An interesting series of experiments have been done with creating surfaces to test for friction coefficients, to the point where the surfaces can be created for specific values. The flat surfaces are designed in order to 'inflate' and control the size of bumps on the surface. This controls the coefficient of friction...one can 'tune' the friction.
Perhaps something like this can be investigated if you have access to 3D printers, and can print patterns of small bumps on a flat surface. You can then use them in a variety of friction experiments. This could develop into a series of experiments, to the point where are
