Interaction of the particles suspended in a droplet with the free surface of the droplet is important in creating a coffee ring. "When the drop evaporates, the free surface collapses and traps the suspended particles... eventually all the particles are captured by the free surface and stay there for the rest of their trip towards the edge of the drop." This result means that surfactants can be used to manipulate the motion of the solute particles by changing the surface tension of the drop, rather than trying to control the bulk flow inside the drop. A number of interesting morphologies of the deposited particles can result. For example, an enantiopure poly (isocyanate) derivative has been shown to form ordered arrays of squashed donut structures.