 If you take your coffee black, you're likely missing out on one of the most curious displays in fluid dynamics. Though well known to science as delayed coalescence, this phenomenon is poorly understood. Now, researchers from MIT have discovered how and why it occurs, and the results could help scientists understand important fluid dynamic effects that lie beyond their morning cup of Joe. Since the 1960s, researchers studying coalescence have examined the roles played by properties like density, surface tension, viscosity, and surface charge. Early on, it was discovered that drops levitating on a fluid bath actually rest on a cushion of air, but it remained unclear just how long that air cushion could be sustained before collapsing under the weight of the drop. To address that problem, the MIT researchers focused on one often overlooked property, temperature. Specifically, the temperature difference between a drop and a fluid bath. Using a high-speed camera and silicone oil as their medium, they discovered that drops immediately collapse only below a critical difference in temperature. Above that value, drops tend to linger. What's more, the team discovered that the amount of time the drops remain suspended scales with the difference in temperature. The bigger the difference, the longer the drops delay their plunge. A combination of mathematical modeling and experiments provided a closer look at the crucial role played by temperature. The temperature difference between a cold drop and a hot bath creates flows both in the drop and the air cushion that increase the pressure in the cushion. That high pressure helps support the weight of the drop. The result is an air cushion that is sustained until the drop and bath reach nearly the same temperature. The researchers model closely approximates the delay times observed in the lab, and their results could one day inform many important natural and industrial processes involving droplet interactions. Those include the formation of rain drops in clouds and the dynamics of emulsions, which are used widely throughout the cosmetics industry. In the meantime, when ordering your next cup of coffee, be sure to ask for cream and hope to catch a glimpse of this fascinating physics in action.