Tribology and MEMS Breakthrough: Control of Friction at the Nanoscale Developed by Oak Ridge National Laboratory Scientists

Methods and apparatus for control of friction at the nanoscale have been developed by Oak Ridge National Laboratory (Oak Ridge, TN) scientists, Dr. Jacob Barhen,  Director Center for Engineering Science Advanced Research, Dr. Yehuda Y. Braiman, Group Leader Optical & Quantum Systems Group, Center for Engineering Science Advanced Research  and Dr. Vladimir Protopopescu Deputy Director, Center for Engineering Science Advanced Research and Group Leader of Complex Nonlinear Systems Group.  Their non-Lipschitzian method of controlling the frictional dynamics of nanoparticles is detailed in U.S. Patent Application 20100042266 which is assigned to UT-Battelle/Chicago/BHGL (Knoxville, TN).

A practical application of the invention is as an efficient tool for controlling friction between a plurality of particles and a surface, between sliding surfaces and between sliding surfaces and a lubricant. The methods are applicable to quartz microbalance, atomic force microscope, and surface force apparatus-type experiments. It  is also applicable to cantilevers and arrays of cantilevers, and in particular to micro-electro-mechanical systems (MEMS) where frictional contact and resulting wear are important factors in their design.

Further non-Lipschitzian control is also applicable to fast controls such as optical, or usage of micro/nano cantilevers. The apparatus is also applicable to implementations at time scales slower than the characteristic times of the dynamical system. Indeed, numerical simulations show that the control can be applied at much slower rates, while still maintaining the average value of the velocity close to the target. The "price" of such relaxed requirements are that longer times are needed to reach the target and larger fluctuations from the averaged value are observed. Another practical application is as a tool for synchronizing a plurality of lasers. There are virtually innumerable uses.

Despite great progress made during the past half century, many problems in fundamental tribology (such as the origin of friction and failure of lubrication) have remained unsolved. Moreover, the current reliable knowledge related to friction and lubrication is mainly applicable to macroscopic systems and machinery and, most likely, will be only of limited use for micro- and nano-systems.

Indeed, when the thickness of the lubrication film is comparable to the molecular or atomic size, the behavior of the (film) lubricant becomes significantly different from the behavior of macroscopic (bulk) lubricant. Better understanding of the intimate mechanisms of friction, lubrication, and other interfacial phenomena at the atomic and molecular scales is needed to provide designers and engineers the required tools and capabilities to monitor and control friction, reduce unnecessary wear, and predict mechanical faults and failure of lubrication in micro-electro-mechanical systems (MEMS) and nano-devices.

The ability to control and manipulate friction during sliding is extremely important for a large variety of technological applications. The outstanding difficulties in realizing efficient friction control are related to the complexity of the task, namely dealing with systems with many degrees of freedom under strict size confinement, and only very limited control access. Moreover, a nonlinear system driven far from equilibrium may exhibit a variety of complex spatial and temporal behaviors, each resulting in different patterns of motion and corresponding to different friction coefficient

 The control of the dynamic attribute can be based on the concepts of non-Lipschitzian dynamics and the use of a non-Lipschitzian global feedback control term.

Optionally, the invention can include maintaining the control until the deviation is reduced to zero whereupon the target has been reached. The device includes a method (and/or apparatus based on the method) to control sliding and frictional properties (such as friction coefficient, friction force, sliding velocity, slip time) of a plurality (e.g., array) of atoms and/or molecules towards a pre-assigned (pre-determined) value of a target (average sliding velocity, slip time, friction coefficient and friction force).

The invention can also include an apparatus to control shear forces and static forces, viscosity, and adhesion forces towards a pre-assigned value of a target (shear and static forces, viscosity, and adhesion forces).It also includes an apparatus based on the method to control sliding trajectory, speed, direction and diffusion of atomic and molecular chains and polymers sliding on surfaces towards a pre-assigned value of a target (sliding trajectory, speed direction and diffusion coefficient).

Implementation of the non-Lipschitzian friction control technique is applicable but not limited for slip time and velocity control in a quartz micro balance apparatus, friction coefficient and friction force control in an atomic force microscope, and friction forces, loss and elastic moduli control in a surface force apparatus.

Implementation of non-Lipschitzian control algorithm can be achieved either through imposing controlled vibrations of the sliding surfaces and/or the AFM tip (normal and/or in-plane) or electromechanical, electro-optical, or optical excitations applied to the sliding system and/or the lubricant according to the proposed algorithm.

Implementation of control algorithm can be also achieved by imposing controlled vibrations of the sliding surfaces with a surface force apparatus, a quartz microbalance and/or using cantilevers and/or cantilever arrays. In addition, electromechanical, electro-optical, and optical control can be utilized in conjunction with (applicable for) all the previously described friction measurement apparatuses.

As in the generic case, this control can be based on the concepts of non-Lipschitzian dynamics and the use of a (terminal attractor based) non-Lipschitzian global feedback control term. Extensive numerical simulations, some of which are described below, have actually proven the robustness, efficiency, and convenience of the invention applied in the context of controlling friction.