This dissertation explores two independent projects in biophysics. Part one focuses on the liquid crystalline phase transition of F-actin. The cytoskeletal protein actin can be found in both an unordered isotropic state and an aligned nematic state, and plays an important role in cell structure and motility. The phase transition between these states is predicted to be a first order transition, yet previous results have found it to be continuous. We show here that the transition is truly first order under the proper conditions of high concentration and short average filament lengths. Under this regime, the solution spontaneously separates into ordered domains in the shape of tactoidal droplets. We explore the formation and growth mechanisms of the tactoids and find that the system is only metastable. These findings help clarify the phase separation and co-existence of actin, and may be of relevance to its biological functions.

Part two examines the interaction of leukocytes with their substrate. We use polyacrylamide gels to explore the role of the mechanical environment in determining neutrophil behavior. We find that neutrophils spread more and migrate slower but more persistently on stiff substrates. Inhibition of phosphatidylinositol 3-kinase (PI3K) removes this ability of the neutrophil to sense the substrate stiffness, suggesting a role for PI3K in the mechanism responsible for neutrophil mechanosensing. Furthermore, we find that neutrophil force generation is strain limited and independent of substrate stiffness within a limited range. The greatest forces are found in the posterior of the cell, suggesting that the neutrophil pushes, rather than pulls itself forward. Finally, we use polarized T lymphocytes observed under total internal reflection fluorescence (TIRF) microscopy to explore the localization of integrins, the molecules responsible for adhesion to the extracellular matrix. Active integrins are found localized in the anterior of the cell during migration. These findings elucidate the many complex interactions between the cell and substrate, which drive leukocyte behavior. Future studies based on these findings may lead to insights concerning the physiological functions of leukocytes, and in particular how these functions are regulated by the mechanical properties of the surrounding tissues.