Post-doctoral Position Available

Inquiries regarding this position can be sent to muller2<at>

Research Interests In the Muller Lab

Flow Dynamics of Mixed Suspensions

The flow dynamics of mixed vesicle/particle suspensions are still largely unexplored. A better understanding of how vesicles and other microparticles behave in different flow regimes will provide the foundation for more accurate drug delivery designs and better suspension flow models. This project aims to study both the individual dynamics as well as the collective interactions of vesicles, capsules, and capsule/particle mixtures. Single vesicle responses to different flow regimes will be tested using the microfluidic four-roll mill. New microchannel designs are being developed to test the collective dynamics of suspensions of vesicles.

Figure: (Left) Flow focusing device by rapid microfluidic prototyping. (Center) Measuring channel profiles. (Right) Droplet formation.


Figure: Break up of a
vesicle in planar
extension over time

Suspensions of blood exhibit margination under specific flow conditions. The driving forces behind this migration behavior are not yet well understood. Vesicle suspensions can serve as a model system for red blood cells while allowing for the potential to adjust different suspension parameters such as particle size, membrane deformability, or suspension volume fraction. These vesicle suspensions can be observed under different flow conditions to elucidate the mechanism by which margination occurs. Unilammelar vesicles are generated using electroformation. The polydisperse suspensions are separated by size using microfluidic devices. The separated vesicle suspensions are used in channel flow experiments for the migration study.

Figure: (Left) Electroformed vesicles. (Center) Inertial separation device. (Right) Filter separation device.


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Flow of Biological Fluids in Micro-Fluidic Devices

Microfabrication processes enable the design and manufacture of systems on the microscale. Such microsystems are capable of processing and analyzing biochemical samples and offer multiple advantages compared to conventional systems and protocols. Scaling down the system and increasing the number of samples processed per chip would result in heightened efficiency and reduced cost.

Figure: Microfluidic Device Characterization

The realization of these benefits, however, requires understanding fundamental physical principles on this scale. Any change in conformation or stability accompanying the processing of a macromolecule in a microsystem must either parallel that which occurs during conventional laboratory testing, be benign to the end analysis, or be accounted for in the interpretation of data. Toward this goal, this work seeks to characterize the behavior of biological macromolecules during flow through a microdevice and to identify the critical parameters influencing this behavior. We perform DPIV analysis on the flow of complex biological fluids such as   Figure: Schematic of Micro Check Valve
DNA solutions in microchannels and other microscale
geometries to obtain the velocity profiles. From these
profiles we may learn how to optimize the design of
microfluidic devices

Figure: Composite image of streamlines. Discontinuities do not reflect variations in depth.



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Stagnation point studies for single-molecule genotyping

The device depicted at right combines a flow focusing element to reduce reagent volume and a cross slot, which provides a stagnation point flow in which DNA may be trapped and extended. Such a device may be used to study the effect of extension on binding or to determine long-range sequence patterns on DNA.  

Markers must be created to bind to specific DNA sequences and to allow observation using fluorescence microscopy. DNA-binding probes are biotinylated and attached to 40nm fluorescent Neutravidin-coated beads. (seen below-left)  

Glass slides are modified to promote the stretching of DNA on the surface. This permits confirmation of the binding specificity of the markers. (seen below-right) We have also observed binding on stretched DNA in flow. (not shown)

Figure: Functional DNA-binding markers (left) and Stretching hybrids on surfaces (right).


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Taylor-Couette Instabilities

The effect of a little elasticity on Newtonian flows can be profound. Consider drag reduction: trace quantities of polymer can result in much lower pressure drops (and pumping costs) relative to Newtonian fluids. We are studying the effects of trace amounts of polymer not on pipe flow, where the flow transition is from laminar to a fully turbulent flow, but in Taylor-Couette flow (flow between concentric cylinders).  Here transitions from the base flow occur through a rich series of flows of increasing complexity (seen right). 

Instabilities occur in Newtonian fluids in the Taylor Couette flow due to centrifugal or inertial forces.  The stability of these flow states varies under the influence of elasticity forces.  These changes are captured using flow visualization and dynamic frequency analysis.  For example, the plots below show a stabilization of temporal frequencies at high Re as the elastic forces increase.

Figure: Elasticity effects on Taylor-Couette stability boundaries.

To view some additional images and QuickTime movies of the Taylor-Couette instabilities, please click here.

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Dynamics of Micro-scale Objects

Figure: Microfluidic four roll mill device

The microfluidic four-roll mill, which allows continuous changing of the flow type in a device, is useful for controlling the conformation of DNA molecules in MEMS device. Similar to drop deformation studies (e.g. Bentley and Leal, J. Fluid Mech. 1986), the microfluidic four-roll mill device can be used to study vesicle deformation under varied flow type.

Figure: Flow type parameter.





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This page last updated 09/11/2013 by Kari