ABSTRACT
Colloid physics has been a meeting place of physics and biology long before the word ‘biophysics’ emerged. In this context, the discovery of Brownian motion (by Robert Brown in 1827), is not a good example because the crucial aspect of his observation was that pollen particles appeared to move, even though they were not motile. Hence, certainly at the time of Brown, the biological relevance of Brownian motion was not clear. But things became different with the discovery of osmotic pressure. Osmotic pressure is very important in biology — it is responsible for the turgor of plant cells, and animal cells usually do not survive in a medium with the wrong osmotic pressure. The systematic study of osmotic pressure, started by a biologist (Hugo de Vries), was taken over by physiologists (Hamburger and Donders) and was given a theoretical foundation by a chemical physicist (van ’t Hoff), who showed that the osmotic pressure of macromolecules in solution depends on their concentration in the same way that the pressure of a noble gas depends on the number density of the constituent atoms. This is a surprising nding, because the law holds for macromolecules of any size. In particular, it applies to the class of macromolecules called ‘colloids’.
