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How do dishwashers actually work?

Your dishwasher works via a very simple equation: water + dirty dishes + soap = clean dishes.

A hundred years ago, however, the dishwashing process wasn’t so easy. Cleaning up after a meal was a laborious process that required gathering water from a pump or well, heating it on a stove or another heat source, and scrubbing, scraping, and towel-drying each delicate dish by hand.

Nowadays, the process has been streamlined: You load the dishes, add detergent, close the door, hit a button, and walk away. The rest is magic, as far as we can tell. But inside our machines, manufacturers ceaselessly tweak the interior configurations, exploiting the same physics and chemistry concepts in slightly different ways, with the goal of getting you cleaner dishes in a less time.

Below, we break down the dishwasher equation and reveal the underlying scientific principles that save us so much time and effort.


Even the very first dishwashers sprayed water on dishes, rather than suspending them in water. Why are well-placed jets a better way to clean than simply soaking? Simple: While standing water may be enough to remove lighter, wetter stains, tougher stains require additional force.

You might be asking yourself: Why not fill the tub and use jets, like a hot tub? But there are several very good reasons why that wouldn’t work.

First, it would be extremely costly to reinforce and seal the machine to prevent it from flooding your kitchen. Second, it would use a prohibitive amount of water. Third, it’s much more difficult to drive jets of water through water than it is to spray them through the air. Liquids are dissipative, which means any force or energy exerted on water molecules tends to be absorbed, rather than transmitted.


A jet of water moves through air much more easily than it would through water.

A garden hose will shoot water much further through the air than it will in a swimming pool. That’s why a dishwasher more closely resembles a bunch of hoses spraying water into an empty pool than it does a Jacuzzi with the jets turned on. By spraying water through air, the water can go further and carry greater force when it hits your dishes.

This is also why dishwasher spray arms use relatively tiny holes to create the jets of water. Though it might seem like larger holes would deliver more water and thus better cleaning performance, it’s not so.

The reason? Pressure, aka the ratio of force to area.

Let’s go back to the garden hose metaphor: In your hose, force is the volume of water moving through it. Area is the hole where the water comes out. If you put your thumb over the hole, area is reduced. That means pressure goes up, and the water shoots out with greater force. In other words, assuming a steady flow of water, a smaller hole will always produce a more powerful water jet.

But if every manufacturer uses spray arms with small holes, why do so many spray arms look so different? The answer leads us to the next part of the equation…

Dirty Dishes

Dirty Dishes

Dishwasher racks may all seem the same at first glance, but the orientation and distribution of the rack tines varies from model to model and manufacturer to manufacturer. This is because each model is designed to be loaded a specific way for ideal cleaning. Pictures and descriptions of the ideal dish load are usually found in the user manual.


The recommended dish loading style for the bottom rack of the Kenmore 14753 Dishwasher

A combination of spray arm design and proper loading can greatly impact how clean your dishes get. Drastically deviating from the recommended loading configuration can prevent the spray arms from doing their job.

Imagine you have a plate with a baked-on lasagna stain, and a mug with a three-week-old coffee stain. How do you place them in the dishwasher? Chances are, the plate gets placed between two tines, standing upright, and the mug gets placed upside down on top of a tine. Why do we instinctively load these dishes in this way?

Cleaning the plate is like pushing an anvil across the floor. A lasagna stain doesn’t lie flat—it has elevation. With objects (lasagna) on a surface (plate), the best way to remove them is to exert force parallel to the surface. That’s why the best way to remove a lasagna stain from a plate is to shoot a jet of water along its surface.

Cleaning the mug, on the other hand, is like power-washing a window. Does your window get cleaner when you spray water along the surface at an angle, or blast it on the perpendicular? The latter, of course. A coffee stain doesn’t stick up from the bottom of a mug, so the best way to clean it is to shoot a high-pressure jet of water straight up at the bottom.

It’s incredibly complicated to design a spray arm and rack configuration that can get all your dishes spotlessly clean.

These thought experiments are simple, but it’s a lot more complicated to design a spray arm and rack configuration that can get all your dishes spotlessly clean—especially when you take into account the staggering number of possible dish and stain combinations. That’s why there are so many different dishwasher interior designs.

Most manufacturers have a pet feature (like Samsung’s Waterwall, or Kenmore’s 360° PowerWash Plus) and the rest of the dishwasher is built around that feature. But while the spray arms and placement of dirty dishes are vital, your tableware won’t get clean without the last part of the dishwasher equation…


Dish Soap

In recent years, a major scientific advancement made it possible for dishwasher owners to skip pre-washing their dishes. The advancement? Harnessing the power of food-digesting enzymes.

The enzymes in your dishwasher detergent simulate the ones that live inside your own body and break food down for digestion and nutrient absorption. Normally, these enzymes require a warmer living environment (like your digestive tract), but further research has led to enzymatic degergents that are effective at a wide range of temperatures.

Additionally, rinse aid (more accurately called a “drying aid”) has become more commonplace. This substance gets rid of water spots and speeds up drying by making the dishes hydrophobic. In other words, it prevents water on your dishes from pooling in droplets and instead forces it to run off in sheets.

Thanks to this kind of chemistry, today’s dishwashers produce cleaner and dryer dishes while simultaneously saving water and electricity. That means they can save you lots of money on utilities in the long run.

So here’s the big takeaway: A dishwasher is at its most efficient—and most effective—when all three parts of the equation are working in sync. For that to happen, the manufacturer needs to design a functional machine, and the user needs to load it correctly and use the right detergent.

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