The inkjet printing of electronics is an area of intense research, and one which has the potential to reduce costs and waste in emerging technologies, such as solar cells. In order to develop this printing technology, an understanding of the drying process of metal nanoparticle solutions is required. We asked Chalmers et al. to discuss their recent paper published in Journal of Physics: Condensed Matter, which makes strides forward in this exciting area of research:
These days, inkjet printing is not just about transferring pictures and text onto paper. In manufacturing, inkjet printing is being used to create a variety of complex structures on surfaces. However, this is not without challenges.
Our work is inspired by one particular application of inkjet printing , in which the electrical connections are inkjet-printed onto a solar panel. The ink itself is a suspension of silver nanoparticles, which dries to form an electrically conducting bridge. The surface onto which it is printed is itself a complex structure: it must dry to form a connection from one metallic region on the surface over to another, across a region of insulating polymer. The polymer is also printed onto the surface at an earlier stage. Metals are hydrophilic, which means that the water solvent in the ink is happy to be on this part of the surface, but the polymer is hydrophobic, meaning it repels water. Thus, the ink has a tendency to move off (dewet) from the polymer part of the surface, so that when it dries there is no bridge and no electrical connection.
We have developed a Monte Carlo model to simulate the ink drying process on such heterogeneous surfaces. Within the model we can study the influence of different surface wettability, i.e. the strength of the attractions between ink solvent/nanoparticles with the different parts of the surface and also variations in the height of the surface. One striking finding from our work is that having a step down from the metal to the polymer part of the surface can significantly enhance the ability of the ink to dry in an unbroken connection. The figure above shows a sequence of snapshots as the ink dries. On the left it is on a flat surface and the bridge breaks during drying, whilst on the right, the step in the surface enables the bridge to remain as the ink dries. Our results show it is important to carefully control the ink and surface interactions to obtain the best connections.
It turns out the popular simile is wrong: watching ink (or paint) dry is not boring, but is a complex and interesting interplay of thermodynamics and fluid dynamics that requires careful control to get right.
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About the authors
The authors are all members of the Mathematical Modelling group at Loughborough University.
Chris Chalmers is a PhD student at Loughborough University, aiming to finish this summer. His interests are in using computers to solve problems.
Roger Smith is a Professor of Mathematical Engineering at Loughborough University, with a long track record of research involving applications of mathematics to industry, including molecular dynamics and long time scale problems, materials modelling, especially radiation damage phenomena and thin film growth.
Andrew Archer is a Senior Lecturer at Loughborough University and also the current Head of the Department of Mathematical Sciences. His research interests lie in the field of soft condensed matter and liquid state theory. He is particularly interested in the behaviour of liquids at interfaces, such as the wetting properties of a liquid and being able to predict the structures and patterns that are formed when particle suspensions dry. He also does research on understanding how and why quasicrystals form.
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