Vijaykumar B. Varma

Spreading of a ferrofluid core in three-stream micromixer channels

Spreading of a water based ferrofluid core, cladded by a diamagnetic fluid, in three-stream micromixer channels was studied. This spreading, induced by an external magnetic field, is known as magnetofluidic spreading (MFS). MFS is useful for various novel applications where control of fluid-fluid interface is desired, such as micromixers or micro-chemical reactors. However, fundamental aspects of MFS are still unclear, and a model without correction factors is lacking. Hence, in this work, both experimental and numerical analyses were undertaken to study MFS. We show that MFS increased for higher applied magnetic fields, slower flow speed of both fluids, smaller flow rate of ferrofluid relative to cladding, and higher initial magnetic particle concentration. Spreading, mainly due to connective diffusion, was observed mostly near the channel walls. Our multi-physics model, which combines magnetic and fluidic analyses, showed, for the first time, excellent agreement between theory and experiment. These results can be useful for lab-on-a-chip devices.

Droplet Merging on a Lab-on-a-Chip Platform by Uniform Magnetic Fields.

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

Control of Ferrofluid Droplets in Microchannels by Uniform Magnetic Fields

Magnetic droplets are versatile tools for a range of lab-on-a-chip (LoC) applications. The combination of a uniform magnetic field and magnetic droplet offers wireless and programmable manipulation. We performed LoC experiments and numerical studies on ferrofluid droplets under the influence of a uniform magnetic field. The dynamic behavior of flowing ferrofluid droplets was examined. The droplet size, shape, interdroplet spacing and velocity could be controlled by tuning the magnetic susceptibility of the ferrofluid, the viscosity of the carrier medium, and the flow rates. Our droplet-based micromagnetofluidic numerical model is in good agreement with our experiments. These studies are useful for magnetic droplet control and mixing in a LoC using a uniform magnetic field.

Magnetic Droplet Merging by Hybrid Magnetic Fields

Wireless and programmable manipulation of droplets is a challenge. We addressed this challenge by a combination of magnetic fluids and hybrid magnetic fields. We investigated the remote, wireless and programmable manipulation of ferrofluid droplets in a capillary microfluidic platform by a combination of uniform and non-uniform magnetic fields. The time-dependent motion of droplets under the influence of magnetic field was studied. Actuation and inter-droplet spacing of the droplets could be controlled by tuning the magnetic field strength. The influence of viscosity on the inter-droplet spacing was investigated. The time-dependent merging of (a) ferrofluid-ferrofluid and (b) ferrofluid-rhodamine droplets was demonstrated. Simulation and experimental results are in good agreement. The present work can be used for magnetically controlled, remote, wireless and programmable droplet actuation and merging relevant to biomedical assay, cell manipulation, tissue culture, drug efficacy, and synthesis of magnet-polymer composite particles.

Tuning magnetofluidic spreading in microchannels

Magnetofluidic spreading (MFS) is a phenomenon in which a uniform magnetic field is used to induce spreading of a ferrofluid core cladded by diamagnetic fluidic streams in a three-stream channel. Applications of MFS include micromixing, cell sorting and novel microfluidic lab-on-a-chip design. However, the relative importance of the parameters which govern MFS is still unclear, leading to non-optimal control of MFS. Hence, in this work, the effect of various key parameters on MFS was experimentally and numerically studied. Our multi-physics model, which combines magnetic and fluidic analysis, showed excellent agreement between theory and experiment. It was found that spreading was mainly due to cross-sectional convection induced by magnetic forces, and can be enhanced by tuning various parameters. Smaller flow rate ratio, higher magnetic field, higher core stream or lower cladding stream dynamic viscosity, and larger magnetic particle size can increase MFS. These results can be used to tune magnetofluidic spreading in microchannels.

Instability-Induced Mixing of Ferrofluids in Uniform Magnetic Fields

The advantages of ferrofluids in microfluidic lab-on-a-chip applications include remote control of the fluid flow within the chips, e.g., mixing of the species using an external uniform magnetic field. Hence, three-stream flow systems consisting of a ferrofluid core clad by two streams of diamagnetic silicone oil were studied. The instability of the ferrofluid, subjected to an external uniform magnetic field, was also studied. When the strength of this magnetic field was increased to a critical value, the ferrofluid was spread toward the silicone oil and a transient instability developed at the ferrofluid-silicone oil interface. Further increasing magnetic field resulted in periodic instability structures and permanent instability. The effect of magnetic field strength, flow rate, and flow rate ratio were determined. With a higher flow rate ratio, the permanent instability was observed only at the larger magnetic field strength. Our modeling results were consistent with these experimental results. Our work shows that an external uniform magnetic field of only a few millitesla can lead to instability and mixing, thus it is relevant to mixing in practical microfluidic devices.

Control of Magnetofluidic Laser Scattering of Aqueous Magnetic Fluids

Optofluidics combines optics and fluid dynamics to integrate photonic capabilities with various fluidic platforms. However, fluidic control for photonic applications is a challenge. Magnetofluidic laser scattering (MFLS) offers a novel approach with wireless, programmable control. We report the control of MFLS of aqueous magnetic fluids for a range of applied magnetic fields. The MFLS was observed in the form of vertical streaks due to scattering from self-assembled ordered structures of magnetic nanoparticles. The time evolution of the streaks was investigated at different magnetic fields. The role of hydrodynamic parameters was determined by nanoparticle tracking analysis. Faster MFLS response was observed for magnetic fluids with higher susceptibility, higher drift velocities, smaller hydrodynamic diameters, and higher magnetic fields. These investigations are useful for wireless, programmable control of magnetic fluids relevant to optofluidic sensing and detection capabilities for microfluidic and lab-on-a-chip applications.

Synthesis of Silicon Nanostructures Using DC-Arc Thermal Plasma: Effect of Ambient Hydrogen on Morphology

Silicon nanoparticles (Si-NPs) were synthesized using thermal plasma assisted gas phase condensation at different compositions of argon and hydrogen. The content of hydrogen in argon was varied from 0 to 15 mole percent. Synthesized nanoparticles were characterized by Transmission Electron Microscopy (TEM) and Fourier Transform Infrared spectroscopy (FTIR). Noticeable change in the morphology of nanostructures was observed with changing hydrogen content. Si-NPs synthesized in the presence of argon consisted of flake like structures, mostly amorphous. With increase in hydrogen concentration, flake like structures disappeared and prominent spherical structures and nanowires were observed. On further increasing hydrogen content spherical crystalline nanostructures with a tail of nanowire were formed and then nanoplatelets of SiC along with silicon nanostructures were observed. Different parameters that changed owing to different hydrogen concentration, were calculated and it is attempted to predict the cause of changing morphology.

Effect of shield gas on the size distribution of aluminum nanoparticles synthesized in thermal plasma reactor

Synthesizing nano-particles with narrow size distribution in the gas phase process, via thermal plasma reactor is a challenge. The paper addresses this problem by introducing a crucial modification in thermal plasma reactor, used to produce nano-aluminum particles for propellant applications. The size distribution is assessed by transmission electron microscopy.