I have a bottle of vodka that has a load of gold flakes suspended in it. It has been sat still for over 24 hours and the flakes are all still suspended within the liquid: they have not risen to the surface or sunk to the bottom. Any ideas as to the physics behind this?
The viscosity of water ethanol mixtures isn't especially high, though the wetting properties of vodka may make it seem oily. Actually water ethanol mixtures are highly non-ideal: both water and ethanol have a viscosity of about 1 mPa.s at room temperature, but a mixture can achieve a viscosity of over 3 mPa.s. See this paper or Google for many such tables.
The real reason gold leaf will stay suspended for so long is that it is extraordinarily light. The thickness of gold leaf is around 100nm, and since the density of gold is 19300 kg/m$^3$ a flake of gold leaf 1mm by 1mm weighs just 2 $\mu g$ so the downward force due to gravity is 20 nano-Newtons. At such low forces water is viscous enough to keep the gold suspended for long periods.
I'd imagine the viscosity of the vodka is pretty high, and this is why the gold flakes are not rising or sinking within the bottle. Moreover, the viscosity of Vodka has no absolute numerical value as brands vary, but it's pretty high.
I diluted some vodka with flakes and found that they sank. Also if you spin the bottle and let go the liquid inside rotates forward and then backwards. Must be a non newtonian fluid. I suggest it has long chain molecules formed by an organic addative. These may be trapping the gold in suspension. My bottle has been standing for over a month and there is no sign of movement. May have to taste some more tonight.
My theory. The gold weighs the same as the vodka it displaces. Therefore it suspends rather than sinks or floats.
protected by Qmechanic♦ Aug 3 '14 at 14:44
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