Convective current driven by momentum transfer between magnetic nanoparticles (MNPs) and their surrounding fluid during magnetophoresis process under a low gradient magnetic field (<100 T m(-1)) is presented. This magnetophoresis induced convective flow, which imposed direct hydrodynamic effects onto the separation kinetics of the MNPs under low gradient magnetic separation (LGMS), is analogous to the natural convection found in heat transportation. Herein, we show the significance of the induced convection in controlling the transport behavior of MNPs, even at a very low particle concentration of 5 mg L(-1), and this feature can be characterized by the newly defined magnetic Grashof number. By incorporating fluid flow equations into the existing magnetophoresis model, we reveal two unique features of this convective flow associated with low gradient magnetophoresis, namely, (1) the continuous homogenization of the MNPs solution and (2) accompanying sweeping flow that accelerates the collection of MNPs. According to both simulation and experimental data, the induced convection boosts the magnetophoretic capture of MNPs by approximately 30 times compared to the situation with no convection.
The
migration process of magnetic nanoparticles and colloids in
solution under the influence of magnetic field gradients, which is
also known as magnetophoresis, is an essential step in the separation
technology used in various biomedical and engineering applications.
Many works have demonstrated that in specific situations, separation
can be performed easily with the weak magnetic field gradients created
by permanent magnets, a process known as low-gradient magnetic separation
(LGMS). Due to the level of complexity involved, it is not possible
to understand the observed kinetics of LGMS within the classical view
of magnetophoresis. Our experimental and theoretical investigations
in the last years unravelled the existence of two novel physical effects
that speed up the magnetophoresis kinetics and explain the observed
feasibility of LGMS. Those two effects are (i) cooperative magnetophoresis
(due to the cooperative motion of strongly interacting particles)
and (ii) magnetophoresis-induced convection (fluid dynamics instability
originating from inhomogeneous magnetic gradients). In this feature
article, we present a unified view of magnetophoresis based on the
extensive research done on these effects. We present the physical
basis of each effect and also propose a classification of magnetophoresis
into four distinct regimes. This classification is based on the range
of values of two dimensionless quantities, namely, aggregation parameter N* and magnetic Grashof number Gr
m, which include all of the dependency of LGMS on various physical
parameters (such as particle properties, thermodynamic parameters,
fluid properties, and magnetic field properties). This analysis provides
a holistic view of the classification of transport mechanisms in LGMS,
which could be particularly useful in the design of magnetic separators
for engineering applications.
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