Browsing by Author "Sinha, Ashok"

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    Characterizing Magnetic Particle Transport for Microfluidic Applications
    Sinha, Ashok (Virginia Tech, 2008-09-25)
    Magnetic particles with active functional groups offer numerous advantages for use in μ-TAS (Micro Total Analytical Systems). The functional site allows chemical binding of the particle with the target species in the fluid sample. Selection of the functional group establishes the target molecule and vice versa under assumptions of highly specific biding. The particles hence act as mobile reaction substrates with high surface to volume ratios owing to their small size. The concept of action at a distance allows their use as agents for separation in microchannels based on relatively simple design. It is possible to manipulate magnetic particles and bound target species using an externally applied magnetic field. Hence, the particles can be effectively separated from the flow of a carrier fluid. Magnetic fields create dipolar interactions causing the particles to form interesting structures and aggregates. Depending upon the applied field, the microstructure evolution of the aggregate is interesting in its own right, e.g. related to improvements in material properties and bottom-up self assembly. The shape of the aggregates can be determined a priori if the interaction between the particles is well characterized. The dominant competing forces that influence magnetic particle dynamics in a flow are magnetic and viscous. There are a number of physical parameters such as viscosity, magnetic susceptibility, fluid velocity, etc. which are varied to study their individual effects. Initially dilute suspensions are studied experimentally and numerically using a particle based dynamics approach. Once established, a force model for particle interaction is investigated for concentrated suspensions. A Lagrangian particle tracking algorithm that returns positions of the particles is used for this work that focuses on studying the dynamics of these particles. A mathematical model is proposed and investigated for functionalization between magnetic and non-magnetic particles. Having characterized the collection of magnetic particles, the effect of relative concentrations is investigated on the collection of the non-magnetic species.
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    Magnetic Microsphere-Based Mixers for Microdroplets
    Roy, T.; Sinha, Ashok; Chakraborty, Shibaji; Ganguly, Ranjan; Puri, Ishwar K. (AIP Publishing, 2009-02-01)
    While droplet-based microfluidic systems have several advantages over traditional flow-through devices, achieving adequate mixing between reagents inside droplet-based reactors remains challenging. We describe an active mixing approach based on the magnetic stirring of self-assembled chains of magnetic microspheres within the droplet as these stirrers experience a rotating magnetic field. We measure the mixing of a water-soluble dye in the droplet in terms of a dimensional mixing parameter as the field-rpm, fluid viscosity, and microsphere loading are parametrically varied. These show that the mixing rate has a maximum value at a critical Mason number that depends upon the operating conditions.
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    Numerical investigation of flow-through immunoassay in a microchannel
    Sinha, Ashok; Ganguly, Ranjan; Puri, Ishwar K. (American Institute of Physics, 2010-02-01)
    Immunomagnetic separation (IMS) is a method to isolate biomaterials from a host fluid in which specifically selected antibodies attached to magnetic particles bind with their corresponding antigens on the surface of the target biological entities. A magnet separates these entities from the fluid through magnetophoresis. The method has promising applications in microscale biosensors. We develop a comprehensive model to characterize the interaction between target species and magnetic particles in microfluidic channels. The mechanics of the separation of target nonmagnetic N particles by magnetic M particles are investigated using a particle dynamics simulation. We consider both interparticle magnetic interactions and the binding of the functionalizing strands of complementary particles. The temporal growth of a particle aggregate and the relative concentrations of M and N particles are investigated under different operating conditions. A particle aggregate first grows and then exhibits periodic washaway about a quasisteady mean size. The washaway frequency and amplitude depend on the initial fractional concentration of N particles while the aggregate size scales linearly with the dipole strength and inversely with the fluid flow rate.
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    Single Magnetic Particle Dynamics in a Microchannel
    Sinha, Ashok; Ganguly, Ranjan; De, A. K.; Puri, Ishwar K. (AIP Publishing, 2007-11-01)
    Functionalized magnetic particles are used in micrototal analysis systems since they act as magnetically steered mobile substrates in microfluidic channels, and can be collected for bioanalytical processing. Here, we examine the motion of magnetic microbeads in a microfluidic flow under the influence of a nonuniform external magnetic field and characterize their collection in terms of the magnetic field strength, particle size, magnetic susceptibility, host fluid velocity and viscosity, and the characteristic length scale. We show that the collection efficiency of a magnetic collector depends upon two dimensionless numbers that compare the magnetic and particle drag forces. (c) 2007 American Institute of Physics.