Dr. Piotr Habdas

We are dragging a probe particle through gels probing locally gel network strength and promoting particle migration and strand breakage. We want to experimentally establish the connection between the dynamics of individual particles participating in the gel structure and their local connectivity. We trust this will provide information about the structural origin of dynamical heterogeneity in colloidal gels and the link between structure and complex dynamics of colloidal suspensions with short range interparticle attraction at a range of volume fractions.

If smaller particles are added to the hard-sphere colloidal suspension, a short-range inter-particle attraction is turned on between the large particles. Exclusion of the small particles from the region between the surfaces of two nearby particles creates an unbalanced osmotic pressure pushing the particles together. This is known as the "depletion attraction". The range and the depth of the attraction potential can be varied independently by changing the size and the number of the small particles. Since the "sticky" hard-sphere system takes into account attraction it is a more truthful model of molecular glasses than the hard-sphere system. We are using various microscopy techniques to observe the behavior of the colloidal particles and thus gain more insight into the nautre of the glass transition.

A colloidal suspension, a model system in which micrometer sized particles are suspended in a liquid, exhibits the glass transition and can be studied using microscopy. Spherical colloidal particles have been used in the past, however molecules in nature are mostly anisotropic. Thus, we are using ellipsoidal colloidal particles to study particle dynamics in dense colloidal glasses.

We are testing the effectiveness of a new photosensitizing drug, (4-pyridyl) porphyrin (4PP). Currently in the United States, hospitals that offer Photodynamic Therapy use the photosensitizing drug, Photofrin. Both Photofrin and 4PP are free-base porphyrin complexes that differ only differ in their hydrophobic and hydrophilic side chains. Porphyrin complexes are very effective photosensitizers for Photodynamic Therapy for several reasons: they have intense absorption in the visible light region, their aromatic structure creates a very stable complex, and they generally have low toxicity in the dark. Despite these advantages, the commercially used drug, Photofrin, leaves patients photosensitize for up to six weeks after the procedure. The aim of my project is to determine if 4PP can be just as effective as Photofrin but at a lesser concentration. This way, less drug is administered to the patient, thus shortening recovery time.

Non-Newtonian liquids are something that everyone is familiar with. Some examples of non-Newtonian liquids are paint, creams, ketchup and mud. In general, the viscosity and thus the behavior of non-Newtonian liquids changes with mixing rate. Water is a prime example of a Newtonian liquid; its viscosity doesn't change no matter how fast you mix it. We dragged spheres of various size through tubes of different sizes filled with a non-Newtonian liquid. We measured the drag force on the spheres moving with different velocities and thus determined indirectly when the tube walls start affecting the motion of the sphere.

Some of the projects in our lab have an educational aspect. Students who are planning to become physics teachers are especially interested in such projects. We are designing various experiments that introduce non-Newtonian liquids and can be used in introductory physics laboratories.