Popping the top on house paint usually draws people to look inside the can. But Princeton researchers have turned their gaze upward, to the underside of the lid, where it turns out that pattern of droplets could inspire new ways to make microscopically small structures.
The trick comes in controlling the droplets, which form under competing influences like gravity and surface tension. A new study, published Oct. 26 in the journal Nature Communications, explains how a deeper understanding of these highly dynamic, sometimes unstable forces can be harnessed to cheaply and quickly fabricate objects that normally require a more expensive and time-consuming process.
"We've done away with the molds," said Pierre-Thomas Brun, assistant professor of chemical and biological engineering at Princeton and the principal investigator for the study. "We don't need a clean room or any fancy equipment, so engineers have much more freedom in the design process."
Using a silicone common in medical devices, the team poured a thin liquid film over the surface of a plate, about the size of a compact disc, which they then flipped upside down for several minutes while the film cured.
Without intervention, the liquid silicone congeals into an irregular array of droplets -- much like the paint under a lid. But by etching the plate with mathematical precision, using lasers to cut the marks, the researchers "seeded" the droplets into a lattice of perfect hexagons, each with a uniform dimension.
"Gravity wants to pull the fluid down," said Joel Marthelot, postdoctoral research associate at Princeton and lead author on the paper.
"Capillary forces want the surface to deform minimally. So there is a competition between these two forces, which gives rise to the length scale of the structure."
More sophisticated versions of the experiment used a centrifuge in place of gravity, which allowed the team to vary the size of the drops with an indefinite range. Instead of plates, in this version they used plastic cylinders that look like clear hockey pucks.
The excess fluid spun off and left their predictable pattern of cured drops. The technique worked down to the limit of their machinery, which produced a lattice of structures that were each around 10 microns, a fraction of the width of a human hair. The structures, which are prototypes, simulate the kinds of soft lenses that are a common part in smartphones.
"The faster it spins, the smaller the drops," Marthelot said, noting that they could make structures even smaller than what they had achieved so far. "We don't really know the limit of our technique. Only the limit of our centrifuge."
According to Brun, the kinds of mechanical instabilities that cause this behavior are usually regarded by engineers as a kind of nemesis. They are the physical thresholds that determine weight loads or heat capacities. "In this case," he said, "we took advantage of something that is normally seen as bad. We tamed it and made it functional by turning it into a pathway to fabrication."
The technique can be easily expanded to large-scale manufacturing, the researchers said. As their methods evolve, they plan to create biomimetic devices, like an inflatable compound lens that mimics the eye of an insect, or soft robots that can be used in medical technologies.
"One can envision a wide range of potential future application," said Jörn Dunkel, associate professor of mathematics at the Massachusetts Institute of Technology, "from drag-reducing or superhydrophobic surfaces to micro-lenses and artificial ciliary carpets."
In addition to Brun and Marthelot, two other researchers contributed to the study: Elizabeth Strong, formerly a student at MIT and now a Ph.D. candidate at the University of Colorado, Boulder; and Pedro M. Reis of the Ecole Polytechnique Fédérale de Lausanne.
Scott Lyon | EurekAlert!
Risk of infection with COVID-19 from singing: First results of aerosol study with the Bavarian Radio Chorus
03.07.2020 | Klinikum der Universität München
Age research: A low level of the stress hormone cortisol contributes to the ageing process
01.07.2020 | Universität des Saarlandes
Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells. An international team led by Stefan Weber from the Max Planck Institute for Polymer Research (MPI-P) in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these "electron highways" could make perovskite solar cells even more powerful.
Solar cells convert sunlight into electricity. During this process, the electrons of the material inside the cell absorb the energy of the light....
Empa researchers have succeeded in applying aerogels to microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation over a wide frequency range – and they are unrivalled in terms of weight.
Electric motors and electronic devices generate electromagnetic fields that sometimes have to be shielded in order not to affect neighboring electronic...
A promising operating mode for the plasma of a future power plant has been developed at the ASDEX Upgrade fusion device at Max Planck Institute for Plasma...
Live event – July 1, 2020 - 11:00 to 11:45 (CET)
"Automation in Aerospace Industry @ Fraunhofer IFAM"
The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM l Stade is presenting its forward-looking R&D portfolio for the first time at...
With an X-ray experiment at the European Synchrotron ESRF in Grenoble (France), Empa researchers were able to demonstrate how well their real-time acoustic monitoring of laser weld seams works. With almost 90 percent reliability, they detected the formation of unwanted pores that impair the quality of weld seams. Thanks to a special evaluation method based on artificial intelligence (AI), the detection process is completed in just 70 milliseconds.
Laser welding is a process suitable for joining metals and thermoplastics. It has become particularly well established in highly automated production, for...
02.07.2020 | Event News
19.05.2020 | Event News
07.04.2020 | Event News
03.07.2020 | Life Sciences
03.07.2020 | Studies and Analyses
03.07.2020 | Power and Electrical Engineering