Transport and Dynamics of Swimming Microorganisms in Time-Periodic Flows

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Nontechnical Abstract: The main goal of this proposal is to understand the dynamics and behavior of motile (i.e. swimming) microorganisms in flows. Many microorganisms live and function in environments in which fluid flow is present. Examples include algae in lowland rivers and ocean, bacteria in the gut and intestines, phytoplankton in oceans, and sperm cell in human reproductive tracts. Here, the PI is interested in the transport and mixing of microorganisms in flows in order to gain insight into many poorly understood phenomena, some of which are mentioned above. From a technological point of view, motility and flow interactions are of much interest in applications that include fermentation processes for vaccine & food production, sewage treatment plants, and production of biofuels. These processes stand to greatly benefit from a better understanding of the nontrivial coupling between flow and motility. Here, the PI proposes a systematic experimental investigation on the effects of (i) flow on the transport & mixing properties of swimming microorganisms and (ii) of active stresses on the imposed 2D time-periodic flows. The research team is composed of a graduate student who is receiving training in fluid dynamics, biophysics, experimental & statistical methods, and nonlinear dynamics. The research team also includes undergraduate students, who are supervised by the PI and the graduate student. The fundamental knowledge obtained from this investigation can be useful in the development of new models for the transport and mixing of active matter and of forced active flows. Technical Abstract: The main goal of this proposal is to develop fundamental understanding on the transport, mixing, and dynamics of swimming microorganisms in flows with complex spatiotemporal structures. These processes are experimentally investigated in well-controlled flows in an electromagnetically driven thin fluid layer placed atop an array of magnets. A time-periodic current that travels horizontally through the fluid layer results in Lorenz forces that drive a (time-periodic) flow in the fluid. Spatially- and time-resolved velocity fields are obtained using particle tracking methods and differentiated to obtain the flow stretching fields or Lagrangian structures. Stretching fields are intimately related to the rate of divergence of initially nearby-points, which in chaotic flows is exponential in time (t) on the average. These stretching fields have been used to characterize the mixing dynamics, predict mixing rates, and the transport of passive impurities and particles, and are applied to study active matter (i.e. swimming microorganisms) under flow. Experimentally computed stretching fields are instrumental in understanding the Lagrangian dynamics, transport, and mixing of self-propelled microorganisms such as the bacterium V. cholerea and the alga C. reinhardtii. The knowledge obtained from the proposed work can be potentially useful for the successful design of controllable underwater autonomous vehicles (micro-swimming robots), the prevention of waterborne disease outbreaks associated with drinking water, and development of accurate models for the dispersion of planktonic matter in oceans. Using such methods, the PI hopes to address many outstanding questions such as: (i) What are the main flow parameters governing the transport and mixing of swimming microorganisms in time-periodic flows? (ii) How are the dynamics of the swimming suspension affected by flow? Does 'bacterial superfluidity' leads to enhanced transport? (iii) Do microorganisms align with regions of high stretching and unstable manifolds? (iv) Is mixing enhanced or hindered by the microorganisms' swimming action? How pullers or pushers affect the flows finite time Lyapunov exponent?