Wetting, adhesion and friction at patterned interfaces – WAFPI

The ability to control friction and reduce wear has long been a major factor for increasing the efficiency and working life of machines and moving parts. With an increasing demand for surfaces that exhibit low friction forces or adjustable adhesion, we need to gain a better understanding in the relationship between the three phenomena of adhesion, friction, and wetting, both at the practical and theoretical level. Recently a new route has started to be explored, based on the observation of nature using patterned surfaces.
In this program, we propose to examine the incidence of micropatterning on the behavior of a moving contact, either a solid-liquid or a solid-solid contact, comparing dynamic wetting, adhesion and friction experiments performed on the same surfaces, in order to trace back the detachment mechanisms associated to the motion of the triple line in the case of liquid –solid contact or to the fracture front in the cases of adhesion and friction.
The originality and the potential impact of our project rely on the fact that we shall first couple the results of three approaches, wetting, adhesion and friction, usually considered as related, but quite rarely quantitatively compared, and second that we deliberately make the choice of varying in a precise and controlled manner the geometric characteristics of the patterns, keeping the surface energy constant (same surface chemistry). To do so, patterned surfaces obtained by two complementary techniques will be fabricated, so that patterns with exactly the same overall geometrical characteristics (wavelength, height and size of the features) but very different shapes (abrupt edges or smooth sinusoidal undulations) could be produced, still keeping the same surface chemistry. The precise control of the surface will be done using either e-beam lithography or wrinkling instabilities on surfaces with gradient of mechanical properties.
Several new experimental devices will have to be developed to push this program at the front head of the present researches conducted in these areas worldwide.
It is also now possible to create patterned surfaces with anisotropic patterning: for example, cylindrical pillars whose axis is inclined with respect to the normal to the surface. Such surfaces will alow one to produce answers to other questions such as: What is the impact of the anisotropy of the pattern on the dynamic wetting behavior and on the friction coefficient? Since the degree of anisotropy is an adjustable parameter which can be tuned by varying the inclination or aspect ratio of the pillars or stripes, this question will be carefully studied.
The aim of the project is not only to produce experimental results, but also to benefit from the possible correlations between the results obtained in the three different areas of wetting, adhesion and friction, along with the possibility of varying in a systematic manner the geometrical parameters of the patterning, to develop a full, joined modeling of the mechanisms governing the motion of a contact. The role of the theoretician group in the consortium will be essential in that respect. Outcomes are expected, both of fundamental and applied character: the project should bring real advances in our understanding of the mechanisms governing surface properties, and should also open the route to the design of surfaces with adjusted wetting, adhesion and friction properties.