11/25/2012 21:05:01

## Seeing the World in a Grain of Sand

Watson Lecture Preview

José Andrade has got the dirt on dirt. An associate professor of civil and mechanical engineering at Caltech, Andrade will discuss how the actions of a few grains of sand can affect landslides, earthquakes, and even Mars rovers. He will be speaking at 8:00 p.m. on Wednesday, November 28, 2012, in Caltech's Beckman Auditorium. Admission is free.

Q: What do you do?

A: I study the behavior of granular materials. These can be many things: granular materials are the second-most-manipulated materials on Earth, after water. Some of the best examples are soils, like sand; rocks; and construction materials, like concrete. I make computational models that try to capture the behavior of these materials—for instance, to simulate landslides, or the beginning of an earthquake, or the interaction of a rover wheel and martian soil.

Q: Why is this cool?

A: It's cool because these materials are very complex, although they look very simple and innocent on a small scale. When you look at sand, the grains interacting with each other seem very simple. But their behavior as a bulk material is very complex. So even though at a fundamental level they may be governed by an innocent-looking equation—let's say F = ma, or force equals mass times acceleration—once they get together in a landslide it's not that simple. This dichotomy is really attractive to me, and leads to some really cool work in terms of modeling. You have millions of grains in an avalanche, so we want to keep the F = ma for each grain but somehow do the calculations at a coarser scale. The challenge is to capture the essence of the physics without the complexity of applying it to each grain in order to devise models that work at the landslide level.

Q: How did you get into this line of work?

A: I took apart a lot of stuff as a kid, to the great displeasure of my parents. Doorknobs were a big specialty for me. I was fairly successful reassembling them, but I also used to take apart clocks. They were amazing devices, but most of the time I couldn't put them back together. And then I got into radios, and then TVs. I usually failed to put them back together to their pristine state, and that's where I used to get in trouble. One of my big hobbies was to open cassettes—remember them?—and make mix tapes. I was fairly successful cutting up and reassembling tapes and being able to play them.

In college I started out as a civil engineer, a structural engineer, but I developed a passion for mechanics, which is the relationship between forces and deformations. And in grad school I discovered my passion for geologic materials, for granular materials, and for modeling. But it was still geared toward engineering—I just went from thinking about what's on top to looking at the things underneath. And that's a cool, and at the same time a sad part about what we do: when we're doing a good job, nothing interesting happens. As one of my colleagues says, our best work is underneath the building, where nobody sees it. It's only when things fall apart . . .

It wasn't until I came to Caltech that I started to move toward the science side—landslides, planetary science, rovers crawling around. I never thought 10 years ago I'd be doing work on Mars. Not at all.

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.

Written by Douglas Smith