By Catherine Whitby, University of South Australia
Do you buy expensive moisturisers in a bid to combat the ravages of age, or does catching a mid-afternoon whiff of your pits have you reaching for the roll-on?
We smear various lotions and potions on our skin every day and, while we might take note of their active ingredients, we may not consider how they’re delivered.
Those ingredients are often expensive but in order to work, they need to remain suspended, separate to the lotion, until they come in contact with skin. How is this achieved?
Liquid marbles are one type of delivery vehicle for expensive ingredients in, for example, moisturisers and deodorants.
A drop of liquid is covered by a shell of “hydrophobic”, or water-repellant, powder.
The shell protects the liquid from the outside world, so you can transport the liquid by rolling the drop along surfaces.
They observed that if hydrophobic lycopodium powder spores are poured onto the surface of water, an object inserted into the water will come out covered with powder, but perfectly dry.
Stephen Fry demonstrates the properties of hydrophobic sand.
From this, Aussillous and Quéré realised that a powder-coated drop of water no longer sticks to surfaces. Instead it rolls and bounces.
A major problem for developing liquid marble products has been the limited number of powders hydrophobic enough to coat drops of common liquids.
This changed when the Australian scientist Karen Hapgood started to use liquid marbles to make capsules from powdered drugs. She found less hydrophobic powders, of drugs such as salicylic acid (which is also used in acne treatment), could at least partially coat drops of water.
This is what happens if you coat your finger in superhydrophobic powder and dip it in water.
Recently my colleague Rossen Sedev at the Ian Wark Research Institute and I showed how the particle wettability (the extent to which a liquid wets the particles) determines what happens when a liquid drop is mixed with powder. This will help people designing liquid marbles choose the best combination of powder and liquid.
Dry skin? Try dry water
Although liquid marbles were discovered in 2001, inventors already knew that large volumes of water could be mixed with some powders to produce a fluffy, apparently dry powder, called “dry water”.
Rubbing the cream into skin releases the water and leaves the finished makeup to be spread evenly in a matte finish.
It’s almost oil-free and is found in cosmetics used by people with sensitive skins who avoid oily, emulsion-based foundations.
But practical applications for liquid marbles and dry water aren’t limited to the cosmetic industry.
Liquid marbles are being developed by scientists excited by their potential to act as sensors.
A drop of water coated in fluoropolymer particles will float on a pool of water until the pollutant penetrates into the powder and breaks the shell apart.
The powder coating is also permeable to gas. Wei Shen filled powder shells with indicator solutions and showed that the liquid marbles can detect ammonia and hydrochloric acid gas.
So next time you smooth your pricey moisturiser onto those fine lines and wrinkles, spare a thought for the ingredients released from their powdery prisons to work their magic on your skin.
Catherine Whitby acknowledges receipt of an Australian Research Council Future Fellowship. Her research has been funded by the Australian Research Council, University of South Australia, Australian Academy of Science (COST), FABLS Support Scheme for Emerging Research Projects, Australian Institute of Nuclear Science and Engineering and the National Institute of Standards and Technology (NCNR).
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