Regensburg 2022 – wissenschaftliches Programm

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DY: Fachverband Dynamik und Statistische Physik

DY 4: Wetting, Droplets and Microfluidics (joint session DY/CPP)

DY 4.3: Vortrag

Montag, 5. September 2022, 10:30–10:45, H19

Viscosity-induced Destabilization of a Liquid Sheet in Inertial Microfluidics — •Kuntal Patel and Holger Stark — Institut für Theoretische Physik, Technische Universität Berlin, Berlin, Germany

Lab-on-a-chip devices based on inertial microfluidics function between Stokes and turbulent regimes, which enables to achieve high throughput. In the present work, we study the motion of a liquid sheet of thickness t_s and viscosity µ₁ in a two-dimensional microchannel of width w, filled with a viscous fluid (viscosity µ₂, m=µ₂/µ₁>1). At finite Reynolds number Re, a small perturbation at the interfaces separating the sheet from the surrounding fluid can result in a rapid destabilization of interfaces and may lead to a break-up. The present work gives a proof-of-concept of how viscosity-driven interfacial instability can be exploited for a controlled droplet production in inertial microfluidics. Such microfluidic droplets are utilized in food and pharmaceutical related applications and as chemical reactors in Lab-on-a-chip devices.

In our computational linear stability analysis based on the Orr-Sommerfeld equation, we observe that the growth rate of the fastest growing mode ξ^* increases with Re. Furthermore, the dependence of ξ^* on m and t_s is quantified by the scaling law ξ^*∝ mt_s^2.5, which is valid for thin sheets in moderate Re flows with relatively weak interfacial tension Γ. In the second part, our lattice Boltzmann simulations starting from a single-mode perturbation of wavelength λ reveal that the interfacial instability causes the liquid sheet to break up and ultimately to form droplets when λ≥0.5w. We also identify different interface breakup mechanisms leading to droplet formation.