Martin van Hecke
Folding a map or a package insert can be difficult. The order in which you make the folds matters. Martin van Hecke designed a rubber object which does not have that problem. When you push on the sides, it elegantly folds itself step-by-step into a small, neat package. This self-folding object is one of the clever metamaterials Van Hecke designed.
Programming mechanical metamaterials
A cube that shows a smiley face on its side when you compress it. A material that contracts more when you compress it less. And a rubber block that pops inwards instead of outwards when you squeeze it together. Metamaterials have properties that do not occur in nature. By designing these materials with strategically placed holes and shapes, Martin van Hecke can ‘programme’ them to respond to input in a specific way.
In September 2018, Van Hecke and colleagues published a paper in Nature about a multi-step, self-folding metamaterial. The article describes a square-shaped object, consisting of a regular pattern of interconnected rubber blocks with some space in between and bendable connections. If you push on the sides, the structure folds into a smaller regular pattern. Push again, and the rubber blocks become neatly aligned, removing all open spaces and making the object even smaller. The order in which the folds occur is programmed into the material.
The folding looks easy, but designing the metamaterials to behave like this takes time. The research is an interplay between theory, computer simulations and experimenting with rubber models, adjusting the locations and thickness of the different connections. ‘We had to discover how thin the connections that fold first should be compared to those that have to bend in the second folding step’, explains Van Hecke.
The research field of metamaterials is relatively new: 10 years old at the most. ‘When we started working on it five of six year ago, I could hardly explain it to my students. It did not even have a name’, tells Van Hecke. ‘But I thought it would be interesting. And I was not disappointed.’
Van Hecke has always enjoyed working on systems which behave in complex and unexpected ways. His earlier research entailed foams and granular materials. During this research it became clear that spatial structure, such as how sand grains are stacked, influences the behaviour of materials. ‘I started wondering whether that could be turned around: designing a structure in a certain way to get a specific behaviour’, says Van Hecke. ‘That is what we have been working on with metamaterials.’
The self-folding materials perform a step-wise process after a simple push. ‘Next, we would like to design the inverse’, says Van Hecke. ‘A material that reacts a certain way after you perform a step-by-step action.’ This requires programmable materials which remember in what order you performed the steps. Inspiration comes from complex materials that have a type of memory, such as sand. ‘This way, my old and new research are connected.’
Van Hecke’s metamaterial research is curiosity driven. The goal is to understand the possibilities and the fundamental principles behind metamaterials in order to design completely new materials with unexpected properties. It seems easy to imagine applications for these type of materials, such as self-(un)folding solar panels for spacecrafts. Van Hecke: ‘History has taught us that future applications will probably be completely different than anything we could try to predict today.’
Martin van Hecke (Amsterdam, 1967) started as a theoretical physicist. After he obtained his PhD in Leiden in 1996, he worked as a postdoc at the Niels Bohr Institute in Copenhagen, where he switched to experimental research with granular materials. This was a new research field at the time. He returned to Leiden in 2008 to start a granular research group. Since 2014, he leads the research group ‘Mechanical Metamaterials’, which is split between Leiden University and research institute Amolf in Amsterdam. ‘I think it is important that a national institute such as Amolf has a good connection with universities.’