Friction is a force rising between two surfaces when they move in opposite directions or when one moves over a stationary surface. The scale of the movement does not matter: from the movement of continental plates and glaciers to the motion of the atoms, friction forces are always there. On the other hand, the magnitude of these forces is determined by many factors, including but not limited to surface topographies, presence of lubricants or contamination, operating conditions, etc.
Since the times of primitive humans, people wanted to reduce (or in general to control) friction: invention of the wheel, first lubricants (water, mud, animal fat) – these and other attempts were all driven by the necessity to reduce friction. During the industrial revolution, with the development of trains and other machinery, the need in such control increased dramatically. It became critical to understand what makes friction and how to control it. At this period, the field of lubrication (and Tribology in general) gained a large attention and by the middle of the 20th century, lubrication mechanisms in non-conformal contacts, i.e., in gears, bearings, etc., were more or less understood.
Today, the World faces new challenges such as sustainability, climate change and a general degradation of the environment. Research has shown that around 23% of the world’s total energy consumption occurs at the tribological contacts (moving parts subject to friction and wear, such as bearings, gears, etc.). Today, this energy is lost, however, the use of the best tribological practices is capable of reducing the energy consumption by up to 40% in the long term. In general, use of the best tribological practices can contribute towards several of the 17 United Nations sustainable development goals (United Nations, 2015), particularly, affordable and clean energy (reduced friction for efficient energy use), responsible consumption and production (reduced material waste), and climate action (reduced emission due to lower friction). These challenges require new research, solutions and innovative development.
Friction can be both beneficial and disadvantageous. For car breaks, high friction is a must, the car have to stop as fast as possible. For bearings, high friction is an enemy, it leads to energy losses and wear, thus reducing the life time of a bearing or a machine element. However, sometimes, you need both high and low friction, depending on the operation cycle of the machine. An example of such tool is a robot arm gripper. The friction has to be high when the robot grips an object and has to be low when its necessary to let the object go.
Clearly, increase or decrease of friction on demand for the optimal performance of a machine element would be great to reduce the energy losses and to increase efficiency of tribological systems. Recently, researchers from Germany demonstrated a set-up that can control friction in a lubricated system using Ionic Liquids (ILs) .
IL is a liquid salt consisting of anions and cations. ILs have been proposed for use as lubricants back in 2001 and since then there has been tremendous research performed. Particularly, switchable friction has been already demonstrated for low contact pressure, low shear rate and for atomically smooth surfaces. However, for the macroscopic contacts encountered in real life, achieving controllable friction is still a challenge.
According to the authors, Gatti et al (2020), at nanoscale, adsorbed near-surface structure of ILs molecules determine friction. This layer can be altered using electrical potential: the surface charges lead to reformation of the strong interface layer and thus the fluid structure-interaction and changes friction. It was even demonstrated that using this approach one could reach a superlubricity regime. The authors propose to mix different ILs to adjust dielectric properties of the lubricants and thus precisely control friction.
Using an experimental set-up, the authors describe the dynamics of a system which resembles a clutch transmission as a typical application of controllable friction. For a clutch, it is necessary to have high friction in order to transmit power. The friction has always to be high and if it gets low, it means that the lifetime of a clutch is over. In order to maintain high friction, the researchers proposed to use their technique. High friction ensures minimum energy loss and prevents shuddering in clutch power transmission.
They demonstrate an experiment that was set up to externally polarize the lubricant film of an IL mixture and control the dynamics of COF. According to the researchers, this is achieved by lubricating the contact with an ionic liquid mixture comprising of long-chain cation and two different anions containing ILs of high charge/mass ratio. When an external electric impulse is applied, it induces a permanent change of the frictional response. Polarization influences molecular adsorption, exchange of adsorbed ions, and molecular orientation of the ions. This changes the value of COF according to the type of polarization used. In the experimental set-up anodic polarization increases anion concentration in the friction gap while forcing cations to the center of the lubricant film. Friction increases with anodic polarization.
There is an electrochemical explanation of this phenomenon. According to the authors, cations are larger than anions and have oil-like side groups. When cathodic polarization is applied, cations are drawn to the friction gaps on both sides of the lubricant film. Such properties decrease COF. Relative to non-polarized state, anodic polarization increases static COF by 30% while cathodic polarization reduces it by 10% in dynamic systems. In practical scenarios, anodic polarization is suitable for high friction applications such as clutch transmission while cathodic polarization is suitable in plain and bearing rollers.
COF is controlled by the polarizing potential, which should change with the friction conditions to meet the operational requirements of the system. When the system is programmed to automate the dynamics of polarization potential to match with those of friction conditions without user’s intervention, the tribosystems is said to work with programmed friction. Programming friction ensures an automatic balance and sustenance of a desired COF by maintaining synchrony of polarization potential and current with friction conditions in the system.
Further details can be found in the original article .
Key Words: Friction, coefficient of friction (COF), lubrication, tribosystems, programming friction, electric tribocontroller.
 Gatti, F. et al. (2020). Towards programmable friction: control of lubrication with ionic liquid mixtures by automated electrical regulation. Scientific Reports, 10(17634), https://doi.org/10.1038/s41598-020-74709-2