The Times Australia
The Times World News

.
The Times Real Estate

.

Slip, slop, slurp! The surprising science of sunscreen, sand and ice cream

  • Written by Shane Keating, Senior Lecturer in Mathematics and Oceanography, UNSW
Slip, slop, slurp! The surprising science of sunscreen, sand and ice cream

Ahh, summer at the beach! The sun on your face, sand between your toes, an ice cream in your hand.

For scientists young and old, a trip to the beach is also a perfect opportunity to explore the peculiar properties of some fascinating fluids.

Through thick and thin

Take sunscreen. When you first squeeze sunscreen from the bottle, it spreads easily over your skin, providing an even protective layer against the Sun’s rays. But once on your skin, sunscreen gains a thicker consistency – it has higher viscosity – preventing it from dripping off.

Read more: Explainer: how does sunscreen work, what is SPF and can I still tan with it on?[1]

Viscosity is the ability of a fluid to keep its shape when a force is applied. Sunscreen is what’s called a shear-thinning fluid, which means rubbing it makes its viscosity decrease so it flows more freely.

This effect typically occurs in fluids containing chain-like molecules called polymers. At rest, the polymers are tangled up in an irregular pattern; but when they are pushed around, they rearrange themselves into layers that slide past each other more easily.

Sunscreen is a ‘shear-thinning fluid’, which means it flows more easily under pressure. Shutterstock

Shear-thinning fluids are quite common. Ketchup is a classic example: it has high viscosity at rest, making it stick to the sides of the bottle until you shake it so its viscosity decreases and it flows out the nozzle.

When the ketchup lands on your plate, its viscosity increases again so it forms a satisfying dollop. (If this is starting to make your mouth water, you’ll be interested to know that saliva is also a shear-thinning fluid.)

Footprints in the sand

The opposite of a shear-thinning fluid is a shear-thickening fluid, a material whose viscosity increases with applied force.

A familiar example is very wet sand: if you pick up a handful, it will flow between your fingers like grainy custard. When you squeeze it, however, the sand becomes firm and, counter-intuitively, appears dry.

This behaviour, called the wet-sand effect, occurs because the compressive force of your hand pushes apart tiny grains of sand, creating space that lets water drain away from the surface.

Wet sand is a ‘shear-thickening fluid’: under pressure (like from a footstep) it becomes firmer and less runny. Shutterstock

The same effect allows you to run on wet sand, producing firm and dry patches where your feet land. But if you stand still and gently wiggle your toes, the wet sand reverts to a liquid state, allowing your feet to sink in – and make a pleasing slurp when you pull them out.

Newton on the beach

Simpler fluids, such as water, have a more or less constant viscosity. These are called Newtonian fluids, after Isaac Newton, who first wrote down the mathematical law to describe them in his famous 1687 book Principia.

To understand viscosity, imagine drinking water through a straw. When you suck, you create lower pressure at the top of the straw than the bottom, drawing water upwards.

The fluid near the walls of the straw experiences friction, so it flows more slowly than fluid near the centre. Newton reasoned the fluid separates into thin layers that slide over each other with a relative speed that depends on the applied force.

Read more: Kitchen Science: the many wonders of humble flour[2]

The viscosity measures the amount of friction between these different layers. The greater the viscosity (think of a milkshake), the more force you must apply to suck the fluid up the straw.

Newton’s law of viscosity, as it is known, is a mathematical ideal. No real fluid behaves exactly this way, but common fluids like water, alcohol, and vegetable oil come pretty close.

By contrast, non-Newtonian fluids — including shear-thinning and shear-thickening fluids — do not obey Newton’s law of viscosity: their viscosity changes depending on how much force is applied to them.

The scoop on ice cream

Time for some ice cream. Ice cream is a frozen mixture of cream, milk, sugar, and flavourings, but it is the unique behaviour of cream that is responsible for the dribbly joy of really good ice cream.

Cream is peculiar stuff. It is the fat-enriched portion of milk, separated from its watery base.

The resulting emulsion of fat globules and a small amount of liquid gives cream its silkiness. When cream is whisked, the applied force breaks the membranes of the fat globules, which glom together around trapped air, producing a suspension of bubbles and cream: whipped cream.

