Prof. Walz is the Chairman of the Department of Chemical Engineering

Prediction and Measurement of Surface Forces
Our research is aimed at understanding the various forces that can act between colloidal particles in solution and how these forces (such as electrostatic interaction between two electrically charged surfaces and the ubiquitous van der Waals interaction) can alter the stability of a colloidal dispersion. We use the optical technique of total internal reflection microscopy (TIRM) to measure the interaction energy profile between a single, freely-moving, microscopic particle and a planar interface.

We are investigating:  

The effect of nonadsorbing polyelectrolytes on colloidal stability.

It has long been known that a nonadsorbing polymer added to an otherwise stable dispersion of colloidal particles can cause the particles to reversibly flocculate. The attractive force, depletion attraction, between the particles results from the exclusion of polymer from the gap region at sufficiently low particle separation distances. We focus on predicting the effect of nonadsorbing polyelectrolytes (charged polymers, micelles, small charged particles) on colloidal stability. Predictions made using a force balance model indicate that not only is the magnitude of the interaction energy greatly increased by the presence of charge but the interaction is no longer purely attractive and can actually become oscillatory at larger separation distances. Experiments performed with TIRM as well as traditional stability measurements have confirmed these results.

 

The effect of nonadsorbing polyelectrolytes on the dynamics of particle interaction.

In addition to altering the interaction forces between particles, the presence of nonadsorbing polymers (either charged or uncharged) can also affect the dynamics of particle interactions at close separations. Consider, for example, two particles whose gap width (distance of closest separation) is less than the characteristic size of the free polymer in solution. If the gap width increases slightly, then the liquid medium must flow into the gap region without the polymer. We are currently collaborating with Prof. Jerzy Blawzdziewicz in the Department of Mechanical Engineering to develop a thorough understanding of this effect. Our particular focus will be on direct experimental measurements of the dynamics of motion for a single particle located very close to a solid interface.

 

The interaction forces between Cryptosporidium parvum oocysts and mineral surfaces.

Cryptosporidium parvum is a single-cell protozoan that can produce severe intestinal problems in humans. The organism is commonly found in dairy run-off and is thus frequently found in surface waters. In the natural environment, C. parvum produces a hard outer covering, termed an oocyst, that makes the organism resistant to typical drinking water disinfection methods like chlorination. The most common removal method is thus physical filtration. Our project, which is supported by the U.S. Department of Agriculture, is aimed at understanding the interaction force between C. parvum oocysts and mineral surfaces such as those found in porous soil. Since many people around the world, including in the U.S., drink untreated drinking water (in 1993 in Milwaukee, WI, untreated drinking water resulted in over 100 deaths and thousands of illnesseses), understanding these interactions and being able to predict how these oocysts to travel through underground aquifers is of global major importance.

 

Development of a new experimental tool for measuring colloidal forces.

Perhaps the most commonly used experimental technique for directly measuring colloidal forces on individual particles is the atomic force microscope (AFM). In this approach, a single colloidal particle is glued to the AFM cantilever and the interaction force between this particle and a flat surface is measured by detecting the deflection of the cantilever, which can be treated as a simple linear spring. One of the limitations with this technique, however, is accurately determining the particle-surface separation distance.

We are collaboring with Prof. William Ducker, one of the pioneers in measuring forces using the AFM and currently in the chemistry department at Virginia Tech, to improve the method for measuring forces using the AFM. In our new method, termed the Colloidal Force Microscope, the particle-surface separation distance will be measured using the scattering from an evanescent surface wave, similar to the approach used in the technique of total internal reflection microscopy (TIRM).

The John Walz Group

 

Selected Publications

"Atomic Force Microscopy Colloid-Probe Measurements with Explicit Measurement of Particle-Solid Separation," S.C. Clark, J.Y. Walz, and W.A. Ducker, Langmuir, 20, 7616 (2004).

"The Stucturing of Nonadsorbed Nanoparticles and Polyelectrolyte Chains in the Gap between a Colloidal Particle and Plate," M. Piech and J.Y. Walz, Journal Physical Chemistry B, 108, 9177 (2004).

"Direct Measurement of Depletion and Structural Forces in Polydisperse, Charged Systems," M. Piech and J.Y. Walz, J. Colloid Interface Sci., 253, 117 (2002).

"A Model for Calculating Electrostatic Interactions between Colloidal Particles of Arbitrary Surface Topology," Ning Sun and J.Y. Walz, J. Colloid Interface Sci., 234, 90-105 (2001).