NASA scientists are making these organic thin films on the Space Shuttle to overcome problems caused by convection. Convection is a circular motion in air or in a liquid created from uneven heating. On Earth's surface, when a gas or liquid is heated it expands, becoming lighter and less dense. This lighter material rises, mixing with cooler and denser material from above. Such turbulence occurs in the world's weather patterns or even in a pot of water boiling on the stove.
Convection creates difficulties when trying to create a uniform film.
A UV lamp or laser light will raise the temperature of the film solution, causing the hotter solution to rise. Aggregates of solid polymers often form in the solution, and convective flows that develop in the solution can carry these aggregates to the surface of the quartz. Because aggregates on optical films can cause light to scatter, the goal is to make the films as smooth and uniform as possible. David N. Donovan. Convection is actually caused both by heating and the Earth's gravity.