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Stanford Report, August 25, 1999

Barroom analogy for operation of membrane ratchet

Brownian motion is an example of what scientists call a random walk, a movement whose steps are determined at random. A colloquial term for such a path is a drunken sailor's walk. So an appropriate analogy to understand how the membrane ratchet works is a waterfront bar visited by a stream of heavily drinking sailors, Boxer says. The bar stretches all the way across the back wall, and the front entrance is on the left side. If the space between the door and the bar is empty, then the sailors will head straight for the bar. Although the sailors spread out a bit as they stagger, they end up predominantly on the left side of the bar. But if the floor is filled with tables that are angled from left to right, they will deflect the staggering sailors further down the bar to the right.

The furniture-filled bar also would sort the sailors depending on how thirsty they are. Extremely thirsty sailors would head to the bar the most quickly and so their staggering course will not be deflected as much, on average, as that of the sailors who are only mildly thirsty. So more of the thirsty individuals would wind up at the left end of the bar and more of the mildly thirsty would end up on the right end. The amount of deflection is also affected by a sailor's degree of drunkenness. The drunker the sailor, the more pronounced his stagger and the more his course is deflected to the right. The distribution of the sailors at the bar also can be changed by altering the angle of the tables. SR