Carol Hirschmugl uncovers the invisible world where surfaces meet
Imagine watching a heated basketball game. Now, step outside the arena and try to describe what's happening without the benefit of seeing or hearing it unfold. It would seem like an impossible task.
But it's similar to what physicist Carol Hirschmugl does every day in her research into the invisible worlds of surface science and biophysics. Instead of following the athletes and the ball, though, Hirschmugl, an associate professor of physics, tracks what happens to molecules when they meet the surface of a particular material or move around in a living cell.
An object's molecules and electrons are always in motion, vibrating and wiggling. Hirschmugl's novel imaging methods take advantage of these vibrations to investigate very small particles—and map the movement of chemicals within them.
She hopes her work will lead to new ways of addressing environmental pollution. Hirschmugl starts this long process by tracking molecular changes that occur inside a cell when it comes in contact with pollution or when toxic substances touch soil and water.
Beyond the microscope
But before she can witness any action, she has to detect all the parts involved. Using a device called a synchrotron, Hirschmugl can probe what she could not with a normal microscope. The synchrotron emits energy at all spectral frequencies, from infrared (IR) to X-rays. In IR, which is what Hirschmugl uses, the light it gives off is brighter than the sun, although it is not visible with the human eye.
IR reveals the unique vibrations of specific molecules within a living cell, which act as "signatures," allowing Hirschmugl to identify the material she's working with. She is using the technique to observe how algae digest carbon dioxide (and give off oxygen), something that has implications for controlling air pollution.
In her work with algae, she studies the distribution of proteins, lipids, and carbohydrates, molecules that play a major role in metabolizing the organism's food (photosynthesis). It's important in fully understanding a process that is vitally linked to human respiration and environmental health.
Recently funded by $1 million grant from the National Science Foundation, Hirschmugl will be developing new ways to "see" how alga reacts to its environment.
"Since the alga uses up a lot of CO2," she says, "we're interested in what happens when you change its environmental conditions. We want to look at how its biological makeup changes when exposed to say, runoff pollution.
"I'm taking the question one step further and seeing how the distribution of its parts changes because of interactions with contaminants like nitrates or ammonium, which come from fertilizer runoff or sewage."
Her ultimate goal is to be able to measure changes in the distribution of carbohydrates, lipids, and proteins with a living alga sample to observe the internal changes actually taking place.
Electrons behaving madly
In a second imaging project, Hirschmugl tracks the arrangement of specific molecules on a solid surface, again enlisting the wave properties of electrons.
"What we are looking at is way smaller than the wavelength of light," says UWM physicist Dilano Saldin, who collaborates with Hirschmugl. "So we need to study the energy distribution from electrons scattered from the surface."
The technique Hirschmugl uses is a modified method of low-energy electron diffraction (LEED). By shooting a minute beam of electrons onto a surface, and using a sensitive detection plate, she creates a visual picture of the electrons as they are spread out in all directions and eventually hit the plate. After sophisticated analysis, the resulting pattern can reveal the structure of the surface material.
Why go to all this trouble? One of the challenges of science today is to reveal the workings of the atomic world, says Saldin, whose expertise includes the interpretation of the patterns made by the scattered electrons.
Most interactions of a solid with its environment take place at the surface. Forces can influence the surface that won't affect the interior, causing unusual atomic rearrangement that changes the way a material behaves. This kind of transformation is behind the process of corrosion in metals, for example.
The goal of these surface studies is to examine the behavior of water molecules when they come in contact with an oxide surface, the dynamics of which are not well understood, but could be valuable in predicting groundwater propagation and determining how contaminants flow through soil.
Driving Hirschmugl's inquiry is the fact that water and an oxide surface (like soil) meet in unpredictable ways.
The interactions between water and an oxide will vary, she says, depending on which atoms are touching the oxide surface—and whether it makes contact with the oxygen atom, or one or both of the hydrogen atoms.
"Water and soil present a really different interface," she says as if describing a good novel. "I want to know what happens next. Do the water molecules break down or do they remain intact?
"With these techniques, we're getting access to the dynamics of the molecules and the statics (location) at the same time."