Sheared bioconvection in a horizontal tube

Area of Science:

• Fluid dynamics
• Microbiology
• Bioengineering

Background:

• Microorganisms exhibit taxes (phototaxis, chemotaxis) influencing distribution.
• Cell density differences and taxes can induce hydrodynamic instabilities known as bioconvection.
• Bioconvection patterns and their interaction with imposed flow are not fully understood.

Purpose of the Study:

• To investigate how imposed fluid flow affects bioconvection patterns in Chlamydomonas suspensions.
• To quantify the impact of flow rate and cell concentration on bioconvection patterns and cell dispersion.
• To determine if bioconvection is suppressed by imposed flow and its effect on the mean flow profile.

Main Methods:

• Experimental quantification of bioconvection patterns in horizontal tubes with imposed flow.
• Use of the green biflagellate alga Chlamydomonas.
• Varying cell concentrations and flow rates across different tube diameters.

Main Results:

• Established the dependence of pattern wavelength on cell concentration without flow.
• Observed cell plumes bowing with small imposed flows.
• Documented plume fragmentation into diagonal patterns at higher flow rates, with constant angles and speeds.
• Found that pattern wavelength generally increases with flow rate, with concentration-dependent critical transitions.
• Demonstrated that bioconvection is not suppressed at high flow rates and perturbs the flow field.

Conclusions:

• Imposed flow significantly alters bioconvection patterns, leading to fragmentation and diagonal structures.
• Bioconvection remains a relevant phenomenon even under flow conditions, influencing fluid transport and flow profiles.
• The study provides novel insights into the interplay between active microbial swimming and external fluid dynamics.