Molecules squeezed and stroked

摘要

How does a pen write on paper? As you blink your eyes, how do tears lubricate your cornea? As you type, how does the hard disk store information? All of these apparently disparate actions share a central feature ― surfaces, separated by fluids, are squeezed together and the molecules sandwiched between are stroked by sliding. Sliding is one of the most common processes of the everyday world, but we know little about the interplay between surfaces and their confined lubricants. Shinji Yamada has now looked at the effect of sliding on the behaviour of the silicone oil polymer PDMS ― a flexible threadlike chain molecule that normally exists in a tangled, randomly coiled form. Writing in Langmuir, he reports that the degree of friction between two sliding surfaces depends on the number of layers of PDMS sandwiched between. Remarkably, each layer is the width of the polymer backbone, indicating that sliding might have ordered the polymer chains.