Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor(s)

Jennifer Schwarz

Second Advisor

Shikha Nangia

Keywords

droplet networks, mechanics, membranes, programmability, shape, soft matter

Subject Categories

Physical Sciences and Mathematics

Abstract

This thesis analyzes three different soft matter systems---membranes, polymers, and droplets---to answer questions about shape, mechanics, and programmability.

For membranes, my collaborators and I have developed a theoretical model of endocytosis in yeast. Endocytosis is the process by which a cell membrane deforms to surround extracellular material to draw it into the cell. Endocytosis in yeast involves clathrin, actin, and Bar proteins. Our model breaks up the process into three stages: (i) initiation, where clathrin interacts with the cell membrane via adaptor proteins, (ii) elongation, where the membrane is then further deformed by polymerizing actin filaments, followed by (iii) pinch-off. Our results suggest that the pinch-off mechanism may be assisted by a pearling-like instability. In addition, we potentially rule out two of the three competing models for the organization of the actin filament network during the elongation stage. For polymers, the actin cytoskeleton network at the leading edge of the cell becomes anisotropic with filament alignment favoring the direction of motion of the cell. To begin to capture the mechanics of this anisotropic filament network, my collaborators and I have constructed an effective medium (mean field) theory of an anisotropic, disordered spring network. We find that increasing the anisotropy increases the filament density required for a nonzero shear modulus (rigidity). We also conduct numerical simulations and find good agreement with the effective medium theory. We then extend our analysis to include the mechanics of coupled disordered spring networks to study force transmission between the actin cytoskeletal network and DNA via the lamin filament network and potentially begin to establish a microscopic basis for the mechanical regulation of transcription via the actin cytoskeleton. For droplets, we study numerically a collection of aqueous droplets joined by single lipid bilayers to form a cohesive, tissue-like material. The droplets in these droplet networks can be programmed with different osmolarity gradients. These osmolarity gradients generate internal stresses via local flows and the network then folds into designed structures. In other words, global change is driven by local osmolarity gradients. Using molecular dynamics simulations, we study the formation of shapes ranging from rings to spirals to tetrahedra and determining the optimal range of parameters for such structures. By adding an osmotic interaction with a dynamic environment, a folding-unfolding process can also be realized. This latter result is a step towards osmotic robotics.

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Open Access

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