Control and synchronization of capillary flows in stepped microchannels

TL;DR

This paper presents a combined experimental, theoretical, and simulation study on controlling and synchronizing capillary flows in microchannels with geometric steps and offsets, enabling passive flow regulation.

Contribution

It introduces a novel geometric control method using steps and offsets to passively switch and synchronize capillary flows in microchannels.

Findings

Reversible switching between pinned and flowing states achieved.

A scaling model explains flow transitions based on surface tension and pressure.

Synchronization of flows in parallel channels demonstrated.

Abstract

Capillary-driven transport offers a simple, self-sustained alternative to externally pumped microfluidic systems, yet achieving precise control of such flows remains challenging. We experimentally and theoretically investigate capillary flow in rectangular microchannels when a liquid meniscus encounters a geometric step with varying channel width and height. Depending on the contact angle and step dimensions, the meniscus either pins or advances, defining distinct flow and no-flow regimes. Introducing a lateral offset between channel sidewalls provides an additional geometric control that enables reversible switching from a pinned to a flowing state, even for liquids with relatively high contact angles. We rationalize these transitions using numerical simulations and an energy-based scaling model that captures the balance between surface tension and Laplace pressure at the step. Finally, we demonstrate synchronization of capillary fronts in parallel microchannels by combining stepped and offset geometries. These results establish a simple, geometry-mediated mechanism for passive control and synchronization of capillary flows, expanding the design space for microfluidic systems.