How swimming algae enhance particle mixing in tiny fluid spaces

What is it about?

In micro-fluidic systems—such as micro-devices used for patient screening, drug discovery, and DNA analysis—mixing fluids is a major bottleneck because the tiny volumes prevent turbulent flow, leaving mixing limited to slow diffusion. Bioconvection is a phenomenon where motile micro-organisms, such as gyrotactic algae, spontaneously form large-scale fluid flows as they swim upward toward regions of downflow. In this paper, we created a mathematical model to investigate how bioconvection affects a mixture containing two different types of small solid particles suspended in a fluid chamber. Our model accounts for fluid motion, cell swimming, gravitational settling of particles, and random Brownian diffusion. Through computer simulations, we observed how the upward swimming of algae creates density instabilities that drive fluid currents. These bioconvective currents rapidly redistribute both types of solid particles, speeding up their mixing and settling compared to suspensions without swimming organisms. Furthermore, we discovered that the presence and density of one particle type directly alters the spatial distribution of the second particle type.

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Photo by masakazu sasaki on Unsplash

Photo by masakazu sasaki on Unsplash

Why is it important?

Controlling fluid mixing and particle sedimentation in micro-volumes without heavy, costly mechanical stirring equipment is a critical challenge in modern biotechnology. Our research demonstrates that self-propelled biological cells can serve as effective, natural micro-mixers to stir bidispersed solid suspensions. This offers a novel, low-energy alternative to traditional diffusion-limited mixing in lab-on-a-chip technologies. What makes this work especially timely is our key finding that introducing a second particle phase allows researchers to fine-tune the spatial distribution of primary solid particles. Because particles with different densities settle at different speeds and interact with bioconvective plumes differently, scientists can leverage these multi-particle interactions to adjust particle concentration profiles without destroying the biological convection itself. This insight provides a foundational step toward engineering controlled biological transport systems.

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