Carnegie Mellon University
February 15, 2012

RESEARCH: Soft Particle Suspensions

RESEARCH: Soft Particle Suspensions

While engineers often take a top-down approach to understanding materials, starting from the large-scale macroscopic material properties, there are others, like, Craig Maloney, assistant professor of Civil and Environmental Engineering, who start from the bottom up, trying to predict macroscopic material response from the microscopic makeup of the material.

One class of materials that Professor Maloney studies is soft particle suspensions. The soft particles can either be droplets of one fluid suspended in another – as in an emulsion – or solid particles suspended in fluid – as in an industrial paste or slurry. Soft particle suspensions are used every day in industries ranging from agriculture to construction, yet there is surprisingly little fundamental understanding of their behavior.  This lack of fundamental understanding makes designing products with tailored properties – such as the viscosity or yielding strength of the suspension – very challenging.

Maloney is studying these suspensions using computer simulations that operate at the level of individual particles.  These simulations require specialized computing hardware and can involve several millions of particles at a time.  The patterns that emerge in the motion of the particles impact the macroscopic properties – similar in some respects to the complex patterns that arise in turbulence.

One of the most important microscopic parameters governing the macroscopic behavior is the density of particles.  At low densities, these suspensions act like simple fluids and flow through a pipe even at very tiny driving pressure.  At higher densities the particles start to crowd each other out, the viscosity becomes larger and larger, and, finally, the suspension develops all the typical properties of a solid: the flow will stop, and the suspension becomes elastically rigid. 

“Imagine that you pack a crate tightly with grapefruits.  If you shake the crate, the grapefruits aren’t going to budge.   Now, suppose they suddenly turn into oranges.  If you keep shaking the box, the oranges are going to fly all over the place” Maloney explains. “By being able to understand and control this remarkable rigidity transition, similar to melting, we can predict and manipulate the behavior of these industrially important suspensions.” 

Professor Maloney was recently awarded a National Science Foundation Career Award that will provide additional funding for his work with these soft particle suspensions. In addition to continuing his research Maloney will also support additional educational activities by developing undergraduate and graduate level coursework to look at these theoretical models. Lecture modules will relate modern viewpoints to traditional engineering theories creating a “virtual laboratory” using interactive software in lieu of a physical laboratory. 

Maloney will also work with the Pittsburgh Supercomputing Center (PSC) to provide outreach for local secondary school instructors. Graphically-based modeling tools will train the teachers to not only use the simulation modules as classroom learning tools but to teach them enough about these simple modeling tools to modify existing simulations and even design their own from scratch.  A new program at PSC is currently being developed which will focus on Pittsburgh high schools, including the new science academy.

Please visit Professor Maloney's website for more information about his research.

Materials, Mechanics and Computing research group