Welcome to the Meyer Group at Brandeis

We study the physics of complex fluids. These are systems that are generally intermediate in their degree of ordering, between liquid and crystal states. Examples are liquid crystals, colloidal suspensions, molecular aggregates in solutions, and polymer solutions. These systems are studied by various methods, and with several goals. One goal is to understand the macroscopic properties and the fascinating phenomenology of these systems, and the other major goal is to understand how these systems order, the role of the forces that are important, and how their microscopic properties and the nature of their ordering dictate their macroscopic properties.

In the case of liquid crystals, macroscopic phenomena involving the response to electric and magnetic fields, surface orienting forces, and flow fields are especially interesting. A variety of complex effects are observed, by optical microscopy and other optical methods, and analyzed in terms of the elastic and viscous properties of the liquid crystal. In turn, these phenomena are used to measure the elastic constants, viscosities, and other basic macroscopic parameters describing the liquid crystals. Then, starting from knowledge and ideas about the molecular properties of the system under study, and how its molecules are ordered, models are developed to account for the macroscopic parameters that are measured. For example, liquid crystals based on solutions of rigid polymers are studied. The microscopic variables that are important, and can be varied experimentally, are the length of the polymer chains, their concentration in solution, and the degree of rigidity of the polymer. These all affect the macroscopic elastic and viscous coefficients of the liquid crystals, and change the nature of its response to applied external forces, such as electric and magnetic fields.

In the case of colloidal suspensions of Tobacco Mosaic Virus particles, again the interplay between microscopic and macroscopic properties is very rich. These rigid rod-like particles are stabilized in suspension in water by repulsive electrostatic interactions. The ionic strength of the solution, and the concentration of virus particles, change the nature of the ordered phases exhibited by this system. One observes isotropic liquid-like order, a nematic phase, a smectic A phase, a columnar hexagonal phase, and a crystalline phase, in all of which the virus particles constitute only a small fraction of the volume of the system. The detailed understanding of this rather simple physical system is important for deepening our general knowledge about how small particles in solution interact and spontaneously order.

For solutions of molecular aggregates, there are more complex questions concerning molecular interactions and ordering, because order exists at several scales. First, the molecules order into rod-like aggregates, due to their basic attractive interactions with one another, and their complex interaction with the water surrounding them. Then, the rod-like aggregates interact to form various liquid crystal ordered phases. Again, ionic strength, concentration of the solution, and temperature all play roles in determining the macroscopic properties of the system. In addition to the macroscopic techniques used to study the systems mentioned above, x-ray diffraction and computerized molecular modeling play a crucial role in developing an understanding of these systems.