This project investigates the dynamics of chiral particles and chiral organisms in shear flows, using a combination of microfluidic experiments and mathematical modeling. Chiral particles are particles with shapes that cannot be superimposed upon their mirror images, and are of broad interest for both basic science and technology. Chiral particles are ubiquitous in biology: for example, amino acids are chiral molecules. Since the intermolecular reactions involved in life processes depend on the geometry of participating molecules, different enantiomers of chiral drugs, pheromones, pesticides, and odorants have different biological effects. As a consequence, there is much interest in devising ways to separate pure enantiomeric samples out of a mixture of both enantiomers. Furthermore, in biology chirality occurs not only at the molecular scale, but also at the cellular scale. Many swimming microorganisms have chiral helical structures, such as the propulsion-generating flagella of most bacteria. At the same time, the watery environments of microbes are constantly subject to fluid flow, and hence shear, such as in the oceans, groundwater, industrial pipe flow, and catheters. This project has two specific aims: (i) exploiting the chirality-dependent motion of particles in shear flows to separate a mixture of enantiomers by handedness in a microfluidic channel, and (ii) understanding how chiral morphologies of microbes affect their swimming and foraging. These aims are unified by the ultimate goal of better understanding the behavior of chiral particles in shear flows, and hence amenable to the same experimental and theoretical research tools. The proposed research has the potential to (i) transform our technological capability to separate a racemic mixture into component enantiomers, by developing a simpler and less expensive separation strategy than those currently available, and (ii) improve our understanding of microbial motility, with applications ranging from the recycling of limiting elements in natural systems to the spreading of disease and biofilm formation.
The PIs investigated the interaction of chirality and flow, with applications to microbial swimming, chiral separation, and artificial microswimmers. Chiral particles are particles with shapes that cannot be superimposed upon their mirror images, and are of broad interest for both basic science and technology. Chiral particles are ubiquitous in biology: for example, amino acids are chiral molecules. Since the intermolecular reactions involved in life processes depend on the geometry of participating molecules, different enantiomers of chiral drugs, pheromones, pesticides, and odorants have different biological effects. As a consequence, there is much interest in devising ways to separate pure enantiomeric samples out of a mixture of both enantiomers. Furthermore, in biology chirality occurs not only at the molecular scale, but also at the cellular scale. Many swimming microorganisms have chiral helical structures, such as the propulsion-generating ?agella of most bacteria. At the same time, the watery environments of microbes are constantly subject to ?uid ?ow, and hence shear, such as in the oceans, groundwater, industrial pipe ?ow, and catheters. Finally, the fact that rigid bacterial flagella are chiral has led researchers to construct artificial microswimming robots that also employ chirality in order to generate propulsive flows. There are five major outcomes from the award that address intellectual merit. 1) The PIs published a paper (Marcos, Fu H.C., Powers T.R. and Stocker R., 2009, "Separation of microscale chiral object by shear flow", Phys. Rev. Lett., 102, 158103) describing the preferential drift of chiral objects in shear flows. This paper proposed that the drift could be used to separate chiral objects in microfluidic devices. 2) The PIs published a paper (Marcos, Fu H.C., Powers T.R. and Stocker R., 2012, "Bacterial rheotaxis", PNAS, 109, 4780-4785) describing a bias in swimming direction caused by the interaction of ambient shear flows with chiral flagella of swimming bacteria. 3) The PIs published a paper (Hyon Y., Marcos, Powers T.R., Stocker R. and Fu H.C., 2012b, "The fluid mechanics of wobbling bacterial trajectories", J. Fluid Mech., 705, 58-76) which describes bacterial swimming trajectories caused by asymmetrical flagellar configurations, and the implications of observed trajectories for the propulsion mechanism used by B. subtilis. 4) One of the PIs submitted a paper (Rusconi R., Guasto J.S. and Stocker R., 2013, "Hydrodynamic shear traps bacteria, hindering chemotaxis and promoting surface attachment", Nature Phys., under review) that describes how flows can lead to bacterial aggregation. 5) One of the PIs submitted a paper (Cheang U.K., Meshkati F., Kim D.H., Kim M.J., and Fu H. C., 2013, "Minimal geometric requirements for nano- and micropropulsion via rotating magnetic field", ACS Nano, under review) that describes how chirality is not required for propulsion of rigidly rotated artificial microswimmers, in surprising contrast to bacterial propulsion. Furthermore, two papers in the Annual Reviews (Guasto, Rusconi and Stocker, Ann. Rev. Fl. Mech. 2013; Rusconi, Garren and Stocker, Ann. Rev. Biophys., in review) are directly related to work performed as part of this project. There are seven major outcomes which address broader impacts. 1) The award has led to ongoing work developing a simpler and less expensive chiral separation strategy than those currently available. 2) The research supported by the award has improved our understanding of microbial motility, with applications ranging from the recycling of limiting elements in natural systems to the spreading of disease and bio?lm formation. 3) The award has supported the training of two graduate students at the University of Nevada, Reno. 4) The award has supported the training and mentorship of one postdoctoral research associate at MIT and one postdoctoral research associate at the University of Nevada, Reno. 5) The PIs have included topics related to the award in classes for undergraduate and graduate students as well as pedagogical lectures for fellow researchers. 6) The PIs have developed and deployed outreach demonstrations which illustrate the principles of microbial swimming to K-12 students and the public. 7) The PIs and the postdocs have broadly disseminated results from this award at scientific meetings in diverse disciplines, ranging from physics to engineering to biology and to oceanography, thereby contributing to the interdisciplinary impact of these findings.
