Besides being unhealthy, is Ranch dressing liquid or solid?
By Rae Robertson-Anderson, Professor of Physics
& Biophysics, University of San Diego
.webp)
So, you shake the bottle, and suddenly, a blob of dressing
plops onto your plate. Now it seems more like a liquid.
But unlike water or milk, the ranch doesn’t spread out
across the plate. Instead, it keeps its shape, almost like a pile of mashed
potatoes. That makes it seem more solid again.
Then, when you dip a carrot or celery stick into the blob,
it changes shape. You can smear it around, but it doesn’t resist like a true
solid would. Instead, it moves and spreads, much like a thick liquid.
So, what is ranch dressing? Is it a liquid, a solid, or
something in between?
The Physics of Ranch Dressing
I’m a professor
of physics and biophysics, and I study materials that don’t behave like
traditional solids or liquids. Scientists call these materials soft matter:
things that can be both rigid and flexible at the same time. In my lab, we explore squishy materials
like skin and mucus to understand their unique properties and even design new
materials inspired by them. I also run a social media channel, Physics Mama, where my two
kids and I explore the physics behind everyday life.
The Basic States of Matter
To figure out what’s going on with ranch dressing, you need
to understand what the different
states of matter are and what makes each one unique. “Matter” is just
the scientific word for “stuff,” and it is anything that is made up of the
microscopic building blocks called atoms and
that has mass.
You probably learned in school that there are three states
of matter: solid, liquid and gas. Think ice cube, a puddle of water, and steam.
Maybe you also learned about a fourth state, known as plasma.
These different states are defined by how the extremely
tiny molecules making up the matter interact with each
other. These molecules are so small that you can’t see them with your naked
eye. But their invisible interactions determine the properties of the materials
that you can see.
Molecules and Movements
Molecules in a solid are physically attached to each other
in a way that keeps them from moving around relative to each other. This is
what makes solids rigid and able to keep a fixed shape.
The molecules in a liquid, on the other hand, are not
connected to each other. They can move around, slide past each other, and mix
themselves up. This freedom of movement is what allows a liquid to take the
shape of whatever container it is in.
The molecules in a gas are completely free to move around
without really bumping into the other molecules in the gas too much. Like a
liquid, a gas will take the shape of any container it is in and has no fixed
shape. But unlike liquids and solids, gases can also change their size or
volume.
A plasma is similar to a gas but has much more energy. This
energy causes the electrically charged parts of the molecules, called protons
and electrons, to break apart. The Sun and stars are examples of plasma, as is
the material that makes neon signs
glow.
Understanding Elasticity and Viscosity
While solids hold their shape, they are not completely
rigid. The connections between the molecules behave like tiny springs, which makes solids elastic. If you
push on a solid, it will deform – but it will bounce back to its original state
when you stop pushing, kind of like your mattress when you bounce on your bed.
Of course, this happens at the molecular level, so you can’t see it happening.
And even though liquids easily change shape, they do resist
this change due to the friction between the liquid molecules as they try to
move past each other. This friction is called viscosity. Liquids such
as honey or syrup are much more viscous than liquids such as milk or water,
making them harder to stir. Imagine trying to swim in a swimming pool of honey
– delicious but difficult.
If you haven’t made Oobleck yet, mix 2 cups cornstarch and 1
cup water – hours of fun await you!
Introducing Soft Matter: A Fifth State
Ranch dressing is actually a fifth state of matter known as
soft matter. Soft matter can have properties of both liquids and solids, so
materials scientists say it is viscoelastic
– a combination of viscous and elastic. Other common examples of soft
matter include yogurt, cookie dough, shampoo, toothpaste, silly putty, snot,
slime and frosting.
These substances aren’t quite solid and aren’t quite liquid
– they’re a little of both. You can pour shampoo out of a bottle, but if you
put a bit between your fingers and pull them apart, it will stretch between
your fingers. Cookie dough can hold its own shape, but if you push on it, it
deforms and doesn’t bounce back.
The Science Behind Shear Thinning and Thickening
Many viscoelastic materials exhibit
shear thinning, which means that their viscosity decreases the more you
agitate them. This is why shaking your bottle of ranch dressing or ketchup
allows you to pour it out – even though before shaking it was too solid-like to
leave the bottle. It’s also why yogurt that seems quite solid and able to
maintain its shape becomes more liquid-like when you stir it quickly.
Squishy materials can
also exhibit shear thickening – they become more rigid the harder you
try to deform them. This is how Oobleck,
a simple mixture of cornstarch and water, works. You can slowly pour it and
submerge your hand in it, like any other liquid, but if you squeeze it or shake
it up it solidifies.
The Role of Polymers in Soft Matter
The reason these squishy materials have both liquid and
solid properties is that they’re
made of polymers: long, chainlike molecules. These long chains get all
tangled up, like
a bowl of spaghetti, so they are sort of connected, like the molecules in a
solid, but also sort of free to move past one other, like molecules in a
liquid.
Most store-bought ranch dressing contains xantham gum, which
is a natural polymer used to thicken and stabilize many foods.
So the next time you try to pour your ranch dressing out of
the bottle, you can imagine the xantham gum polymers all tangled up with one
another, making the dressing act like a solid. When you shake the bottle,
you’re disentangling the polymers so they slide and flow past each other,
allowing the dressing to flow easily out of the bottle and onto your plate.
