Dynamics of magnetic modulation of ferrofluid droplets for digital microfluidic applications

Abstract

Active control of droplet generation in a microfluidic platform attracts interest for development of digital microfluidic devices ranging from biosensors to micro-reactors to point-of-care diagnostic devices. The present paper characterises, through an unsteady three-dimensional Volume of Fluid (VOF) simulation, the active control of ferrofluid droplet generation in a microfluidic T-junction in presence of a non-uniform magnetic field created by an external magnetic dipole. Two distinctly different positions of the dipole were considered-one upstream of the junction and one downstream of the junction. While keeping the ferrofluid flow rate fixed, a parametric variation of the continuous phase capillary number, dipole strength, and dipole position was carried out. Differences in the flow behaviour in terms of dripping or jetting and the droplet characteristics in terms of droplet formation time period and droplet size were studied. The existence of a threshold dipole strength was identified, below which the magnetic force was not able to influence the flow behaviour. It was also observed that, for dipoles placed upstream, droplet formation was suppressed at some higher dipole strengths, and this value was found to increase with increasing capillary number. Droplet time period was also found to increase with increasing dipole strength, along with droplet size, i.e. an increase in droplet volume.

Two-Step Numerical Approach To Predict Ferrofluid Droplet Generation and Manipulation inside Multilaminar Flow Chambers

“…8, which has been successfully employed in the past for describing the magnetization of magnetic particles, and it is similar to other models employed in the literature for describing ferrofluids. [18][19][20] In this case, the magnetic properties of the DP (χd=6.79 and Ms,d= 5.25•10 4…”

“…Different authors have addressed the modeling of droplet generation, mostly by passive control by using different flow rates for the continuous phase (CP) and dispersed phase (DP), 15,16 but active ferrofluid droplet formation has been reported as well. 17,18 Numerical studies have also been developed for studying magnetic droplet splitting or breakup at T-junctions, 19 including magnetic droplet transport on a digital microfluidic platform. 20 Still, a significant part of numerical studies on ferrofluid droplets are devoted to the investigation of the motion and deformation of a single droplet under the influence of magnetic fields [21][22][23][24][25][26][27][28] or electric fields.…”

“…36,37 Fluid configurations are defined in terms of a VOF function, F, which satisfies the following equation: 38 The Journal of Physical Chemistry tracking the F function through the entire flow domain. 18 The fluid velocity field is governed by the mass continuity and momentum equations, which are solved under the assumption that the fluids are incompressible, i.e. :…”

Flexible Ferrofluids: Design and Applications

“…The flow rates of fluids as well as sizes and features of droplets are controlled in a consecutive integral way, while the manipulation of discrete droplets could hardly be achieved. In this system, ferrofluids could serve as either a dispersed phase or a continuous phase . For example, as a dispersed phase, ferrofluids experience both magnetoviscous effect and magnetic drag effect, which impact the formation of ferrofluid droplets and are determined by relative flow rates and intensity of external magnetic fields; as a continuous phase, ferrofluids could facilitate fabrication of polymer droplets, adjust the droplet shape, and aggregate droplets into chains with certain length, relying on the magnetic buoyancy force and dipole–dipole interactions produced by a designed magnetic field .…”

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

“…During the squeezing stage, the curvature of the growing droplet at the neck is gradually decreased as the tip of the water thread penetrates further into the main channel, leading to increases in the pressure P d . 67 Then it shows a peak because of droplet breakup. Generally, P d is always higher than P c at the same timescale as expected.…”