Sunday, July 14, 2013

LIQUID ON A ROUGH SURFACE: A THIN FILM OR DROPLETS

The phenomenon of liquids coating rough surfaces in the form of films or droplets is commonplace. But how can we tell in what conditions a liquid will form a continuous film or just isolated drops? Existing theory generally describe ideally smooth surfaces, which are not so relevant in the real world. Now for the first time, scientists have developed a general theory that provide an answer to the question of film or droplets for rough surfaces. Liquids in large amounts are extraordinarily adaptable. They will fill any type of vessel and adapt their outer shape of any surface or border. Microscopically thin films of liquid, however, are a different story. They must mediate between the rough surface of an object and the free surface of the liquid, which always aims to be as smooth as possible due to the influence of surface tension. Now, researchers have succeeded in establishing a general theory of the phenomenon, which is valid for the vast majority of natural surfaces. In the calculation, they describes the roughness as statistical height distribution, since the peaks and troughs occur in disordered arrangements. The only conditions are that the roughness is shallow, and similar over the whole surface. This leads to amazingly simple expressions that requires not more than school Mathematics. The new equations reveal that the behavior of liquids depends quite simply on two parameters: the contact angle, which gives the angle of the surface of the liquid against the substrate, and the vapour pressure in the surrounding air (i.e. the humidity). In these cases, physicists speak of a transition between two phases. In the wet phase, the liquid forms a practically continuous film over the rough surface. Almost all the microscopic troughs are filled, and only occasionally does a particularly high peak break the surface. In the dry phase, however, large portions of the surface remains untouched by the liquid, as it clumps together into droplets. “ The system always selects the option that requires least energy in function of the vapour pressure and contact angle ".

Ranjna Bhardwaj

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