Emergent oscillations and chaos in non-compliant microfluidic networks

Abstract

Incompressible fluids in microfluidic networks with non-rigid channels can exhibit flow rate oscillations analogous to electric current oscillations in RLC circuits. This is due to the elastic deformation of channel walls that can store and release fluid, as electric capacitors can store and release electric charges. This property is quantified through the compliance of the system, defined as the volume change relative to the pressure change. In systems with rigid walls and incompressible fluid, compliance vanishes and no oscillations can occur through this mechanism. Here, we show that not only oscillations but also chaos can emerge in the flow-rate dynamics of non-compliant microfluidic networks with incompressible fluid. Notably, these dynamics emerge spontaneously, even under time-independent driving pressures. The underlying mechanism is governed by the effect of fluid inertia, which becomes relevant at moderate Reynolds numbers observed in microfluidic systems exhibiting complex flow patterns. The results are established using a combination of direct numerical simulations and a reduced model derived from modal analysis. This approach enables us to determine the onset of oscillations, the associated bifurcations, the oscillation frequencies and amplitudes, and their dependence on the driving pressures. These findings can inspire novel studies and applications of previously unexplored oscillatory and chaotic regimes in non-compliant microfluidic systems.