A discovery made some 250 years ago by physicist Johann Gottlob Leidenfrost has been exploited by researchers at the Christian-Albrechts-Universität zu Kiel in Germany as a template-free synthesis and patterning method for nanostructures.
In a manuscript titled De Aquae Communis Nonnullis Qualitatibus Tractatus (A Tract About Some Qualities of Common Water), Leidenfrost described a phenomenon in which a liquid in contact with a mass much hotter than the liquid’s boiling point can produce an insulating vapour layer which slows the evaporation of the liquid.
Thanks to the Leidenfrost Effect, it is now possible to bypass the usual 100°C limit for water-based synthesis of nanostructures, and make use of chemical reactions which occur only at much higher temperatures.
“Wet-chemical strategies utilising fluid mechanics appear to be the simplest and most effective way of template-free, self-assembling nanostructuring,” says Rainer Adelung. “However, structuring of nanocluster arrays or wire-like morphologies from a droplet still faces certain challenges.”
In the Lotus Effect, dust particles are removed from the surface of lotus leaves when gathered into droplets moving across the surface. What Adelung and his colleagues have done is reverse this process and deposit nanoparticles carried in a Leidenfrost droplet moving across a surface such as silicon.
The simplest pattern that can be achieved is straight lines, or nanowires, with the liquid droplets allowed to move across the substrate under the influence of gravity. Another possibility is concentric circles.
Semiconducting wires are currently being formed in this way, and the Kiel researchers are working with a company to develop drinking water sensors using their patented Leidenfrost deposition method.
When asked if the method could be used to fabricate arrays of aligned nanotubes, Adelung said that this had not yet been tried, as the focus was on particles and their in-situ synthesis. “Directing nanotubes by flow was already a topic. But here with the overheated droplets, it could lead to new possibilities of structuring on large areas.”
Figure: Schematic of the Leidenfrost deposition process, together with a scanning electron micrograph of zinc oxide nanowires formed in this way (© Rainer Adelung/Christian-Albrechts-Universität zu Kiel).
Further reading: Anti-Lotus Effect for Nanostructuring at the Leidenfrost Temperature, Elbahri et al., Adv. Mat. 19, 1262 (2007).
Article first published in Nanomaterials News.