text: The coffee-ring pattern originates from the capillary flow induced by the evaporation of the drop: liquid evaporating from the edge is replenished by liquid from the interior. The resulting current can carry nearly all the dispersed material to the edge. As a function of time, this process exhibits a "rush-hour" effect, that is, a rapid acceleration of the flow towards the edge at the final stage of the drying process.
  Evaporation induces a Marangoni flow inside a droplet.  The flow, if strong, redistributes particles back to the center of the droplet.  Thus, for particles to accumulate at the edges, the liquid must have a weak Marangoni flow, or something must occur to disrupt the flow.  For example, surfactants can be added to reduce the liquid's surface tension gradient, disrupting the induced flow.  Water has a weak Marangoni flow to begin with, which is then reduced significantly by natural surfactants.
  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.