Speaker
Description
While external flow control is straightforward to implement in microfluidics, it is fundamentally limited in its ability to generate spatially localized flow patterns or complex flow topologies. Programming localized, non-trivial flow patterns remains challenging, which limits the ability to mix local samples or to transport fluid between regions of microfluidic devices.
Here, we introduce a new approach to generate programmable and localized flow around micro-patterned features in microfluidic chips. Our PDMS device consists of a thin and wide microfluidic channel flanked by two air chambers [1]. When vacuum is applied to the air chambers, the ceiling of the microfluidic channel deforms downward, displacing the liquid inside. In parallel, hydrogel structures that span the height of the channel are photo-patterned within the channel with controlled geometry and horizontal extension.
The device is operated by actuating each of the chambers independently, applying oscillating pressure sequences that are out of phase with each other. This forcing breaks the temporal and spatial symmetry of the flow in the microfluidic channel and produces a net fluid flux. The resulting flow pattern depends on the frequency of the pressure sequences and their amplitude, in addition to the hydrogel shape.
The flow production is investigated for a cylindrical hydrogel at $\text{Re} = 10^{-4}$ by performing PIV of the flow field averaged over one period of the forcing. The vertical ceiling displacement generates a circumferential net flow localized around the hydrogel cylinder, with mean velocities in the range of $10$-$50$ µm/s. The phase difference between the actuators provides direct control over both flow direction and magnitude. The latter is maximal when the chambers are actuated with a phase delay of $\pi/2$. The opposite delay reverses the rotation direction, while delays of $0$ or $\pi$ produce no net flow. Flow velocity can be further tuned by adjusting the actuation frequency or amplitude, or by modifying the hydraulic resistance at the channel inlet and outlet.
This phenomenon represents a form of flow rectifier in which the hydrogel structures convert sequential ceiling deformations into rotational flow, through the asymmetric resistance created by the oscillating geometry of the microfluidic chip. It can be applied to generate local flow patterning within a wide channel, without any moving parts in contact with the fluid. The principle may be extended to more complex flow patterns by varying further the shape or number of the features, or by adding additional air chambers with different phase differences, which opens new possibilities in reconfigurable microfluidic flow control.
[1] S. Jain, H. Belkadi, A. Michaut, S. Sart, J. Gros, M. Genet and C. N. Baroud, “Using a micro-device with a deformable ceiling to probe stiffness heterogeneities within 3D cell aggregates”, Biofabrication 16 035010, 2024.