Suspended particles are integral part of many systems and engineering technologies. They can be found in the form of colloidal suspensions, emulsions, polymer precursor solutions, and in biological materials such as blood. The miniaturization of new technologies and the advent of microfludics has made the manipulation of suspended particles in the microscale particularly important for a variety of fields. The ability to easily impart complex chemical environments to suspensions in microfluidic devices enables us to characterize these systems, modify their properties and drive their motion. Nonetheless, precise manipulation of the chemistry surrounding suspended particles has been particularly difficult up until recently. This thesis dissertation shows how microfluidic devices integrated with hydrogel membranes can be used to control the chemical environment of suspended particles for a variety of studies and practical applications.
First, I demonstrate how particles move diffusiophoretically under ionic surfactant gradients. Diffusiophoresis, the motion of particles under concentration gradients, has been known for several decades but it has rarely been studied experimentally outside the context of simple electrolytes. Here, we show that diffusiophoresis in ionic surfactants below the CMC can be understood in terms of the classic theory for electrolytes. Above the CMC, however, the drive for diffsuiophoresis is significantly diminished due to a large drop in the change in chemical potential with added solute.
Next, I show that gradients of dipolar molecules such a zwitterions can drive diffusiophoresis. I derive the diffusiophoretic migration of particles under gradients of dipolar molecules. This theory is backed up by experiments which reveal that, in such systems, particle velocities are directly proportional to the imposed gradient but do not scale with the inverse of the local concentration, as occurs under electrolyte gradients. Furthermore, I show that the diffusiophoretic velocity in zwitterions scales with the square of the intercharge distance.
Finally, I demonstrate further applications of our hydrogel membrane-integrated devices by showcasing several case studies of unique experiments using our technique. I show diffusiophoresis under previously untested solutes such as butanol, acids, glycerol, and sucrose. I demonstrate a proof-of-principle experiment for colloidal tagging in microfluidic devices and for the study of chemotaxis. Lastly, I examine AC electrophoresis in chemically manipulated environments and I show the ability of our device to perform electrophoretic measurements in spatially homogeneous and time-evolving systems.
|Advisor:||Squires, Todd M.|
|Commitee:||Gordon, Michael, Leal, Gary, Pennathur, Sumita|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 78/12(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Physics, Nanotechnology|
|Keywords:||Colloids, Diffusiophoresis, Electrokinetics, Electrophoresis, Microfluidics, Surfactants|
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