The SpillNot allows a single open beverage in a wide variety of container shapes and sizes to be carried easily for long distances with amazingly little risk of spilling and without excessive concentration to avoid spills. The non-intuitive nature of the solution is so striking that in action, the invention is fascinating to observe. It’s so simple, yet history has no record of a SpillNot before this!
The SpillNot provides entertainment and a challenge if desired. For example, some confidence and skill are required to perform tricks (best attempted outdoors in an open area with a small plastic cup of water) like flipping it upside down. However, when used simply to go from point A to point B with a beverage, it is often easier than carrying a full mug. It could also be a social ice-breaker by providing entertainment and an interesting topic for discussion, as the underlying physics are not trivial.
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That is, when people spill coffee while walking with an open mug, the liquid ‘sloshes’ out due to a horizontal acceleration of the cup. When the cup is accelerated, it applies a horizontal force to the liquid. The liquid, of course, has no fixed shape so the horizontal force causes a build-up of liquid at the side of the cup that is applying the force. If the build-up is higher than the side of the cup, there is a spill. If the force ceases without a spill, gravity pulls the liquid back down and the momentum of this flow causes a smaller build-up of liquid on the other side of the cup. With no additional forces, the oscillations eventually die down and equilibrium is reached with the surface of the liquid flat and level. However, when walking with a cup of liquid, a person may inadvertently apply horizontal accelerations to the cup at an interval that matches the resonant frequency of the oscillations described above. When this occurs, the amplitude of the oscillations can increase, causing the liquid to spill even if the person carrying the cup has not applied any single particularly large horizontal acceleration to the cup.
What happens when a person applies a horizontal acceleration to the handle (upper end of the ‘thread’) of the SpillNot? Because the thread is very flexible, no lateral force is applied to the saucer and thus no lateral force is applied to the cup. Instead, as the top of the thread moves horizontally, the saucer and cup swing up slightly in the opposite direction and the thread exerts a force on the cup through the saucer causing acceleration in the only direction in which the thread can exert a force, toward the upper thread end that is held. The direction of this force is parallel to the sides of the cup (by the SpillNot design), and thus does not tend to cause sloshing. At the same time, gravity pulls down equally on the saucer, cup, and liquid molecules in the cup, counter-acting the swing caused by horizontal acceleration of the upper thread end without causing sloshing.
More generally, swinging does not tend to cause the cup to slide off the saucer or liquid to slosh in the cup because the only force acting in a direction that is not parallel to the sides of the cup is the force of gravity, and gravity is equally accelerating all molecules of the saucer, cup, and liquid in the cup.
If the SpillNot is swung all the way around in a loop such that the cup is upside-down at the top of the arc, the liquid does not pour out of the cup due to the pseudo force, centrifugal force, which acts in the opposite direction as gravity when the cup is at the top of the arc.
Interestingly, for the webbing loop handle to lie flat and have no twists, it must be a Mobias strip (for ease of assembly, the strip handle for the standard SpillNot is sewn together)!