The brand of eye make-up remover I use comes in a clear plastic bottle. When new and unused, the liquid settles into two distinct layers, in a 50:50 proportion. I shake the bottle before I use it so the two halves are completely mixed. Despite this, with use, the proportion begins to vary. After I have used about a third of the bottle, the proportions are roughly two-thirds to one-third, with the upper layer becoming twice the depth of the lower. By the time half the bottle has been used up, there is a very thin bottom layer, with the majority of the liquid being in the upper layer. What is going on, and might it in any way alter the efficacy of the product?
鈥� Your correspondent does not say if she inverts her bottle of eye make-up remover to use it but it may be significant. The two immiscible liquids from which it is composed are clearly of different densities. Even after a thorough shaking, globules of the higher-density component will settle to the bottom slightly faster than those of the lighter one. If the bottle is inverted, the top becomes the bottom, and slightly more of the higher-density component will soak into the cotton wool pad than the lighter one. As a result, over the lifetime of the bottle the high-density component will be used up faster than the lighter one.
鈥淓ven after a thorough shaking, globules of the higher-density component of the liquid will settle to the bottom slightly faster than the lighter one鈥�
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A simple experiment to test this would be to shake the bottle and then, still holding it upright, to shake it hard with cotton wool held over the top so that some of the mixture splashes onto the pad. Over time, the lighter top liquid should diminish more than the heavier one or, if the shaking is vigorous enough to maintain an even mixture, both the layers will diminish at roughly the same rate.
Adrian Bowyer, Department of mechanical engineering University of Bath, UK
鈥� One layer in the make-up remover is probably organic, while the other is aqueous. I assume that the writer is splashing the liquid onto tissue or cotton wool to use it. Natural fibres in tissue paper and cotton wool tend to have an affinity for water because they contain celluloses that form hydrogen bonds to the water molecules. If the mixture is tipped onto the tissue or cotton wool, the aqueous layer will be preferentially absorbed, so that over time a skewed ratio appears in the bottle. As most organic compounds are less dense than water, it is the lower that is aqueous, and so is used faster.
Edward Farrington, Gothenburg, Sweden
鈥� The make-up remover comprises two liquids, one of which, xylene for instance, may be more volatile than the other, perhaps water. A greater proportion of the volatile liquid evaporates into the air space in the bottle, so when the cap is removed lots of volatile vapour and only a little non-volatile vapour are lost. Holding the bottle and shaking it helps this process along. Warm it in your hand, and even more of the volatile liquid will evaporate. The more often the bottle is opened, the more quickly the proportions will become uneven.
Rudolf Heitz, North Beach, Western Australia
鈥� Will the changing proportions of the two layers alter the efficacy of the product? I doubt that it will, because water or cream used on their own on a cotton pad will remove some kinds of eye make-up. However, as every cosmetic manufacturer knows, given the right wording, you can persuade people to buy anything.
Gillian Coates, Trefor, Anglesey, UK