Picoliter agar droplet breakup in microfluidics meets microbiology application: numerical and experimental approaches.

Droplet microfluidics has provided lab-on-a-chip platforms with the capability of bacteria encapsulation in biomaterials, controlled culture environments, and live monitoring of growth and proliferation. The droplets are mainly generated from biomaterials with temperature dependent gelation behavior which necessitates stable and size-controlled droplet formation within microfluidics. Here, the biomaterial is agar hydrogel with a non-Newtonian response at operating temperatures below 40 °C, the upper-temperature threshold for cells and pathogens. The size of the produced droplets and the formation regimes are examined when the agar is injected at a constant temperature of 37 °C with agar concentrations of 0.5%, 1%, and 2% and different flow rate ratios of the dispersed phase to the continuous phase (φ: 0.1 to 1). The numerical simulations show that φ and the capillary number (Ca) are the key parameters controlling the agar droplet size and formation regime, from dripping to jetting. Also, increasing the agar concentration produces smaller droplets. The simulation data were validated against experimental agar droplet generation and transport in microfluidics. This work helps to understand the physics of droplet generation in droplet microfluidic systems operating with non-Newtonian fluids. Pathogenic bacteria were successfully cultured and monitored in high resolution in agar droplets for further research in antibiotic susceptibility testing in bacteremia and urinary tract infection.

Dynamics of temperature-actuated droplets within microfluidics.

Characterizing the thermal behavior of dispersed droplets within microfluidic channels is crucial for different applications in lab-on-a-chip. In this paper, the physics of droplets volume during their transport over a heater is studied experimentally and numerically. The response of droplets to external heating is examined at temperature ranges of 25-90 °C and at different flow rates of the dispersed phase respect to the continuous flow. The results present a reliable prediction of the droplet volume and stability when heating is applied to the droplets at the downstream channel in a quite far distance from the droplets' ejection orifice. Increasing the ratio of flow rate resulted in larger droplets; for instance, the flow ratio of 0.25 produced drops with 40% larger diameter than the flow rate of 0.1. For every 10 °C increase in temperature of the droplets, the droplet diameter increased by about 5.7% and 4.2% for pure oil and oil with a surfactant, respectively. Also, the droplets showed a degree of instability during their transport over the heater at higher temperatures. Adding SPAN 20 surfactant improved the stability of the droplets at temperatures higher than 60 °C. The experimentally validated numerical model helped for systemic analysis of the influence of key temperature-dependence parameters (e.g. surface tension, density and viscosity of both phases) on controlling the volume and stability of droplets. Our findings supported to develop highly functional systems with a predetermined droplets performance under high temperatures up to 90 °C. This report provides a preliminary basis for enhancing the performance of droplet microfluidic systems for digital droplet polymerase chain reaction (ddPCR), continuous flow digital loop-mediated isothermal PCR (LAMP), and droplet-based antibiotic susceptibility testing.