Scattering Microscopy in Microfluidic Channels using an External Cavity Diode Laser

Abstract

A proof of concept system for the use of an external cavity diode laser in microscopy-based measurements of scattering particles within a microfluidic channel was demonstrated. Both a highly reflective mirror of reflectivity R = 98% and a blazed reflective diffraction grating of average efficiency 60% were used to form two variations of the external cavity. Varying concentrations of an ethylene glycol based titanium oxide nanofluid were pumped with a syringe pump through a 1µl flow cell at a rate of 0.005±0.001ml per minute. Compared to a control of the system without an extended cavity, the system with the mirror demonstrated a higher performing concentration/intensity relationship, verified with a Z-value statistic test to above a 5σ confidence level compared with the control. This allows for higher precision determination of scatterer concentration, along with potentially a higher dynamic range. The blazed diffraction grating was demonstrated to have less of a benefit than the highly reflective mirror, though still demonstrated potential use cases with a higher dynamic range than the control. Exponential fits were found using χ2 linear regression fitting, although due to reduced χ2 ≈ 350, it is assumed there is an underestimation in the error of titanium oxide concentration. These results suggest that an external cavity diode laser may be an improvement compared to a standard diode laser as a way of measuring concentration of
light-scattering particles, such as droplets in focused flow droplet microfluidics. Inline low concentration scatterer measuring techniques were described, with attempts being inconclusive due to CCD software limitations.