The area around a cell is incredibly cramped: molecules jostle, creating a perpetual crush. It's this pandemonium, not just chemical affinity, that kick-starts particle capture, according to a new model. The force of entropy, of disorder, acts like a crowd on a packed dance floor: the tighter it gets, the more it presses particles against the cell's receptor 'locks'.

The physicists factored in the crowd's pressure, the cell membrane's flexibility, and its internal skeleton. Their model, like a hilly landscape with a capture pit, showed that a particle that's too small won't dent the membrane, while one that's too large simply won't fit. The sweet spot is tens of nanometers, just like many viruses. Stiff cells, such as bone cells, are even pickier about size.

Surprisingly, the cell spends zero energy on the initial approach: it's all done by the free-for-all jostling. That's why HIV, with a diameter of 100 nanometers, is so good at infecting: it fits right into the 'capture window.' This discovery could help engineer nano-medicines of the perfect caliber.