David A. Weitz Professor Experimental Soft Condensed Matter Group Department of Physics Harvard University

David Weitz’s group studies the physics of soft condensed matter, materials which are easily deformed by external stresses, electric, magnetic or gravitational fields, or even by thermal fluctuations. These materials typically possess structures which are much larger than atomic or molecular scales; the structure and dynamics at the mesoscopic scales determine macroscopic physical properties. The goal of this research is to probe and understand the relationship between mesoscopic structure and bulk properties. We study both synthetic and biological materials; our interests extend from fundamental physics to technological applications, from basic materials questions to specific biological problems. The techniques we use include video image analysis, light scattering, optical microscopy, rheology, and laser tweezing. They also often require new experimental techniques; we pioneer the use of multiple scattered waves to study dynamics and mechanical properties of materials.

Problems of current interest include the study of the growth and dynamics of colloidal crystals, binary alloys and glasses; light scattering and confocal microscopy are combined to probe both structure and dynamics of these materials. We also investigate the structure and properties of gels formed with attractive interactions, and search for the universal behavior that can describe such disordered structures. This work is motivated both by fundamental interest and important applications of these systems to create novel structures and to model the properties of individual materials. The Weitz group is developing new techniques to probe mechanical properties of soft materials, combining direct measurements of the properties with analysis of the thermal motion of microscopic probes within the materials. This microrheology will allow us to directly measure the properties of biological systems such as local regions within single cells.

Another area of interest is the engineering of novel structures made from colloids, polymers or lipids that can be used to encapsulate materials. The goal is to create structures whose properties can be controlled and whose structure can be tuned by engineering the components and the fabrication methods. The group studies the mechanical properties and investigates methods to improve the use of these structures as encapsulation media. These structures are also used to mimic fundamental building block of cells, and the group seeks to use these constructs to build artificial cells, and to model mechanical forces within cells.

The Weitz group also studies the properties of suspensions of particles in a fluid, where the gravitational sedimentation is balanced by an upwards flow of the liquid; these fluidized beds are important technologically, but also exhibit complex behavior and pattern formation.

Speaking in the special symposia on Soft Nanotechnologies & Applications and Nanoparticles in Soft Materials - Colloidal Systems.