Bio-polymer networks have mechanical properties that can be remarkably different from those of most synthetic materials. One of these properties is the strain-hardening that is exhibited by most biological tissues. Strain-hardening allows tissues to be soft and malleable during small perturbations while stiffening when they are deformed beyond a certain limit. Clots, which are mostly composed of fibrin, exhibit a large strain-hardening response above ~5% strain. The modulus of the network can increase by nearly 30 times when it is compared to its value at small strains. Purely elastic gels such as cross-linked polyacrylamide do not exhibit any strain hardening. 

     We are using a combination of scattering techniques coupled to sensitive rheometers to probe the relationship between the nanometer structure of fiber networks and their mechanical properties. Furthermore, the combination of neutron and light scattering experiments allows us to probe the structure of the clots from one nanometer to several tens of micrometers. Therefore, we are sensitive to changes at all relevant lengthscales from the individual protein monomers to the whole fibrin network. More recently, we have also started to probe the effect of flow on the initial formation of the fibrin clot. This research is increasing our basic knowledge on blood coagulation while also impacting important aplications in the field of tissue and biomedical engineering (e.g. surgical adhesives and patches).

(In collaboration with Dr. Lionel Procar at NIST-NCNR)

Emulsions are found in numerous products and industrial processes. The performance of these products and processes is tied tightly to their colloidal stability so that the interfacial structure plays a critical role. In many processes, the fluid-fluid interface is stabilized by nanosized particles or proteins that can act as a physical shell preventing drop coalescence. For charged particles, the stabilization can also be a result of electrostatic interactions. Despite the industrial relevance of these systems, the physical interactions that control their phase behavior are not very well known due to the nanoscale dimensions. In this project, we utilize a combination of scattering techniques (SAXS, SANS and light scattering) to probe the organization of nanoparticles at the emulsion interface and correlate this to their interfacial behavior. This project is funded by a generous grant of the Petroleum Research Fund.

     We are also using neutron scattering techniques to improve the materials that are used for electrophoretic separations. Neutron scattering allows us to probe the structure of the polyelectrolytes (Ex. proteins or DNA) during electrophoretic transport. In particular, the use of isotope labeling allows us to explore the structure of multicomponent systems such as those typically encountered during electrophoresis. The figure above shows how various isotope compositions in the solvent and the surfactant can be used to probe the complete structure of protein-surfactant complexes in a polyacrylamide network. The unique approach that we are taking allows us to correlate directly the nanometer scale structure of the migrating molecules to the efficiency of the separation (speed and resolution). Electrophoretic separations using novel sieving matrices, surfactants and electrolytes are being explored in an effort to improve separation efficiency.

    Electrophoresis, the motion of charged particles due to an externally applied electric field, is commonly used to separate biomolecules (e.g. DNA, Proteins) from complex mixtures. Besides its paramount importance in most biological fields, electrophoresis is also used in applications for medical diagnosis and in biosensors. An efficient separation can allow the identification of disease markers before a patient even presents any symptom. Therefore, it can allow faster diagnosis and treatment.

     The flow and rheology of organized materials is intimately related to the nanometer structure. Flow can be used to disturb an organized material or to organize it even more. The figures above are for a cubic crystal of spherical polymeric micelles of ~20 nm in diameter. The micelles are initially packed into a body-centered-cubic crystal with random orientation (powder). When shear flow is applied, the sample re-orients itself and forms a structure that is more like a single-crystal than it is like a powder. As the rate of shear is increased, the diffraction pattern continues to evolve indicating the coexistence of body-centered-cubic crystals and face-centered-cubic crystals in the same sample. This is a recent example of our work on the flow of nanostructured materials.