UC Davis

Soft matter assemblies, such as lipid bilayer membranes, liquid crystalline multilamellar phases, and cytoskeletal protein networks, are stabilized by collective interactions near the kBT scale, rendering them sensitive to environmental perturbations. This dissertation provides experimental evidence that structural reorganization of such assemblies is not governed by the strength of the environmental perturbation but by the physical mechanism of the stressor. Across two broad categories of environmental stressors, chemical and physical, three geometrically distinct model systems are investigated to assess this claim and capture evidence.In the first system, this dissertation reviews the theoretical framework connecting molecular packing geometry to membrane morphology before developing the pH-responsive bolaamphiphile GC18:1 as an experimental case study. When GC18:1 is inserted into DOPC giant unilamellar vesicles, it converts a chemically inert host membrane into a pH-responsive material. These synthetic liposomes bud inward at mildly acidic conditions and outward at basic ones, a direct consequence of the molecule's geometry shifting with its chemical state. This establishes a theoretical and experimental foundation for this dissertation in which the character of environmental stressors, here the molecular geometry, drives membrane remodeling.In the second system, concentric cylindrical multilamellar assemblies treated as lyotropic smectic-A liquid crystals, known as myelin figures, are subjected to osmotic stresses from solutes spanning molecular weights of 92 g mol-¹ to 10000 g mol-¹, with the osmotic pressure matched across solutes by adjusting concentrations. Small-molecule osmolytes produce no sur-face instability at any pressure tested, while larger macromolecular osmolytes drive periodic axial corrugations consistent with the Helfrich-Hurault elastic instability within seconds of exposure. This result demonstrates that the determining variable is the entropic, excluded-volume character of the macromolecular stressor: depletion forces generated at inter-myelin interfaces by solute-excluded coronas couple to the smectic's elastic instability mode, driving symmetry-breaking buckling that colligatively equivalent small-molecule solutes cannot produce.Ongoing work in the third system examines whether this excluded volume mechanism generalizes beyond lipid membranes to cytoskeletal protein assemblies. TActin, one of the most abundant proteins in the eukaryotic cell, is examined within dextran-rich aqueous two-phase system droplets stabilized by small unilamellar vesicles as Pickering agents, approximating cytoplasmic macromolecular volume fractions of 20-40%. Preliminary results reveal that actin preferentially partitions into these crowded droplets and that the dextran-rich environment promotes its assembly into a filamentous state, even in the absence of conventional polymerizing buffer conditions.These results thus far suggests a novel approach for synthetic cell engineering and biophysical design.Taken all together, the completed and ongoing work supports a unifying framework in which stressor character is the primary determinant of soft matter structural response, independent of stressor magnitude. Across multiple geometries and soft matter assemblies, these findings advance a physical basis for understanding and engineering environmentally responsive soft matter systems.