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Applied, Statistical, and Soft Condensed Matter Physics

Realistic physical systems are comprised of an enormous number of atoms and statistical physics must be used to understand their properties. This an interesting area of research for four reasons: (1) it studies diverse physical systems such as superconductors, liquid crystals, bio-membranes, and polymers, (2) there are many new and extremely novel theoretical ideas and approaches nowadays used in this field, (3) there are a huge number of important experiments being done, and many more that could be done, and (4) the entire field stands at the interface between physics and the fields of biology, chemistry, engineering, and materials science.

Biomacromolecular systems, such as the DNA-cationic lipid complexes used in gene therapy applications, are inherently capable of forming interesting condensed matter structures. These complexes have exemplified novel and unusual liquid crystalline states of matter. Prominent role in this area is played by the cationic-lipid membranes supported on solid surfaces, due to the fact that naturally anionic DNA adsorbed on such membranes is highly laterally mobile.

Here, we address a basic physics questions such as to what happens with the large-scale conformations of semi-flexible polymers, such as DNA, once they are adsorbed on curved surfaces, such as archetypical periodically structured surfaces? We explore the behavior of DNA molecules adsorbed on electrically charged, cationic-lipid membranes prepared on grooved, periodically structured substrates. We have revealed a striking ability of these periodically micro-structured membranes to stretch DNA coils.

We elucidated this phenomenon in terms of an interesting DNA localization transition prone to play a significant role in future biophysical and biotechnological investigations. As a contribution to an ongoing quest to unfold the coiled and therefore inaccessible state of DNA in its natural 3-d environment, our study initiates a new venue for controlling conformations of semi-flexible biopolymers by employing their interactions with specially structured biocompatible surfaces. On the practical side, in contrast to presently employed micro-fluidic methods, our new approach to stretch DNA coils avoids any use of fluid flows and the throughput limitations due to the difficulties in entering DNA into micro-fluidic channels. Indeed, the DNA can be easily brought in large amounts onto our periodic membranes. Importantly, to be stretched by our approach, DNA need not be confined into small spaces of micro-fluidic channels. Rather, the stretched DNA molecules are freely exposed to a larger surrounding water medium and all the molecules dissolved in it. Due to this feature, our new way to stretch polyelectrolytes may facilitate more direct, high throughput protocols and experimental studies of fundamental biological interactions between DNA and other bio-molecules.

We also explore fractal behavior of materials. It has by now been observed in a number of seemingly disparate growth phenomena: the advance of a fluid/fluid interface during fluid flow, a variety of schemes for material deposition, the development and growth of cracks during a fracturing process, to name a few. As the field of fractal growth phenomena matures, the emphasis is shifting from merely cataloguing the unstable growth phenomena that obey a fractal geometry towards understanding what features of this growth phenomena cause the fractal character and what modifications will lead to a smoother, compact or Euclidean growth.

Our research on two phase flow in porous media has demonstrated a crossover from fractal to Euclidean advance of the fluid/fluid interface when the ratio of the viscosities between the two fluids is decreased from infinity, so that fractal flow occurs only when the displaced fluid is infinitely more viscous than the injected fluid. Furthermore, we have determined the time scale on which this fractal to compact crossover occurs, and we have characterized the way in which this characteristic crossover time varies with the ratio of the fluid viscosities. Since this characteristic time sets the time scale for the flow, it influences the flow in the regime of compact flow by determining the viscosity-ratio dependence of the usual density and current flow descriptors.

Currently, we are studying the fractal to compact crossover for flow in three-dimensional porous media, thereby extending the previous work in two-dimensional layers. We are also studying the effect of capillary pressure on the crossover. In addition, we are investigating models of the fracture of materials to determine what features of the model systems affect the fractal character of the fracture.

Affiliated Professors
Martin Ferer
Leonardo Golubovic