These may look delicious, but unless you have a hankering for candle wax I wouldn't eat them. This collection represents a series of experiments in spinning magnetically levitating droplets. When spheres are spun up they tend to flatten at the poles and bulge at the equator as a tug of war begins between the force holding it together (in liquid drops it's surface tension, in stars it's gravity, and in atoms it's the strong nuclear force!) and the centrifugal force - the same force that tugs at you on a merry-go-round. Eventually, this pancaking becomes unstable and it evolves into a peanut like shape which then pulls itself apart into two spheres. Here I recreated all these steps along the way by melting candle wax, levitating it, spinning it, and then letting it solidify.
The Rayleigh-Taylor instability
When a light fluid is floated upon a heavy fluid, (such as oil on water), the fluids come to rest as distinct layers. However, if we attempt to floaty a heavy fluid on top of a lighter fluid, the layers will exchange positions through the growth of discrete plumes. Here I showed that we could use a magnet to induce this exchange by having the top layer lighter, but more magnetically attracted to the magnet below. I then further showed that if the layers are rotating as the exchange begins, the plumes grow smaller and slower. This is an effect of the Coriolis force, the same force that gives hurricanes their rotation (but definitely not what makes the water in your toilet go a certain way!).
Magnetically levitating busmuth crystal
Bismuth crystals have both a striking stepladder structure, and also an array of beautiful colours. This is known. What you may not know is that bismuth is highly diamagnetic, ideal for levitating using magnetic fields - google levitating frogs. Do it. I used a black sponge for a starry effect background.
If you happen to have a diamagnetic levitator and an air nozzle lying around, this is easy peasy to reproduce. Just blow on your levitating drop, and it will slowly spin faster and faster. Along the way, we see a series of shapes as it evolves from a spherical to a peanut drop. This doesn't work with high viscosity fluids, so I'll leave it to you to figure out why a water drop would become pentagonal.
Snowflakes on a string
What happens when you create a stable temperature gradient around a string in a humid environment? Pretty ice, that's what.
How I got my PhD
It's pretty well understood how a drop of spilt coffee turns into a dried ring shape. What was not so clear was how a drop of polymer solution can do the reverse; pile up dried material in a central mound. Figuring this very specific and complicatd problem was how I got my doctorate. Yes, I watched things dry and now I'm a Dr. This particular image shows how the polymer behaves at different chain lengths, with short polymers top left, long polymers bottom right. The scale bar represents 2 mm.
Dried droplet series: saline. Two things happen when salt solution dries: large square crystals form, and a thin "creeping" crystallisation front spreads over the surface (particularly if it is glass). Here I've captured both, with the large crystals being slightly out-of-focus due to being significantly taller than the thin creep-layer.
Dried droplet series: Green ink. Despite my obvious expectation of something green-looking, when viewed under a reflective microscope at x10 magnification, green ink appears orange, with spiky crystals in the central region. Under a transmission microscope, it looks purple!
Dried droplet series: colloidal. In many situations in industry it is useful to create mixtures of water (or other solvents) and tiny spheres - spheres small enough that they drift around rather than sediment like sand would. The spheres pack into layers as the drop dries, and under a reflective microscope, the layers gain distinct colours depending on their thickness due to light interference. Here, orange is one layer thick, purple is around 5 layers thick, and green is many layers thick.
That's no moon!
It's a dried droplet of a high concentration, long chain polymer solution. I can't explain the hole. It must have watched Rogue One recently.
Polymer solution 1
Dried droplet series: Polymer solution. The way polymers dry depends on lots of factors, such as the humidity, polymer chemistry, concentration, temperature ... It's complicated - that's why I managed to stretch the study out into an entire PhD. Here I show what one particular solution dries into when viewed under cross-polarized filters, which darkens everything but regions that "twist" light rays (see birefringence for more info.)
Polymer solution 2
Dried drop series: polymers (again!). This time I ramped up the concentration, and rather than a thin crystalline deposit, it dries into a tall central pillar. The ins and outs of how this happens is what my PhD thesis was all about.
Dried drop series: blood. Blood is a pretty complex solution, containing a solvent, polymers, colloids, and miscellaneous. When we let a drop dry, it tends to first stick to the surface, and then form a complex crack pattern at the late stages. Horse blood is especially weird, as circular non-penetrative stress cracks form as well. Why? That's an open question, but similar circular cracks happen in blood from people with hypoxia; a condition where very little oxygen gets to the tissues. I suspect that the old horse blood just doesn't have any oxygen bound to the red blood cells anymore, which is enough of a change to give a different dryout pattern.
Thanks to Will Turner for doing the initial experiment that inspired this image, and for getting me to kick myself into gear and image all these other droplets too!
Dried drop series: fairy liquid. It might look like some kind of alien eye ball, but that thing in the middle is just a bubble that rose to the top of the drop. The rest formed some sort of lamella phase, and became birefringent (hence visible under cross-polarised light filters) as it solidified.
Fairy liquid 2
... More or less the same as the previous, but this one I accidentally smeared across the slide as I put it under the microscope. That's science in action!
... For now, this particular "photo" I've put together is a secret. As soon as the paper is published, I'll update this description and end the suspense.
Is it a clam? Is it a fairground balloon? No, it's water (mostly)! I noticed a long time ago during my PhD days, that if I made a little enclosure to do my drying drop experiments in using superglue, it affected the drops by making them look all wrinkly. Here I tried to take this effect to the extreme. I surrounded a water drop with a ring of superglue, and hastened to take photos while it was wrinkling. Here is the result. It's still almost entirely water, but with a very thin layer of condensed glue vapour on top.
Dried drop series: mica.
There's this stuff called mica which scientists sometimes use to image fluid flows, and v05 use to give their shampoo a bit of shine. Here I added some to water and looked at what was left after a droplet dried. Colourful!
Dried drop series: Egg white!
Okay I didn't have an egg on me when I had this idea, but my lab does have albumen, which is basically the same thing. Mixing it up made the solution quite bubbly, and I think the bubbles are what created the shiny central region. Either way, it's pretty.
Dried drop series: SDS
SDS is basically just an industry standard surfactant; a molecule that, like a soap, likes to sit at the interface between water and air (or water and oil) and reduce the surface tension. Dried surfactant solutions also look particularly good under cross-polarised filters, as demonstrated by the fairy liquid drop! This image seems to have a sort of Maltese-cross effect going on, but I've no idea of the cause of that!
Dried drop series: K2SO4
Not to be confused with K2SO, everyone's favourite overly honest rebel alliance droid, K2SO4 (potassium sulphate) is just a salt, with a slightly different crystal structure and creeping behaviour to table salt. The crystals look like they're falling off the edges and into a deep chasm!