lead to the fastest mixing. In region A of Fig.…”

“…35 Their results predicted that the mixing induced by rotating chains of magnetic microspheres depends on a dimensionless Mason number, which represents the ratio of the viscous torque to the magnetic torque. Although the structure of the predicted rotating magnetic chains has been adequately validated through experimental measurements, 30,35,36 the corresponding results for a quantitative evaluation of mixing are absent from literature. Therefore, we experimentally demonstrate the mixing enhancement in a microliter-size sessile droplet by rotating chains of magnetic microspheres and quantify the mixing rate in terms of the half-life of decay of a normalized mixing index.…”

Dynamics of magnetic chains in a shear flow under the influence of a uniform magnetic field

“…In such applications, magnetic particles are adopted as mobile substrates for bio-assays, stirring agents to achieve efficient mixing, or actuators. [3][4][5][6][7][8][9][10] Under typical operating conditions, due to small length scale, flows in these devices are laminar and inertia of both fluid and particles may be neglected.…”

Lattice Boltzmann method for microfluidics: models and applications

“…Similar phenomena has also been investigated recently by Clime et al (2009) had simulated the splitting process of a magnetic droplet with a hydrophilic magnetic plug inside in electrowettingon-dielectric devices, and the simulation results had been compared with experimental observations with excellent agreement. On the hand, the dynamics of paramagnetic particles under rotating magnetic fields in fluid has also been studied (Calhoun et al 2006;Krishnamurthy et al 2008) for its potential application in microfluidic mixing. Another interesting area that LBM methods could be useful is the electro-magneto-hydrodynamic-based microfluidic devices (Qian and Bau 2009), which involve electrical field, magnetic field, fluid mechanics, ion diffusion-convection, as well as possible heat transfer.…”