Master of Science
Civil and Environmental Engineering
Despite our current understanding of active fluid mechanics, more knowledge is required to fully understand their rheological properties, response to light, and details of the interactions between cells in diluted suspensions.
The focus of this thesis is on investigating the rheological properties of suspensions of cyanobacterium Synechocystis sp. CPCC 534 under shear, induced by stirring. Experiments were conducted at different stirring rates and results are compared with the stationary condition. A notable increase in growth and biomass of Synechocystis sp. under various shear conditions was observed and the yield was nearly doubled. Besides, the data showed Newtonian behavior for suspensions at different cell concentrations. Although cell concentration displayed a perceptible rise in the viscosity of suspensions, this rise is smaller than the one predicted for suspensions of hard spheres.
Moreover, experiments were carried out on diluted suspension of Synechocystis sp. CPCC 534 in a microfluidic chamber to investigate the response of cells to the light and cell-cell interactions. The cells were exposed to the light of the laser pointer and a white LED. Cell tracking was performed by image processing in Python programing language. It was observed that cells display an alternating intermittent motion that consists of high motility “run phases” and immobile “tumble phases”. Comparing the probability distributions of the ensembled-averaged mean square displacement (EMSD), and velocity variations with time, revealed that cells detect light and spend more time running with high-speed motility under white LED light, while spending more time tumbling when exposed to laser pointer light. Moreover, the frequency of cell-cell interactions shows that exposure to any light sources decreases the number of interactions significantly. The preliminary design and experimental setup of a multichannel microfluidic device are provided as well.
Summary for Lay Audience
Active fluid is a fluid whose constituent elements can self-propel for instance, suspensions of bacteria, algae, and sperm cells. Active fluids display properties that differ fundamentally from conventional (passive) fluids. Contrary to conventional fluid flows in which one needs gradients of pressure, velocity, and temperature to drive the flow, in active fluids, cells as the microstructural elements of the fluid convert the chemical energy of nutrients into mechanical energy for driving the flow. It is essential to investigate the hydrodynamics of microorganisms due to their direct and indirect effects on human lives, the environment, and also its vast applications in industrial, medical, food, and energy fields such as harmful algal blooming, photobioreactors (PBR), biofuel production, water treatment industry, food hygiene, etc.
Experiments were conducted on the suspensions of cyanobacterium Synechocystis sp. under shear stress to investigate their rheological properties. A notable increase in growth and biomass production of Synechocystis sp. under various shear conditions was observed. Moreover, the data showed Newtonian behavior for suspensions at different cell concentrations. Besides, experiments were performed on cells exposed to the laser pointer light and white LED to study their response to light and their interactions. Cell tracking revealed that the intermittent motility of cells consists of high motility “run phases” and immobile “tumble phase”. Statistical analysis on displacement and velocity indicates that cells detect light and spend more time running with high-speed motility under white LED light, while spending more time tumbling when exposed to the laser pointer light. Moreover, the frequency of cell-cell interactions shows that exposure to light decreases the number of interactions. The preliminary design and experimental setup of a multichannel microfluidic device are provided as well.
Habibi, Zahra, "Active fluids: rheological properties and response to light of suspensions of Synechocystis sp. CPCC 534" (2021). Electronic Thesis and Dissertation Repository. 8321.