UNIVERSITY PARK, Pa. — From microscopic robots that can carry and deliver drugs inside the human body to tiny particles that can detect and break down microplastics, an emerging field called active matter is looking toward the microscale to solve some of the world's biggest problems.
Stewart Mallory, assistant professor of chemistry and chemical engineering at Penn State, leads a research group that studies active matter, specifically the collective behavior of self-propelled microscopic particles. The group's goal is to develop theoretical and computational tools to control the behavior of matter at the microscale and ultimately design new materials and devices with it.
The team published a paper today (April 22) in The Journal of Chemical Physics describing a solution to a common problem in micro-engineering, a field focused on the design and creation of tiny machines or devices, some so small they are invisible to the naked eye. Mallory spoke about his research and the field more generally in the following Q&A.
Q: What was the micro-engineering problem and what was your solution?
Mallory: A major problem with designing anything that moves, both large and small, is how is its motion altered when placed in a confined environment. In essence, we want to know if an object starts at an initial position how far will it move in a given time interval. We are interested in the problem when objects are confined to a narrow channel and unable to pass one another. If we have the time that something starts moving and we need to know how far away it will be at a later time, then we need to be able to solve this problem.
It's a really old problem in statistical physics called single file dynamics, and it actually pops up in a lot of places outside of chemistry and physics. Think about any time you're standing in line or stuck in traffic, you're not able to pass the people next to you, you're moving in single file and you're asking yourself how long it's going to take to get where you want to go — that's the problem we focused on solving.
When we're talking about small, particle-sized robots, they are going to be used in confined environments, like delivering medication into the bloodstream and different locations within the body. Before we can deploy these systems, we need to first run simulations to understand how these microscopic swimmers behave in complex environments. We have to be able to predict where they will travel and how long it will take them to get there. If they run into a single file situation, we need to be able to factor in that time, so we derived an equation that tells you that.
Q: Has this microscopic advancement changed anything about how you understand the human-scale world?
Mallory: It has definitely changed my perspective as a driver. Driving on a two lane road is a nice example of single-file dynamics as cars are unable to pass one another. If you've ever driven your car and it seems like people are stopping for no reason, which is called a "phantom traffic jam." These slowdowns in traffic emerge spontaneously and are typically caused by small fluctuations in the speed or spacing of cars that amplify over time due to human reaction times and delayed braking or acceleration. In our work on active particles moving in narrow channels, we have observed similar behavior that leads to particles clustering together and slowing down. So yes, this paper has made me think a lot more about traffic.
Q: Before tackling this single file problem, you published a paper showing a potential way to tune Phoretic Janus particles. What are they, why are they significant and why do you want to tune them?
Mallory: About 20 years ago, a team of Penn State scientists invented these self-propelled nanoparticles that they called Phoretic Janus particles. They are these tiny particles, typically micron-sized or 100 times smaller than the width of a human hair, that can propel themselves through a fluid.
Their surface is composed of two chemically distinct regions, which is why they carry the name "Janus," the Roman god of duality and transitions. That duality allows them to create and maintain chemical gradients around themselves that allows for self-propulsion. Imagine a tiny submarine with two sides, one that pushes water out and the other that pulls water in. This creates a flow that propels the submarine forward. That's similar to how these particles work. By tuning them, we can control how and where they move in response to chemical signals.
Q: Why did you want to focus on these particles, and what did you discover about them?
Mallory: First, it was interesting to me that they were discovered and designed here at Penn State and now they are studied all over the world. It also made sense to focus on them, because we're interested in particles that can essentially swim. These little microswimmers have a wide range of applications. They can be put to work within the body, for cases like targeted drug delivery, or they can clean up problems in the environment, breaking down harmful chemicals, bacteria or microplastics. They are relatively new tools, so our group is working to understand how these particles behave, how they self-propel, what kind of fuel they use and how that fuel changes their dynamics.
In general, these particles are ideal for applications where targeted, microscopic movement is needed. And unlike passive particles that rely on external forces to move, Phoretic Janus particles generate their own motion, which means we can figure out how to "drive" them by adjusting the chemical composition of the particle's two surface regions.
Q: In keeping with that "driving" metaphor, what kinds of fuel do the particles use?
Mallory: Depending on the particle's composition, different fuel sources will power their movement. For example, hydrogen peroxide can be used as fuel for particles that have a metallic region, while other enzyme-coated particles can use bio-based fuels like glucose.
But there is also interaction between particles that can affect their movement, which is why our work focuses on understanding particle behavior on two levels: individual and collective. We've talked about the individual level, where we are aiming to control and accurately simulate the behavior of single Janus particles using advanced computational methods.
On the collective level, we study how behavior changes when many particles interact, exploring the dynamics of their collective behavior. Ultimately, our goal is to integrate these approaches, developing highly accurate simulations that capture the interactions of many particles in complex systems.
Q: What are some potential applications for your research?
Mallory: I'll give you a very specific one. There are nanoparticles made of calcium carbonate that respond to pH gradients generated by cancer cells, allowing them to swim toward the cancer cells. With precise particle design, we can build what are essentially microscopic robots that can sense and move toward specific biological signals, such as the molecules emitted by cancer cells. At some point in the not-too-distant future, we could use those particles to carry a payload of medication and target harmful cells like cancer.
This concept can extend to other applications, like using particles to detect and collect microplastics, offering a potential solution for environmental cleanup.
Q: You also study material research, so how does your research apply to that field?
Mallory: This relates to the collective behavior aspect of our work. The nanoparticles are capable of self-assembly, which is the way that nature builds structures, smaller parts making bigger and bigger parts.
Our work demonstrates that self-propelled particles can enhance this process, making self-assembly a more effective tool for building at the microscale. The idea is that you can design building blocks, suspend them in a solution containing self-propelled particles, and ideally, they would spontaneously form the desired structure.
Q: What is your lab currently working on? What are your next steps?
Mallory: We are developing theories and computational modes to better understand how these particles behave in different environments, which is necessary if we want to develop microscale devices for applications like chemical and drug delivery.
The work we're doing with Janus particles contributes to a broader field of study focused on systems composed of self-propelled particles and their collective behavior, so the advances in discoveries that we make in our group will have impacts across the entire field of active matter. Any step we make is a step forward in understanding and manipulating matter at the microscale.
