Phototribology : control of friction by light

Agradecimentos: This work was partially supported by The Royal Society – UK (award "Royal Society-Newton Advanced Fellowship" - NAF\R2\192108), the French National Center for Scientific Research (CNRS), the Brazilian National Council of Scientific and Technological Development (CNPq – projects: 233194/2014-2, 465423/2014-0, and 422372/2016-1), and Research Support Foundation of the State of Rio Grande do Sul (FAPERGS – projects: 17/2551-0001052-3 and 19/2551-0000656-0). F.C. and F.G.E. are FAPESP fellows (projects 2018/24461-0 and 2019/00757-0). B.L.P., S.R.S.M, and L.M.L. are CAPES fellows. C.A.F and F.A. are CNPq fellows. A.C. and T.P. acknowledge the support from the project ?Novel nanostructures for engineering applications? No. CZ.02.1.01/0.0/0.0/16_026/0008396 and The Ministry of Education, Youth and Sports from the Large Infrastructures for Research, Experimental Development and Innovations project ?e-Infrastructure CZ – LM2018140? and the use of VESTA software Abstract: In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO GRANDE DO SUL - FAPERGS FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado