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Research
Surface interactions and control of microbial locomotion Interactions between swimming cells and surfaces are essential to many microbiological processes, from the formation of biofilms to the fertilization of human egg cells. Until recently, however, relatively little was known about the physical mechanisms that govern the scattering of flagellated or ciliated microorganisms from solid surfaces. A better understanding of cell-surface interactions not only promises new biological insights but may also advance microfluidic techniques for controlling microbial locomotion, with potential applications in diagnostics, therapeutic protein synthesis and photosynthetic biofuel production. Our recent work shows that the surface scattering of mammalian spermatozoa and unicellular green algae is dominated by direct ciliary contact interactions. Building on this finding, we were able to construct optimal microfluidic ratchets that maximize rectification of initially uniform algae suspensions. Since mechano-elastic properties of flagella and cilia are highly conserved across species, similar results can be expected for a wide range of swimming microorganisms.
Self-propulsion and collective swimming of microorganisms Bacteria and algae reach respectable swimming speeds of a few times their body length per second. Even more remarkably, ensembles of microorganisms exhibit complex collective behavior and can form coherent structures like turbulent vortices, spirals or bionematic jets. The characteristic length scales of these patterns may exceed the size of an individual organism by several orders in magnitude. Dynamical structure formation in bacterial systems emerges due to a combination of environmental factors (e.g., varying nutrient resources or oxygen gradients), biological competition, chemical communication (deposition and detection of messenger substances), and physical interactions. Part of our research focusses on identifying and understanding physical processes that may trigger collective dynamics in microbial suspensions. We are interested in questions such as: How do individual bacteria and microalgae affect their fluid environment? Which generic or specific mechanisms are responsible and/or necessary for the collective behavior of these microorganisms? What role did hydrodynamic effects play in the evolution from unicellular to multicellular forms of life? How can collective motions be suppressed or enhanced by external manipulation? Some answers can be found here:
Asexual reproduction and regeneration in multicellular organisms Asexual reproduction by fission or budding is a characteristic feature of bacteria and single cell eukaryotes, such as yeast or amoeba. Higher multicellular organisms usually reproduce sexually, because they lack the regenerative capabilities required for asexual reproduction. Exceptions are hydras and planarians (flatworms) which can reproduce both sexually and asexually. Hydras are relatively primitive organisms composed of only a very small number of cell types. By contrast, planarians are bilaterally symmetric animals, possess all three germ layers, a complex central nervous system and many of their genes can also be found in humans. Planarians exhibit amazing regenerative abilities, facilitated by stem cells that are distributed throughout their bodies. These stem cells not only enable the worms to heal without scarring after wounding, but also allow for asexual reproduction: In the course of a fission cycle, planarians can split in two or more pieces and subsequently regenerate the missing body parts within a few days. In collaboration with the Collins lab, we are studying internal and external factors that can affect the fission and population dynamics in the asexual freshwater planarian species Schmidtea mediterranea.
Relativistic diffusion and thermodynamics Einstein's theory of relativity assumes that particles cannot move faster than the speed of light. Standard descriptions of diffusion and Brownian motion processes are in conflict with this postulate. The latter fact is rather unproblematic in most terrestrial applications, but it may lead to inconsistencies in, for example, astrophysical applications if one wants to describe the quasi-random motion of particles in very hot plasmas. In recent years, we have studied different approaches towards formulating diffusion processes in a relativistically consistent manner. A closely related problem concerns the relativistic generalization of thermodynamics. Einstein's theory predicts that observers who are in relative motion measure different length and time intervals. Historically, there has been some debate as to whether or not this also applies to thermodynamics quantities. The problem can be traced to the fact that thermodynamics deals with extended systems which need to be handled with care in relativity. Recently, we proposed a resolution of several conceptual difficulties by introducing definitions of thermodynamic quantities that are guided by photographic measurements.
Efficient Monte Carlo methods for financial risk measures The recent crisis in the global financial markets demands a critical review of current regulatory practice. Substantial efforts are required to devise efficient quantitative methods for a more reliable estimation of financial risks in the future. Unlike the currently used industry standards for risk evaluation, these tools must be able to detect extreme loss scenarios that are unlikely to occur but whose impact may be dramatic. In collaboration with Stefan Weber, we have developed a new Monte-Carlo technique for the efficient estimation of improved risk measures that are sensitive to the tails of loss distributions.
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