TLT: Tell us about the research focus areas you’re working on.
Turner: My research group focuses on problems in which the mechanics of interfaces and materials play a significant role. The specific research projects that are ongoing in my lab are quite broad, and current projects include research on nanocomposite coatings with high strength and toughness, materials for studying mechanobiology, novel structural materials based on nanocellulose, surfaces with tunable adhesion for robotic gripping and manufacturing processes for making flexible hybrid electronics. While each of these research topics seem quite different from one another, they all leverage my group’s expertise in analytical and computational interface mechanics, micro/nanosystems and high-precision experimental mechanics measurements. 

The mechanics of adhesion is very important in several projects in my group. During the past two decades I have done research on many problems involving adhesion, ranging from direct adhesion of very stiff semiconductor wafers to the adhesion of extremely soft-pressure sensitive adhesives. Here are three examples of current projects where understanding of adhesion plays a central role.

First, we have exploited our understanding of adhesion mechanics to create a new class of composite surfaces with tunable adhesion (i.e., surfaces with adhesion that can be changed from high to low on demand). This research includes developing interfaces where adhesion can be controlled through peeling direction, developing interfaces where adhesion can be tuned through application of an electrical voltage and work to apply these novel concepts to robotic gripping and pick-and-place assembly of micro-scale electronic components.

Second, we have research aimed at developing structural materials that exploit the unique properties of sustainable cellulose nanomaterials. In this work related to nanocellulose, we are investigating composite systems as well as pure nanocellulose materials made by additive manufacturing (3D printing). In composites and additive manufacturing, engineering and understanding adhesion at specific interfaces is crucial. 

Finally, we are investigating manufacturing processes to make high-performance flexible hybrid electronics (FHE). The manufacturing of FHE devices requires high yield processes to transfer and assemble very thin (<10 micrometers) layers of brittle semiconductors. Success in transfer processes requires unique strategies to control and manipulate adhesion at different interfaces at different points in the process.


© Can Stock Photo / Ikonoklast

TLT: How did you get involved in nanomechanics and nanofabrication research?
Turner: Nanomechanics and micro- and nanofabrication have been and continue to be a significant aspect of my research. My first exposure to small-scale mechanics and manufacturing was as an undergraduate researcher at Johns Hopkins University. I worked in a lab that was developing techniques to measure the mechanical properties of polysilicon, the key structural material in microelectromechanical systems (MEMS). It was 1997, and there was a lot of active university research in MEMS. I continued working on mechanics, materials and manufacturing problems related to MEMS and microelectronics during my graduate work at MIT. This work in microsystems positioned me perfectly to get involved in nanomechanics and nanofabrication as research in these areas increased. Many of the same tools and techniques that I learned while working in MEMS could be leveraged for research in nanoscale systems.

TLT: What are some of the challenges and opportunities for tribologists in scalable additive nanomanufacturing and/or fabrication of microsystems?
Turner: Interfaces are ubiquitous and extremely important in micro/nanosystems. The high surface area-to-volume ratio found in these systems means that surface interactions can play a critical role in performance of micro/nanosystems. There are well-known challenges related to friction, wear and surface degradation in MEMS devices, with MEMS switches perhaps being the best example of where device performance has been limited by surface failures. I am very interested in manufacturing processes for nanosystems in which an understanding of tribology can lead to improved performance as well as engineering-based design of the manufacturing processes. Chemical mechanical polishing (CMP) and tip-based nanolithography (TBN) are two examples of manufacturing processes in which tribology is critical. In both CMP and TBN, friction, wear and contact mechanics play a significant role, and an understanding of these concepts can be used to improve and optimize these processes.

TLT: Tribologists often talk about friction and wear but not so much adhesion. Could you tell us more about why adhesion matters at the nanoscale?
Turner: Surface forces, such as van der Waals forces, which can cause adhesion, are present on all surfaces. However, these forces are short range and, as a result, are often insignificant in macroscale systems where the roughness is larger than the range of the forces. In nanoscale systems, the surfaces are typically very smooth and, thus, short-range surface forces can cause adhesion. Beyond being smooth, nanoscale systems also have high surface area-to-volume ratio and, thus, surface forces, like adhesion, can be dominant over other loads that act on a system such as gravity and inertial forces. As a result, adhesion is crucial in MEMS and nanosystems.

TLT: What are some of the open questions as far as adhesion and nanoscale adhesion research is concerned?
Turner: There is still a challenge in understanding the adhesion of “real” surfaces that contain patterned features, defects and roughness at the nanoscale. Adhesion between real surfaces is always imperfect, and a deeper understanding of the role that imperfection plays in determining adhesion would enable better design and better engineering at the nanoscale, for example, in design of contacting surfaces in MEMS and semiconductor wafer bonding processes.

While the basic role of roughness on adhesion has been understood for decades and sophisticated models of rough surface adhesion have been developed in recent years, it is still quite challenging to quantitatively predict the adhesion of surfaces based on geometry and knowledge of the forces that are causing adhesion. One reason that it is challenging is that roughness occurs over many length scales and experimentally characterizing and modeling the contact over all relevant length scales is quite difficult. A second reason is that multiple types of forces and bonds contribute to adhesion and, thus, there is a complex interplay between mechanics, chemistry and environment that determines adhesion. Progress is certainly being made, but further research, both experimental and simulation, is needed to overcome this challenge.

TLT: In the context of nanomechanics and nanotribology, what are some of the unique challenges associated with probing soft matter such as soft polymers and biological matter?
Turner: Soft-matter tribology at the nanoscale is challenging for a few reasons. First, soft-matter contacts at the nanoscale are rarely well described by traditional contact mechanics because the soft material often undergoes large deformations that violate assumptions made in classical mechanics models.

Second, from an experimental standpoint, the forces are small and the deformations are large. This is the opposite of what you have in a traditional stiff material contact and, as a result, it is not easy to test soft materials with many conventional tribometers and nanoindentation systems. Experimental systems need to be designed specifically for probing soft-material interfaces. 

Finally, many polymer and biological materials are heterogenous at the nanoscale and, thus, both experiments analysis must consider and address this heterogeneity.

TLT: Are there other open questions about hot button research areas or future research areas?
Turner: As tribologists, we often think about using our knowledge to design systems or devices. This is important, but I think tribologists can play a large role in providing insight into the design of manufacturing processes. Advanced manufacturing is an important and growing research area, and I believe there is a critical role for tribologists to play in pushing this research area forward.