Good Vibrations Help Surfaces Stay Clean

Using numerical simulations and experiments, scientists have found that mechanical oscillations can enhance diffusion rates by ten orders of magnitude.

AsianScientist (Sep. 10, 2015) – In a study published in Physical Review X, researchers from Tsinghua University and Tel Aviv University have shown that vibrations can be used to dislodge contaminant particles between surfaces, thereby making the surfaces self-cleaning.

In contrast to liquids and gases, diffusion on solid surfaces is restricted due to the potential energy landscape. Contaminant particles trapped between solid surfaces tend to cause unwanted friction because they are unable to diffuse away.

To reduce friction, diffusion at the molecular scale can be enhanced by a number of external factors, but such practices are often difficult to implement in nano-scale devices as the small surface areas do not allow for much external interference to the system.

In the present study, a team of researchers led by first author Ma Ming has demonstrated that mechanical oscillations—vibrations—can be used to manipulate molecular diffusion in nano-scale confined regions.

Ma and co-workers used numerical simulations based on a one-dimensional model, with two rigid plates and adsorbed particles embedded between the plates, a common configuration in friction-related studies. Various parameters, such as temperature of the substance, damping conditions, and most importantly the presence and frequency of mechanical oscillations, were taken into account in their study.

By oscillating the rigid plates, the diffusion coefficients were enhanced up to ten orders of magnitude higher than non-oscillating conditions. The marked increase of the diffusion coefficient is related to the changes in energy landscape of the molecules during the movement, according to the researchers’ analytical model which describes the complex relationship between temperature, the conditions of oscillation and the resultant diffusion coefficient.

Oscillation-induced diffusion was able to significantly reduce the concentration of contaminants between the graphene surfaces, suggesting that oscillation could be useful for removing contaminants in nano-scale contacts, a common problem plaguing micro- and nano-devices.

This discovery, however, is not without limitations–various unknown parameters of such surface interactions limit the analysis to a qualitative description. Without the prerequisite knowledge of the system’s characteristics, of which some may remain a mystery, a quantitative prediction may only be possible with further work.

Nonetheless, the success of the research group allows for analytical computational models in testing of molecular adsorption, diffusion, and interaction between nano-scale surfaces. This can prevent costly experiments of micro- or nano-friction, and serve to explain even more mechanisms behind the various factors in micro-tribology. The increase in molecular diffusion due to the oscillation may also have promising applications in directed molecular transport.

The article can be found at: Ma et al. (2015) Diffusion through Bifurcations in Oscillating Nano- and Microscale Contacts: Fundamentals and Applications.

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Source: Asian Scientist Magazine; Photo: American Physical Society.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Jonathan Leong graduated from the NUS-Imperial College Joint PhD Programme at the National University of Singapore. He is interested in all things related to friction, but particularly at the micro- or nano-scale. He is a lecturer at SIM University.

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