Soft Matter Seminar: Nature and Nurture: making soft materials with biomolecules in two ways

Wednesday, May 18, 2011 - 1:15pm - 2:15pm
Reiss 261A
John Crocker
University of Pennsylvania

This talk will cover two topics highlighting the deep connections between soft matter physics and biology (i) a soft material “designed” by nature, the cytoskeleton, that provides our cells with unique properties and functionality and (ii) a colloidal crystal whose structure is nurtured by the careful design of interacting DNA sequences on their surface.

A complete mechanical picture of the cell is lacking the microscopic origins of the rheology of cell structures remain unknown due to the difficulty of developing a viable in vitro model and of studying featureless power-law rheological spectra lacking observable molecular time scales. The rheological response of cells can, however, be readily interpreted in terms of multiple mechanically distinct compartments and structures, each having a different response to pharmacological perturbation. More recently, we have begun to examine the ATP-dependent, random stress fluctuations in cells due to polymerization forces and molecular motor activity. Analyzing the stress and strain fluctuations in the deep interior of cells, across six decades of frequency, we find molecular time scales and spectral signatures consistent with the expected molecular motor activity and polar filament dynamics of a microtubule (MT) based active, polar gel.

DNA is a versatile tool for directing the controlled self-assembly of nanoscopic and microscopic objects. The interactions between microspheres due to the hybridization of DNA strands grafted to their surface have been measured and can be modeled in detail. Knowledge of the potential, in turn, enables the exploration of the complex phase diagram in simulation. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these systems readily form colloidal crystals having a range of symmetries. For interactions that favor alloying between two same-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, crystal growth kinetics and solid-solid transitions.

Host: Daniel Blair