he first concept to understand for anisotropy measurements is the concept of Brownian motion. Although water at room temperature contained in a glass to the eye may look very still, on the molecular level each water molecule has kinetic energy and thus there are a continuous number of collisions between water molecules. A nanoparticle (yellow dot in the figure) suspended in solution will undergo a random walk due to the summation of these underlying collisions. The rotational correlation time (that is, the time it takes for the molecule to rotate 1 radian) is dependent on the viscosity, temperature, Boltzmann constant and volume of the nanoparticle