ILMA Compoundings – November 2018 - 12

INDUSTRY RUNDOWN

Solidifying Long-Awaited
Answers
Researchers settle long-standing
lubricant debate
For decades, engineers and scientists have been debating how lubricants act under extreme confinement.
Does the lubricant go through a phase change from
liquid to solid, and if so, then what kind of solid? Is the
solid a glass-like substance or a crystal-like structure?
Moreover, once the pressure is relieved, does the lubricant remain a solid or does it change back into a liquid?
The answers to these questions are fundamental to the
formulation of any lubricant that could potentially face this
challenge. Understanding how oils perform in these situations can help manufacturers create products that improve
energy efficiency and increase surface protection. Worst-case
scenario, should you come down on the wrong side of the
debate, your lubricant might not protect the equipment to
which it is applied.
Dr. Yongsheng Leng, professor of engineering and applied
science at The George Washington University, wanted to
see if he could settle that debate once and for all. To find
a definitive answer to all those questions, Leng and his
team decided to start their research using a small-molecule
lubricant, cyclohexane.
Why such a simple lubricant? Leng explained that fundamental research always starts by keeping things simple. "We
start with the question: What is the thermodynamic state
of the simple molecules? If we understand this, then we can
begin to tackle the very complicated molecules."
Their research focuses on the fully atomistic molecular
dynamics simulation of cyclohexane confined between
two molecularly smooth mica surfaces. According to
Leng, the cyclohexane lubricant shows very similar normal
compression and shear responses as the OMCTS (octamethylcyclotetrasiloxane) lubricant, which is often used in
surface-force experiments.
What Leng found was that when the lubricant is squeezed
between those two solid surfaces, it does go through a sudden
phase change into a solid - specifically, solidified crystal

NOVEMBER 2018

| COMPOUNDINGS | ILMA.ORG

structures. Outside of the laboratory, this is the type of force
a vehicle's engine might experience when a driver accelerates
quickly or slams on the brakes. As to what happens when
that force is then reduced, Leng suggests the lubricant should
change back to a liquid. All that is needed to cause that phase
change is easing up on the force just a bit.
Leng explained that once you understand these fundamental phase changes, you can begin to determine whether
your product will continue to protect the surface under
extreme conditions or if it will fail.
The team at The George Washington University School
of Engineering and Applied Science wants to continue this
research, and Leng is inviting lubricant manufacturers to
collaborate with them to further study more complex lubricant performance. The team's recent research, "Squeezing
and stick-slip friction behaviors of lubricants in boundary
lubrication," was published in Proceedings of the National
Academy of Sciences of the United States of America (PNAS)
in June 2018.

http://www.ILMA.ORG

ILMA Compoundings – November 2018

Table of Contents for the Digital Edition of ILMA Compoundings – November 2018