G. K. Kurup

Field-free particle focusing in microfluidic plugs.

Particle concentration is a key unit operation in biochemical assays. Although there are many techniques for particle concentration in continuous-phase microfluidics, relatively few are available in multiphase (plug-based) microfluidics. Existing approaches generally require external electric or magnetic fields together with charged or magnetized particles. This paper reports a passive technique for particle concentration in water-in-oil plugs which relies on the interaction between particle sedimentation and the recirculating vortices inherent to plug flow in a cylindrical capillary. This interaction can be quantified using the Shields parameter ([Formula: see text]), a dimensionless ratio of a particles drag force to its gravitational force, which scales with plug velocity. Three regimes of particle behavior are identified. When [Formula: see text] is less than the movement threshold (region I), particles sediment to the bottom of the plug where the internal vortices subsequently concentrate the particles at the rear of the plug. We demonstrate highly efficient concentration (∼100%) of 38 μm glass beads in 500 μm diameter plugs traveling at velocities up to 5 mm/s. As [Formula: see text] is increased beyond the movement threshold (region II), particles are suspended in well-defined circulation zones which begin at the rear of the plug. The length of the zone scales linearly with plug velocity, and at sufficiently large [Formula: see text], it spans the length of the plug (region III). A second effect, attributed to the co-rotating vortices at the rear cap, causes particle aggregation in the cap, regardless of flow velocity. Region I is useful for concentrating/collecting particles, while the latter two are useful for mixing the beads with the solution. Therefore, the two key steps of a bead-based assay, concentration and resuspension, can be achieved simply by changing the plug velocity. By exploiting an interaction of sedimentation and recirculation unique to multiphase flow, this simple technique achieves particle concentration without on-chip components, and could therefore be applied to a range of heterogeneous screening assays in discrete nl plugs.

Shape dependent Laplace vortices in deformed liquid-liquid slug flow

Understanding the hydrodynamics of liquid-liquid slug flow is important in the emerging field of plug-based microfluidics; however, the subtle aspects of the vortex geometry are still not comprehensively understood. This paper discusses the hydrodynamics of deformation dependent vortices that develop inside a water-in-oil slug as it flows through a channel. In contrast to prior studies, our simulations and experiments on slug flow reveal multiple vortices inside the moving slug, caused by the deformation of the hemispherical caps by Laplace pressures. These vortices appear in the front and rear of the plug at capillary number between 10−4 and 10−2. A theoretical and simulation model shows the cause of asymmetry in slug deformation and the resulting vortices. Understanding the relevant parameters helps in optimizing slug flow for mixing and particle manipulation, which is important for plug-based microreactors and bead based assays.

High speed low noise multiplexed three color absorbance photometry

Multispectral photometry is often required to distinguish samples in flow injection analysis and flow cytometry; however, the cost of multiple light detectors, filters, and optical paths contribute to the high cost of multicolor and spectral detection systems. This paper describes frequency division multiplexing (FDM), a simple approach for performing multi-wavelength absorbance photometry with a single light detector and a single interrogation window. In previous efforts, modulation frequencies were <10 KHz, resulting in a detector bandwidth of <20 Hz. This paper presents a high frequency FDM circuit which can increase the oscillation frequencies to several 100 KHz, improving the detection bandwidth by a factor of 10 while still maintaining low cost. Light from 3 different LED sources are encoded into unique frequency channels, passed through the detection cell, and later demodulated using phase-sensitive electronics. Electronic multiplexing couples all light sources into a single optical train without spectral filters. Theory and high frequency considerations are demonstrated. Simultaneous three color absorbance detection is demonstrated in solutions and in flowing droplet microreactors. This technique can potentially reduce the cost of multicolor photometry by replacing expensive optical components with low-cost electronics.

Rolling, aligning, and trapping droplets on a laser beam using marangoni optofluidic tweezers

Optical methods for droplet manipulation are attractive because they offer dynamic control without on-chip structures; however, forces from optical tweezers tend to be in the pN range. We demonstrate the trapping of oil-in-water droplets on a focused laser beam via using Marangoni optofluidic tweezers (MOT). MOR exploits Marangoni flow generated by a laser-induced temperature singularity at the oil-water interface. The asymmetry of the flow aligns the droplet centroid to the laser, at which point the flow becomes symmetric and stabilizes the droplet. MOT forces are several orders of magnitude stronger (µN) than optical tweezers, evidenced by the manipulation of 20–1000 µm oil dropletsat speeds > 1 mm mm/s. The flows within the droplet can also be used for mixing and particle collection within the drop, and adjacent secondary vortices can merge droplets. The self-alignment mechanism is supported by CFD simulations, and experimental data.