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Droplet formation caught on camera

Those pesky raindrops that get you wet on your way to work, are formed high up in the sky from clouds of water vapour. The process of nucleation describes the way this happens. Edgar Blokhuis of the Leiden Institute of Chemistry developed a theory to describe this transition more precisely. Chemists from the University of Amsterdam have now confirmed his theory with experiments. On 14 December the research was published in Physical Review Letters.

Too small for the microscope

Blokhuis had not imagined his theory could be tested by experiment so quickly. ‘The molecules I describe are too small to study with a microscope,' he explains. However, the Amsterdam research group of Peter Schall studied colloidal particles. With a diameter of a few nanometers to micrometers, these are many times larger than molecules. Although they are still not visible to the naked eye, you can study colloidal particles with the microscope. In this way the researchers were able to follow the nucleation process closely. The results turned out to correspond surprisingly well with earlier calculations by Blokhuis.

Refinement of theory

The results will allow researchers to refine the classical nucleation theory. Substances occur in three different phases: solid, liquid and gas. During nucleation, a new phase arises from the old one. This starts with the formation of a small nucleus. Think, for example, of cloud formation when small drops of water (liquid) are formed from the water vapour (gas) in the air. You can start this process abruptly by adding a substance in a third phase. Think of the cola eruption that occurs when you put a Mentos candy in the bottle and suddenly a lot of carbon dioxide gas arises. However, if nucleation takes place without the addition of other substances, it can take a lot longer. Although researchers can experimentally determine this nucleation time accurately, with the classical theory a theoretical underpinning appears problematic.

Surface tension

According to Blokhuis this is because the classical theory does not take into account the fact that the surface tension of the nucleus depends on the curvature of the surface. Blokhuis: 'The formation of a droplet depends, among other things, on the surface tension of the droplet in the making. If the formed nucleus is too small, it decreases in size as due to the surface tension, such as an inflatable balloon shrinks when the air escapes. However, if the nucleus is larger than a certain critical size, the nucleus grows and becomes visible to the naked eye.

Curvature

Blokhuis suspected that the curvature of the droplet influences this surface tension. Therefore, this curvature must be included in calculations. He compares two extremes: 'You cannot calculate the surface tension of a small droplet in the same way as that of a flat liquid surface. A flat surface has no curvature, or: the radius is infinite, while a drop does have curvature. The surface tension depends on the radius, and so the surface tension of a flat surface differs from that of a droplet. The difference gets bigger the smaller the droplet is and disappears when the droplet is infinitely large.

Food and cells

‘The evidence is an important step in the realisation that we must include the curvature dependence of the surface tension in order finally to better explain the experimentally found nucleation times', says Blokhuis. Together with his colleagues, he now extends the theory to systems in which molecules are adsorbed to the surface, such as soap molecules and lipids. Soap molecules settle on the surface of dirt and water and pull the dirt loose and into the water. They are widely used in the food and cosmetics industry. Lipids are the building blocks of membranes that protect the cell from the outside world. Together with Norwegian scientists, we made a breakthrough earlier this year and are now working on calculating all the consequences for experimental systems. A first publication was chosen earlier this year as "Editor's Choice" in The Journal of Chemical Physics.

Article of the University of Amsterdam: Nucleation of liquids visualised

Full paper: V. D. Nguyen, F. C. Schoemaker, E. M. Blokhuis, & P. Schall, 2018. Measurement of the curvature-dependent surface tension in nucleating colloidal liquids. Phys. Rev. Lett. 121:24

The emergence of nuclei filmed by the group of Peter Schall (UvA)

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