Lifting a Wet Glass from a Table: A Microscopic Picture

Why is it so hard to lift a wet glass from a table? Is it easier when there is whiskey between the glass and the table? Macroscopically, the picture is quite simple: two surfaces have to be disrupted that are connected indirectly through hydrogen bonds and/or van der Waals forces. In the beginning, a surface has to be created leading to surface tension, and after that a liquid bridge has to be broken. Here we study the phenomenon at the microscopic level using molecular dynamics simulations. The effective force between two quartz plates is measured at different distances and with different alcohol/water mixtures between them. This allows us to compute the total work necessary to "lift the glass from the table". Different aspects of the process, such as clustering and liquid ordering are discussed. We compare the structure of the liquid/glass interface to that of a liquid/vapor interface, for which we present simulation results, like surface tension, as well. On the basis of the simulations, we are able to provide a detailed description of the energetics during the separation process as a function of alcohol concentration. It is shown that there is a net entropy loss upon separating two plates with water or a 10% MeOH solution between them, whereas for higher alcohol concentrations, there is net entropy gain. These findings increase our understanding of the properties of colloid suspensions which is important for process technology.