Weird Effect Discovered When Heated Particles Ended Up Freezing

The concept of freezing particles by warming them is self-contradictory, to put it mildly. However, physicists have demonstrated how particularly designed mixtures ‘melt’ in the dark by crystallizing the moment the lights are on, because of their extraordinary thermal activity.​

Rather than bouncing the particles and disperse them, the team of scientists showed that by using light to heat up the combination, they could lock particles in place and force them to group together, as if they were frozen.

More than a Marangoni Effect

Researchers from the University of Cambridge in the U.K. performed their experiments on a colloid made up of water, polystyrene, and small droplets of oil, covered in DNA to better see the dynamics occurring between them when warmed by light.

As we learned in school, particles suspended in a temperature gradient depart from hot places towards cooler ones. So it would be logical that if we heated suspensions of oil, focusing on the limits with its watery surrounds, you’d expect the combination of particles to bounce in excitement, jolting and grinding their way towards cooler regions and making the fluids move. There’s even a definition and term for this oil and water flow: the Marangoni effect.

So, to put it simply, the diverse surface tension between oil and water makes each vulnerable to variations in temperature in somewhat different ways, compelling their particles to scatter.

[Image: Caciagli etal., Phys Rev. Lett., 2020]
To analyze the impact light has on suspensions of droplets, soft matter physicist Alessio Caciagli and his team covered 20 to 30 micrometer-wide blobs in oil in a polymer that was thickly dusted with single strands of DNA.

These blurry oil balls were then mixed in a suspension with polystyrene spheres about half a micrometer in diameter. The DNA connected the polystyrene to the outer surface if oil droplets, so when the material was hanged in water, it created a loosely bound colloid.

The Effect Light has on Materials

Then, it followed the real deal. Shining a light in the interface between the oil and the water made a single polystyrene clump to stay in place, absorbed by well-understood optical impacts.

Sitting in the laser’s glow, the polystyrene’s temperature increased by about 5 degrees Celsius (41 degrees Fahrenheit), creating a heat gradient against the surrounding water. Normally, the Marangoni effect should be sufficient to disperse the polystyrene spheres and send the colloid away, but strapped together by a foggy mesh of DNA strands, the material instead floats closer together on the surface of the oil drop.

It has been proven that the heat gradient created by the stuck polystyrene generates flow in the two liquids that pull the other suspended particles in close. The outcome is a weird crystallization of colloidal particles triggered by the warmth of a light beam. To make them melt, darkness is all that’s needed.

​This video demonstrates the experiment:

Even though it is not what we might have anticipated, the physics all seem rather fundamental and was tested by the team’s models. Light is showing itself to be a rather versatile tool for manipulating particles.

As the interminable miniaturization of technology requires ever-more-accurate instruments for pulling and poking incredibly tiny materials, the industry could eventually develop nanoscale forceps, and hammers based on the intricate effects light has on matter.

This study was published in Physical Review Letters.

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