Why Drinks Spill When You Walk, and What a Serving Tray Can Do About It

In 2012 Hans Mayer and Rouslan Krechetnikov of the University of California, Santa Barbara published “Walking with coffee: Why does it spill?” in Physical Review E. They showed that liquid in a cylindrical cup sloshes much like a pendulum, and that in a typical mug about 7 cm wide and 10 cm tall, its lowest sloshing frequency is easily excited by walking at a normal pace.

They also found that small irregular jolts, an uneven step or a twitch of the hand, amplify that sloshing until it goes over the rim, and that people who kept their eyes on the cup spilled less than people who did not.

To be clear, the study looked at a cup held in the hand. It did not test trays. But the same physics helps us think about them. A tray does not stop the liquid inside a glass from sloshing. What it can do is stop the glass from sliding or tipping, and catch whatever does spill before it reaches the floor or the guest.

So the useful questions are: how well does the tray hold the glass in place, and how high does the rim need to be to contain a spill?

A low rim of about 1.5 to 2 cm lets guests lift a glass out without raising the wrist. It suits a round cocktail tray that staff hold out while guests help themselves. A rim of 4 to 5 cm suits a deep-rim room service tray that travels from the kitchen, into a lift and down a corridor: if anything spills, it stays in the tray.

The cost of a higher rim is that plates are harder to lift out, so trays used for short trips across a dining room rarely need one.

A smooth leather tray floor is more slippery than people expect, especially under a wet glass, and glasses sliding into each other is how most tips start. If a tray mostly carries drinks, choose an inner surface with grip, such as a matte leather or a non-slip liner, and add rubber feet underneath so the tray itself does not skate when set down.

Size matters too. A tray much larger than its load gives glasses room to slide. A 40 x 30 cm rectangular tray holds six to eight tumblers without crowding.

The study found that people watching their cup spilled less. In practice that means walking at a steady pace without sudden stops, keeping the tray level, placing the heaviest and fullest glasses in the middle close to your body, and not filling glasses to the brim.

See every size under serving trays, and for in-room use look at minibar trays as well.

Mayer, H. C. & Krechetnikov, R. (2012). Walking with coffee: Why does it spill? Physical Review E 85, 046117. Summarised in Physics (American Physical Society): Science of Slosh.