Buzz
If you’re someone who enjoys baths or perhaps doesn’t prioritize hygiene, let me shed light on the infamous 'shower curtain effect.' Imagine stepping into a hot shower first thing in the morning. Half-asleep, the warm, cocoon-like space feels like the perfect sanctuary to transform from a groggy mess into a functional human. But then, the shower curtain—your supposed ally—betrays you. Drawn inexplicably to the running water, it billows inward. On a good day, it’s a slight nuisance. On a bad day, it’s a nightmare as the curtain brushes against you, cold and possibly even a bit slimy.
Why does this happen? For years, brilliant minds have pondered this question, but until recently, only theories existed. No one had tested these ideas or shared conclusive results—until a few years ago. That experiment provided a solid answer, but the earlier theoretical work is equally fascinating. Let’s dive into the handful of explanations proposed over time.
The Bernoulli Principle Hypothesis We previously discussed Bernoulli’s principle in my earlier piece on bathroom science. In simple terms, it states that as the speed of a fluid (liquid or gas) increases, the pressure around it decreases. In the context of shower curtains, the principle was thought to work like this: The water from the showerhead causes the air inside the shower to flow in the same direction, parallel to the curtain. This movement lowers the air pressure inside, creating a pressure difference that pulls the curtain inward. For years, this was the dominant scientific explanation for the shower curtain effect.
The Buoyancy Hypothesis Warm air rises and escapes from the shower, reducing the air density inside. Similar to the Bernoulli hypothesis, the pressure difference between the shower and the outside causes the curtain to move inward. However, this theory has a major flaw: the curtain still moves inward even when you take an ice-cold shower.
The Coandă Effect Hypothesis Jearl Walker, a physics professor at Cleveland State University and former writer of Scientific American’s “Amateur Scientist” column, proposed that the Coandă effect—the tendency of a moving fluid to stick to a surface or vice versa—could explain the phenomenon.* * * While these hypotheses are plausible and well-reasoned, they remain speculative without concrete evidence. One theory even came from a physicist who famously demonstrated scientific principles through daring experiments, like handling molten lava and pouring liquid nitrogen into his mouth. However, without data, these ideas remain theoretical.
In 2001, David Schmidt from the University of Massachusetts tested his own hypothesis, providing the first evidence-based explanation for the shower curtain mystery that plagues so many.
The Horizontal Vortex Hypothesis Using a computer model of a shower, Schmidt discovered that the showerhead’s spray generates a horizontal vortex with a low-pressure center, which pulls the curtain inward. Schmidt, who won the 2001 Ig Nobel Prize in Physics for this work, elaborated on his findings. As he told Scientific American:
To conduct the analysis, I created a model of a standard shower and divided the space into 50,000 tiny cells. The tub, showerhead, curtain rod, and surrounding room were all included. I ran the modeling software on my home computer during evenings and weekends (when my wife wasn’t using it). The simulation, which performed around 1.5 trillion calculations, represented 30 seconds of actual shower time. The results revealed that the spray created a vortex. The center of this vortex, akin to the eye of a cyclone, is a low-pressure zone. This low-pressure area is what draws the curtain inward… It’s similar to a sideways dust devil, but unlike a dust devil, this vortex persists because it’s continuously fueled by the shower’s spray.
