Talks

1. The Mechanics of Semiflexible Networks: Implications for the Cytoskeleton (PowerPoint and Webpage)

Semiflexible polymer networks are constructed of stiff filaments that are densely cross-linked on the scale of their thermal persistence length. Due to this microscale architecture, these gels can store elastic energy through both the bending and the stretching of their constituent filaments. In contrast, traditional rubber elasticity theory assumes that all elastic stresses are stored in the stretching of filaments. Such assumptions are reasonable when describing the mechanics of natural rubber or gels that are the product of modern synthetic chemistry. The cytoskeleton, on the other hand, is constructed of extremely stiff protein filaments that are densely cross-linked. This biogel is the source of morphological control and force generation in all eukaryotic cells and is the prototypical semiflexible network. Understanding its mechanical properties requires new ideas.  In this talk I discuss the highly nonlinear dependence of the mechanics of this system on network density. In particular I show that there is a abrupt change in the mode of elastic energy storage, the mechanics of this material, and in the geometry of its strain field at a particular network density. We call this the affine-to-nonaffine transition. After discussing our basic understanding of the affine-to-nonaffine transition in semiflexible networks, I then present some data on F-actin gel rheology from Dave Weitz and Margaret Gardel that qualitatively supports the existence of this cross-over. I conclude by discussing briefly the implications of this work for the linear and nonlinear rheology of the cytoskeleton.

2. Sailing the surfactant sea: Dynamics of rigid and flexible bodies in interfaces and membranes (PowerPoint and Webpage)

Here I consider the hydrodynamics in membranes and at fluid/fluid interfaces. Normal hydrodynamics is a scale-free theory, but, as first noted by Saffmann and Delbruck, membrane hydrodynamics is controlled by new length scale in the system given by the ratio of the two-dimensional membrane viscosity to the usual three-dimensional bulk viscosity of the surrounding fluid. In this talk, I discuss the implications of this new length in the hydrodynamic mobilities of extended and flexible objects. These results have been experimentally tested by the groups of Dinsmore (UMASS) and Weeks (Emory); I present some of their data. Finally, I discuss some newer work done in collaboration with Mark Henle on membrane hydrodynamics on curved surfaces.

3. The worm turns: The mechanics of alpha helical polypeptides and the helix-coil transition on the worm-like chain (PowerPoint and Webpage)

What are the mechanical properties of a single alpha-helical polypeptide?  Can one develop a meaningful coarse-grained theory of protein mechanics? In this talk I explore the highly nonlinear elastic properties a polypeptide with alpha-helical secondary structure using a simple model. This model consists of a set of tangent vectors to describe the spatial conformations of the polymer chain and a second set of Ising-type variables to describe the local degree of secondary structure. These two sets of degrees of freedom interact through the bending stiffness (persistence length) of the chain. Where the molecule is ordered in its native state (alpha-helix) it is stiff to bending and has a significantly longer thermal persistence length  than it does in its disordered state. Using this model we calculate the nanomechanics of a single alpha helix. Because of this coupling of internal degrees of freedom to the conformational degrees of freedom of the polymer backbone, an alpha helix is highly nonlinear in its response to torques and forces. For example, under torque the molecule undergoes a buckling transition associated with a localized break in the hydrogen-bonding pattern in its secondary structure. This nonlinearity may explain how proteins are able to undergo controlled, reproducible conformational changes. We briefly consider ongoing work where we seek to apply these ideas to conformational changes in calmodulin. 

4. Static and flowing wet sand: Dragging Mr. Bagnold through the mud (PowerPoint and Webpage)

How does physical sand differ from the sand typically considered by physicists?  In both a geophysical context and from the point of view of processing technologies small amounts of a wetting fluid lead to interparticle attractions. In this talk I discuss a theory developed with Dr. Thomas Halsey (Exxon Mobile) for the dependence of the stability of static sand piles upon both the surface roughness of the particles and the volume of wetting fluid present in the system. This work is strongly supported by a series of experiments by Professor Thomas Mason (UCLA). Finally, I will discuss our more recent work on the dynamics of cohesive sand in the flowing state. Here we find a breakdown of the usual Bagnold constitutive law and the appearance of plug flow near a free surface. I also discuss new attempts to develop a microscopic model for the breakdown of Bagnold scaling.