The light, silky texture of ice cream is all due to tiny air bubbles trapped inside little globules of cream. Shutterstock

Whipped cream is a type of non-Newtonian fluid called a Bingham plastic: at rest, it is semi-solid, forming stiff peaks that are perfect for spooning onto strawberries or scones. But under sufficient force, it can flow like a liquid: through the nozzle of a can of instant whipped cream, for example.

As anyone who has made whipped cream by hand knows, the key ingredient is time. The transformation from liquid to semi-solid is caused by applying force over a period of time.

Air bubbles trapped in the cream give ice cream its pillowy softness. In fact, air can make up to 50% of the total volume of ice cream, which explains why it is less dense than water – and why you can use it to make an ice cream float.

Fantastic fluids

Non-Newtonian fluids are found in all sorts of useful substances from biofuels to body armour to blood plasma, and there is still much about them to discover. As Isaac Newton said:

To myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.

What better way to spend a summer day?

Read more: The story of a wave: from wind-blown ripples to breaking on the beach[3]

Read more https://theconversation.com/slip-slop-slurp-the-surprising-science-of-sunscreen-sand-and-ice-cream-169155

The Times Features

Itinerary to Maximize Your Two-Week Adventure in Vietnam and Cambodia

Two weeks may not seem like much, but it’s just the right time for travelers to explore the best of Vietnam and Cambodia. From the bustling streets of Hanoi to the magnificent te...

How to Protect Your Garden Trees from Wind Damage in Australia

In Australia's expansive landscape, garden trees hold noteworthy significance. They not only enhance the aesthetic appeal of our homes but also play an integral role in the local...

Brisbane Homeowners Warned: Non-Compliant Flexible Hoses Pose High Flood Risk

As a homeowner in Brisbane, when you think of the potential for flood damage to your home, you probably think of weather events. But you should know that there may be a tickin...

Argan Oil-Infused Moroccanoil Shampoo: Nourish and Revitalize Your Hair

Are you ready to transform your hair from dull and lifeless to vibrant and full of life? Look no further than the luxurious embrace of Argan Oil-Infused Moroccanoil Shampoo! In a...

Building A Strong Foundation For Any Structure

Building a home or commercial building can be very exciting. The possibilities are endless and the future is interesting. You can always change aspects of the building to meet the ...

The Role of a Family Dentist: Why Every Household Needs One

source A family dentist isn’t like your regular dentist who may specialise in a particular age group and whom you visit only when something goes wrong. A family dentist takes proa...

Times Magazine

"Eternal Nurture" by Cara Barilla: A Timeless Collection of Wisdom and Healing

Renowned Sydney-born author and educator Cara Barilla has released her latest book, Eternal Nurture, a profound collection of inspirational quotes designed to support mindfulness, emotional healing, and personal growth. With a deep commitment to ...

How AI-Driven SEO Enhancements Can Improve Headless CMS Content Visibility

Whereas SEO (search engine optimization) is critical in the digital landscape for making connections to content, much of it is still done manually keyword research, metatags, final tweaks at publication requiring a human element that takes extensiv...

Crypto Expert John Fenga Reveals How Blockchain is Revolutionising Charity

One of the most persistent challenges in the charity sector is trust. Donors often wonder whether their contributions are being used effectively or if overhead costs consume a significant portion. Traditional fundraising methods can be opaque, with...

Navigating Parenting Arrangements in Australia: A Legal Guide for Parents

Understanding Parenting Arrangements in Australia. Child custody disputes are often one of the most emotionally charged aspects of separation or divorce. Parents naturally want what is best for their children, but the legal process of determining ...

Blocky Adventures: A Minecraft Movie Celebration for Your Wrist

The Minecraft movie is almost here—and it’s time to get excited! With the film set to hit theaters on April 4, 2025, fans have a brand-new reason to celebrate. To honor the upcoming blockbuster, watchfaces.co has released a special Minecraft-inspir...

The Ultimate Guide to Apple Watch Faces & Trending Wallpapers

In today’s digital world, personalization is everything. Your smartwatch isn’t just a timepiece—it’s an extension of your style. Thanks to innovative third-party developers, customizing your Apple Watch has reached new heights with stunning designs...

LayBy Shopping