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. We also develop new techniques to study these materials; we pioneered the use of multiple scattered waves to study dynamics and mechanical properties of materials, and applied these optical methods to measure the rheological properties of materials in what is now called microrheology. There are currently three major research themes in the group. The first is colloid physics, which includes the study of colloidal particles as models systems for the study of materials. We investigatge the growth and dynamics of colloidal crystals, glasses and gels; light scattering and confocal microscopy are combined to probe both structure and dynamics of these materials. We investigate the structure and properties of glasses and 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 materials. A second major theme is biophysics. We focus on the mechanical properties of cells, and develop new methods to probe these properties. We use purified proteins to form reconstituted biopolymer networks, and study rheological properties of these systems. By incorporating molecular motors into these networks, we study the properties of highly non-equilibrium, active materials, and develop models to describe the properties of such materials. We apply the insight gained from these model systems to investigate the mechanical behavior of cells. The third major theme is microfluidics, where our focus is on the use of multiphase fluid flow to create, control and use emulsions with microfluidic devices. We engineer novel structures made from colloids, polymers or lipids that can be used to encapsulate materials. These structures are formed from multiple emulsions which are controllably produced with microfluidic devices. In addition, we are developing microfluidic devices that use individual droplets as microreactors, with volumes as small as femtoliters. The precise control over these droplets afforded by the microfluidic devices enables us to use them as reaction vessels for very high throughput studies of biological reactions. We are developing methods to improve the functionality of enzymes through directed evolution of their function, and to study the properties of large populations of individual cells encapsulated within the droplets. Our work spans fundamental science to direct applications. Many projects are carried out in collaboration with industry; we do the fundamental science that also addresses important technological questions. In addition, some of our work is spun out into start-up companies, providing alternate career opportunities.