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{\bf QUESTIONS}  \begin{enumerate}  \item {\it Why do they from a 2D layer?} Because they are dense! heavy!  \item For now consider the simple case of a single inert particle in a (fixed) concentration gradient. Say it moves towards the source of the chemical. It will follow some mean trajectory, but with plenty of scatter. This is an out-of-equilibrium process.  \begin{enumerate}