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.