What started as a question to the A-exam became a revolutionary discovery in the material science field. Neil Y.C. Lin — a graduate student from Cornell University pursuing a physics Ph.D. — was asked if it was possible to measure forces at the single particle scale, given that the current methods can only measure said forces at bulk scale (in groups) on his A-exam. A-exam is an exam where Ph.D. candidates must attempt to answer questions that not even the professors know the answers to. This question led Lin to work with Prof. Itai Cohen, physics, Prof. James Sethna, physics, Matthew Bierbaum grad and Prof. Peter Schall, physics, University of Amsterdam.
After three years of research, this team of theorists, computer calibrators, and crystal specialists found their answer in SALSA — not the tasty dip, but Stress Assessment from Local Structure Anisotropy.
Most materials have complex force distributions within them that determine their overall mechanical properties. The force distributions are complex, because almost all materials have defects. In the case of crystals, even though there is a periodic arrangement of particles, chances are you can find some areas that have a broken symmetry or another imperfection. Thus, measuring such forces, though made difficult by methods currently available, is crucial to understanding how the bulk mechanics emerge.
A major advancement in the field, SALSA, allows researchers to measure 3D force fields in a colloidal suspension system. Colloids act like atoms, but are 106 times larger than atoms, making them easier to see than actual atoms. For comparison, if an atom was the size of a dodo bird, a colloid would be about the size of Italy. Therefore, a colloidal suspension system provides a model for how atoms behave.
“The challenge in the field is yes, we can see atom-like particles, but we still don’t know how to measure the stress or the force within the suspension,” Lin said.
This challenge stems from the fact that you need to know first, how the particles interact, and second where the particles are located in order to determine the force. Conventional methods can measure how the particles interact, but measuring where the particles are located is far from simple. Lin explained that because the forces between particles are often strong, a slight shift in their locations by the disturbance of experimentation can generate a lot of uncertainties, and those uncertainties lead to major errors in the final calculations of force.
SALSA provides a solution to this dilemma. Instead of calculating the exact moment the particles interact, according to Lin, SALSA looks at how many times the particles collide over a given time, otherweise known as collision probability.
“This gives us a much more robust way of approximating the stresses between particles,” Lin said.
SALSA essentially transforms the microscope into a pressure gauge. The microscope is typically used to image the colloidal suspension structure, but now, with SALSA, images can indicate how the forces are distributed.
Before SALSA, a mechanical probe such as a spring was used to gauge stress, pressure, or any other force. Probes fail to resolve individual particles without causing a disturbance. SALSA, on the other hand, is a noninvasive way of measuring the force of a system, and so looks at the system as is, with no disturbances to the system that a probe introduces. This method provides a look at the true, instantaneous state of the system, which is important to determining the true source of a material’s mechanical properties. In addition, with SALSA, the resolution of the force is not limited by probe size, or is the resolution limited to the surface of the material.
“As long as you can resolve individual particles, you can measure the force at the single-particle scale,” Lin said.
Not only can SALSA resolve particles and the stress at the single-particle scale, but it can also resolve the distribution in three dimensions.
“This is actually the first time you can see a stress distribution in 3D in any kind of material,” Lin said.
SALSA opens the door to answering a of the fundamental material science questions. By measuring the force evolution, SALSA has the potential to answer questions such as: What mechanisms underlie cracking and fatigue? How do crystals flow and when do they break down? With SALSA, researchers can identify the driving force behind a phenomenon at a single-particle scale.
“The ultimate hope is that these forces give you the precursor or help you to predict the behavior of materials,” Lin said.