Blood Flow in the Microcirculation
The behavior of blood cells in the circulation is complex, and although many important phenomena have been observed, the principles that underlie and couple these phenomena remain poorly understood. Many serious medical conditions, from hemorrhage to coronary artery disease to diabetes, are associated with disruptions in blood flow. It has been observed that dramatic beneficial effects on blood flow arise from addition to blood of low concentrations of long-chain polymer molecules known as drag-reducing additives (DRAs). These polymers are so named because of their drag-reducing effects on turbulent flow, but since blood flow is not for the most part turbulent, the effects of these polymers on blood flow must have a separate origin from turbulent drag reduction. We aim to gain a fundamental understanding of the effects of DRAs on blood flow in the microcirculation. Aside from its direct application to the issue of drag reducing additives in blood, this work forms the foundation for studying other important issues regarding blood flow in the microcirculation, including effects of RBC shape and deformability on blood flow and the in vivo dynamics of blood-borne drug delivery vehicles including liposomes, polymersomes and filomicelles. Our studies are enabled by the development of efficient methods for simulating the dynamics of confined multiphase flows.
Locomotion of Microorganisms
Fluid mechanics plays an important role in many aspects of locomotion in microorganisms. For example, many microorganisms propel themselves though their fluid environment by means of multiple rotating flagella that self-assemble to form flagellar bundles. This self-assembly process is complex and poorly understood. At a larger scale, the fluid motions generated by individual microbes as they swim affect the motions of their neighbors. Experimental observations indicate the presence of long-range order and enhanced transport in dense populations of bacteria -- these phenomena may be important in many aspects of bacterial dynamics including chemotaxis and development of biofilms.
We are developing mathematical models, computational methods and theoretical frameworks that allow treatment of the solid and fluid mechanics of multiflagellar bacteria at the single and multiple organism levels. These approaches are being used to understand the flagellar locomotion process as well as the intercellular hydrodynamic interactions that lead to collective phenomena in bacterial populations.
Many researchers are working toward the capability to manufacture microscopic artificial swimming machines. Therefore, understanding microbial locomotion is important both for its fundamental impact on biology and also because this understanding will guide the design of nanofabricated swimmers -- it is important to know whether evolution has found solutions that technology should emulate.
Turbulence and Drag Reduction in Simple and Complex Fluids
At low speed, flow in a pipe or over an aircraft is smooth and steady. At higher speeds, flow becomes turbulent—the smooth motion gives way to fluctuating eddies that sap the fluid’s energy and make it more difficult to pump the fluid through the tube or to propel the aircraft through the air. For flowing liquids, adding a small amount of very large polymer molecules can dramatically affect the turbulent eddies, reducing their deleterious effects on energy efficiency. This phenomenon is used, for example, in the Alaska pipeline, but it is not well understood, and no comparable technology exists to reduce turbulent energy consumption in flows of gases, in which polymers cannot be dissolved.
We aim to gain a fundamental understanding of the near-wall turbulent dynamics underlying the turbulent drag reduction phenomenon. Recent work indicates the presence of two kinds of turbulence in flows of solutions containing polymer additives: “active” turbulence, which dominates flows without additives and leads to substantial energy consumption, and “hibernating” turbulence, which drains much less energy from the fluid. Hibernating turbulence is prevalent at high levels of additives, but still occurs occasionally in their absence. Current research aims at gaining further understanding of the interaction of polymer dynamcis and turbulence as well as developement of flow control strategies to coax Newtonian turbulent flows into the hibernation state and thus reduce drag in the absence of additives. This work has a potential for broad impact on the degree of energy consumption (friction drag) in many aerodynamically important flow processes including flow over wings, rotor blades and fuselages.
Dynamics of Confined DNA in Flow
In modern genomics applications, individual molecules of DNA are manipulated not in bulk solution but in microfluidic devices whose dimensions approach those of the molecule. In collaboration with the Schwartz and de Pablo groups at UW-Madison, we have been working to understand and predict the dynamics of confined DNA in nonequilbrium situations such as flow or electrophoresis. We have developed a coarse-grained bead-spring chain model of DNA in bulk solution that, by including hydrodynamic interactions between chain segments, quantitatively captures diffusion, relaxation and conformations of this long molecule in ﬂow. This model has been incorporated into highly efficient algorithms that uniquely combine Brownian dynamics and computational fluid dynamics methods to simulate the dynamics ofsolutions of DNA molecules in these micron scale geometries. With such algorithms, the effect of conﬁnement and ﬂow on the conformations and transport of individual conﬁned polymer chains in solution can be studied. In related work, the combined impact of tethering and confinement has been studied, showing the existence of multiple metastable conformations that can act as switches or valves.