The nature of forces
Water drops can bead on a surface and roll around if you tilt it. They can even cling to a surface when turned upside down. Why don’t they fall off? What happened to gravity? Rafael Tadmor, an associate professor in the Dan F. Smith Department of Chemical Engineering, realized that the effect of gravity on liquids was not completely understood, so he started experimenting with small drops of oil on movingsurfaces. Liquids, he found, did not respond to gravity in the predictable ways that solid objects responded. Heavy solid objects require more force to move than light solid objects. This isn’t necessarily the case for heavier drops of liquid on solid surfaces. Tadmor delved deeper into research on drop adhesion on surfaces and made some innovative discoveries.
In 2004, he was the first to write equations to relate the value of the equilibrium contact angles of drops on surfaces to measured values. His method is still the only way to make this calculation. In 2008, Tadmor wrote another paper that combined his conclusions from 2004 and finally explained theoretically why sometimes drops flow down and sometimes they don’t.
But proving a theory on paper and seeing the physical reality of it with your own eyes are two different things. To measure the forces associated with retaining a drop on a surface, Tadmor, David Day of the Department of Mechanical Engineering and Tadmor’s team of students built the first Centrifugal Adhesion Balance (CAB) machine.
(Tadmor currently has funding from the National Science Foundation to make adjustments and improvements to the machine.)
According to Day, a laboratory technician, Tadmor first came to him asking for help with equipment for an experiment. “I never really knew exactly what he was doing,” Day said. The technician would make a piece to the professor’s specifications, and, when Day checked back, he was told it worked perfectly. Then Tadmor would return to the mechanical engineering lab for a few more things. When the machine was finally complete, he demonstrated it to Day and explained its purpose. “I really enjoyed working with him,” Day said of Tadmor. “I really enjoy R&D.”
With the help of the CAB device, the researchers discovered in 2009 that drops hanging from a surface require more force to slide than drops resting on a surface. In December 2009, "Measurement of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate" was published in Physical Review Letters, a prestigious scientific journal. “We made a huge discovery that echoed throughout the world,” Tadmor said of his team that included students Prashant Bahadur, Aisha Leh, Hartmann E. N’guessan, Rajiv Jaini and Lan Dang.
By measuring liquid drops on surfaces, Tadmor and the students observed a lowered lateral force despite a larger normal force and an increased contact area. Key to their observations was the ability to decouple the normal and lateral forces while monitoring the drop, thanks to the CAB machine. To achieve that separation of forces, the researchers mounted the sample at an adjustable angle in a horizontal centrifuge arm that could be rotated about the vertical axis at a variable speed. A video camera linked to a computer allowed the researchers to compare a drop on top of a horizontal substrate to a drop hanging below a horizontal substrate. Tadmor found that the hanging drop had the larger lateral retention force, despite a smaller contact area and a smaller normal force. According to Tadmor, that seemingly counterintuitive result agrees with theories that incorporate the effects of surface deformation and molecular reorientation.
Tadmor’s investigation is ongoing and continues to be of great interest to other researchers. His advance online article “Approaches in Wetting Phenomena” in the journal Soft Matter was among the top-10 most-read articles in November, according to the journal’s blog post in January. The article was printed in the February 2011 edition.
The drop-adhesion discovery can lead to possible applications in bioengineering for tissue regeneration and pharmaceutical research on inhalation drugs. Tadmor, who was the Distinguished Faculty Lecturer for 2010, will let other researchers deal with the practical applications. He’s more interested in proving scientific foundations. “Only research for the sake of understanding can lead to a scientific breakthrough,” he said.