Galaxies under glass
They may look like the latest images from Hubble, but these works are very much here on Earth. They’re what ROMAN HILL creates when art meets the fundamental laws of science.
ROMAN HILL’S CANVAS is 8mm2 of glass; his media commonly available: glycerine, ink, paint and alcohol. Combined, they are aesthetically awesome, and fascinating examples of the principles of fluid dynamics – the field concerned with the movement of liquids and gases, and thus encompassing everything from the evolution of stars and plate tectonics to blood circulation and turbulence (when fluids behave as a chaotic mess).
The latter is a particular interest of Australian fluid mechanist Sophie Calabretto, a senior lecturer in applied mathematics at Macquarie University, “Fluid flows make up a huge part of our reality; they can be dynamic, interesting, incredibly complex.”
We asked her to explain the amazing science at play in the images on the following pages. The principles remain consistent regardless of scale, but still it’s a common occurrence that’s hard to predict and even harder to fully explain.
This is an example of Marangoni convection, which is a surface-tension effect driven by temperature and concentration gradients, says Calabretto. When inks and alcohol are allowed to move over a surface, the alcohol is contributing to a lower surface tension. As it evaporates, the concentration gradient increases, and so does the surface tension. This higher-tension fluid (here on the left of the images) then pulls aggressively on the surrounding lower-tension fluid, drawing towards it the fire-like fans.
For a liquid, the lowest energy state is the smallest surface area to volume, which is a sphere. (It’s why raindrops form the shape they do.) All systems seek a minimum energy state, and in this case, a water-based fluid mixed with alcohol is dropped into an oil mix. The water-based fluid can’t mix with the oil (known as immiscible) but the alcohol can. As the drop lands, it flattens, forcing the alcohol to the surface
– the marbling effect (top) is a surface-tension effect, caused by the alcohol being uneven in spots. As the alcohol is drawn to mix with the oil, the more viscous fluid contracts to its minimum energy sphere.
This fractal veining effect is caused by the Saffman–taylor instability, says Calabretto. Imagine pressing a drop of liquid between two panes of glass: blood on a lab slide, for example. The fluid pushes out in a consistent way, forming a regular shape. When a less viscous fluid (the yellow blob at top left) is introduced to a more viscous (denser) fluid in a confined space, the Saffman–taylor instability – viscous fingering – occurs. The less viscous fluid cannot disperse the more viscous fluid in a uniform way, and creates little finger instabilities as it pushes into the denser fluid in a flat layer. By adding a third, even less viscous fluid (the blue and purple drop, above) the effect is repeated.
This quartet show an optics effect called thin-film interference. When incoming light hits a thin transparent surface (such as these soap bubbles), some light will reflect off its top boundary and some will travel through it to reflect off whatever’s underneath. The reflected rays have travelled different distances, so they may be out of phase and could overlap and interfere with each other. White light comprises all the different wavelengths of light, which correspond to different colours. Depending on the thickness of the bubble, the distance the light has travelled and the angle it hits, there’ll be destructive interference for some wavelengths, removing those colours, and creating patterns with what remains.