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Wednesday, August 7, 2013

Chemists explain why it's so hard to lift a wet glass from a table. - Seriously, Science?

Photo: flickr/bradleygee
Ever try to lift your wet glass off the coffee table, only to end up just sliding it around because it seems suctioned down? In this study, three physical chemists did some extensive molecular modeling to explain this phenomenon, including asking whether alcohol has a stronger or weaker “seal” than water.  Turns out that the surface tension caused by molecular interactions between the water and/or alcohol molecules requires a certain amount of force to break up, and less force is required for alcohol than for water. This has important implications, according to the authors: “This work was done in compliance with ethical guidelines for basic research. Nevertheless, the results may have severe social implications. We note that the energy required for lifting a glass from a wet table is lowest for hard liquor (over 40% alcohol, Figure 10).Hence, intoxicated persons may be tempted to drink, e.g., whiskey rather than water as it requires only half the effort to pick up the glass. The impact of this finding on alcohol consumption patterns falls beyond the scope of this work, however.
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.
Bonus figure from the main text:
A molecular model of the glass and table (both red) and a water/alcohol mixture (green) during the process of lifting a wet glass from a table.

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