Almost everyone is familiar with the problem of coffee or tea sloshing over the sides of a mug as one walks, but this may be the first time researchers have systematically studied the problem. The results show that the typical frequency of the human stride closely matches the natural frequency for back-and-forth sloshing of a low-viscosity liquid in a cylindrical container the size of a typical coffee mug. Even though our natural side-to-side motion plays a role in coffee sloshing, its effect is small in comparison. A person’s maximum acceleration, which usually happens early on when walking, sets the initial sloshing amplitude, which is subsequently amplified by the stepping frequency. The researchers did find that the time to spill increased substantially if the subject was focused on not spilling the coffee, though it was unclear if this was due to the subject decreasing their acceleration and step frequency, or whether they were actively damping the oscillations with adjustments in the wrist. If you’re a perpetual coffee spiller, there’s still hope: the authors suggest that flexible cups and/or cups with a series of concentric rings–baffles–could help reduce sloshing in spite of our natural tendency to induce it. (Photo credit: dongga/Flickr; Paper: Mayer and Krechetnikov; submitted by @__pj)
Tag: coffee
How Coffee Rings Form
Coffee rings (an ubiquitous feature of academia) are formed by the deposition of particles as the liquid evaporates. When a coffee drop evaporates, capillary action draws the coffee particles toward the edges of the drop, where they congregate into a ring. Research now suggests that this is due to the spherical nature of the particles. Ellipsoidal particles, in contrast, clump together and result in a uniform stain once their carrier liquid evaporates. The effect seems to be due to the particles’ effects on surface tension; the ellipsoidal particles deform the surface of the droplet as it evaporates such that they are not pulled to the edges. Adding a surfactant, like soap, that decreases surface tension caused the ellipsoidal particles to form rings just as the spherical particles do. (submitted by Neil K) #
Drinking Coffee in Space
Operating in microgravity disrupts a lot of everyday routines. However, astronauts have learned to take advantage of surface tension to enable them to drink their daily coffee from a cup still.
