HomeNewsChemistryThe stunning science of sunscreen, sand and ice cream

The stunning science of sunscreen, sand and ice cream


Sunscreen is a ‘shear-thinning fluid’, which implies it flows extra simply beneath stress. Credit: Shutterstock

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


For scientists younger and previous, a visit to the seaside can be an ideal alternative to discover the peculiar properties of some fascinating fluids.

Through thick and skinny

Take sunscreen. When you first squeeze sunscreen from the bottle, it spreads simply over your pores and skin, offering a good protecting layer in opposition to the Sun’s rays. But as soon as in your pores and skin, sunscreen beneficial properties a thicker consistency—it has increased viscosity—stopping it from dripping off.

Viscosity is the power of a fluid to maintain its form when a drive is utilized. Sunscreen is what’s known as a shear-thinning fluid, which implies rubbing it makes its lower so it flows extra freely.

This impact usually happens in fluids containing chain-like molecules known as polymers. At relaxation, the polymers are twisted up in an irregular sample; however when they’re pushed round, they rearrange themselves into layers that slide previous one another extra simply.

Shear-thinning fluids are fairly widespread. Ketchup is a traditional instance: it has excessive viscosity at relaxation, making it persist with the perimeters of the bottle till you shake it so its viscosity decreases and it flows out the nozzle.

When the ketchup lands in your plate, its viscosity will increase once more so it varieties a satisfying dollop. (If that is beginning to make your mouth water, you may have an interest to know that saliva can be a shear-thinning fluid.)

Footprints within the sand

The reverse of a shear-thinning fluid is a shear-thickening fluid, a cloth whose viscosity will increase with utilized drive.

A well-recognized instance may be very moist sand: in case you choose up a handful, it would stream between your fingers like grainy custard. When you squeeze it, nonetheless, the sand turns into agency and, counter-intuitively, seems dry.

This behaviour, known as the wet-sand impact, happens as a result of the compressive drive of your hand pushes aside tiny grains of sand, creating space that lets water drain away from the floor.

The identical impact permits you to run on moist sand, producing agency and dry patches the place your toes land. But in case you stand nonetheless and gently wiggle your toes, the moist sand reverts to a , permitting your toes to sink in—and make a lovely slurp once you pull them out.

Newton on the seaside

Simpler fluids, equivalent to water, have a kind of fixed viscosity. These are known as Newtonian fluids, after Isaac Newton, who first wrote down the mathematical legislation to explain them in his well-known 1687 ebook Principia.

To perceive viscosity, think about ingesting water by means of a straw. When you suck, you create decrease stress on the prime of the straw than the underside, drawing water upwards.

The fluid close to the partitions of the straw experiences friction, so it flows extra slowly than fluid close to the centre. Newton reasoned the fluid separates into skinny layers that slide over one another with a relative pace that relies on the utilized drive.

The viscosity measures the quantity of friction between these totally different layers. The higher the viscosity (consider a milkshake), the extra drive you could apply to suck the fluid up the straw.

Newton’s legislation of viscosity, as it’s identified, is a mathematical perfect. No actual fluid behaves precisely this fashion, however widespread fluids like water, alcohol, and vegetable oil come fairly shut.

By distinction, non-Newtonian fluids—together with shear-thinning and shear-thickening fluids—don’t obey Newton’s legislation of viscosity: their viscosity adjustments relying on how a lot drive is utilized to them.

The scoop on ice cream

Time for some ice . Ice cream is a frozen combination of cream, milk, sugar, and flavourings, however it’s the distinctive behaviour of cream that’s answerable for the dribbly pleasure of actually good ice cream.

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

The ensuing emulsion of fats globules and a small quantity of liquid offers cream its silkiness. When cream is whisked, the utilized drive breaks the membranes of the fats globules, which glom collectively round trapped air, producing a suspension of bubbles and cream: whipped cream.

Whipped cream is a kind of non-Newtonian known as a Bingham plastic: at relaxation, it’s semi-solid, forming stiff peaks which might be excellent for spooning onto strawberries or scones. But beneath ample drive, it will possibly stream like a liquid: by means of the nozzle of a can of instantaneous whipped cream, for instance.

As anybody who has made whipped cream by hand is aware of, the important thing ingredient is time. The transformation from liquid to semi-solid is brought on by making use of over a time frame.

Air bubbles trapped within the cream give ice cream its pillowy softness. In truth, air could make as much as 50% of the total quantity of ice cream, which explains why it’s much less dense than water—and why you need to use it to make an float.

Fantastic fluids

Non-Newtonian fluids are present in all types of helpful substances from biofuels to physique armour to blood plasma, and there may be nonetheless a lot about them to find. As Isaac Newton mentioned: “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 higher method to spend a summer time day?


Researchers develop a viscosity measurement technique for both liquids and gases


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