Experimental determination of van der Waals forces between two-dimensional materials in air and water

Two-dimensional (2D) materials have rather diverse properties and are therefore of high interest for applications in material science, physics, chemistry, and biology. Since many of these applications rely on interfacial stacking, a fundamental understanding of the interactions between 2D materials is of paramount importance. In this work, we describe a strategy which enables rapid and high-throughput determination of the forces between 2D materials (both in air and in liquids) using atomic force spectroscopy. While our experiments demonstrate strong adhesive forces between two graphene sheets with binding energies close to theoretical values, the scaling of the van der Waals (vdW) force with the separation distance differs quite substantially from theoretical predictions. In addition, our experiments in purified water demonstrate that the water confined between two hydrophobic surfaces leads to repulsive interactions, with increasing ionic strength reverting this behavior. These experiments challenge our current understanding of the vdW force and provide much needed experimental benchmarks for the further development of theoretical vdW methodologies.