Most manufactured things break when you subject them to 90% strain; however Professor Xiaoyu Rayne Zheng of the Department of Mechanical Engineering at Virginia Tech has developed additively-manufactured metamaterials that completely recover from being deformed to this level. Strains are usually defined as the change in length divided by the original length and is limited in most engineering structures to less than 2%, which is the level at which steel experiences permanent deformation. Professor Zheng has developed a microstructure with a recurring architecture over seven orders of magnitude that allows an extraordinary level of elastic recovery; and then his team manufactures the material using microstereolithography. Stereolithography is a form of three-dimensional printing. Professor Zheng presented some of his research at the USAF research review that I attended last month [see ‘When an upgrade is downgrading‘ on August 21st, 2019 and ‘Coverts inspire adaptive wing design’ on September 11th, 2019]. He explained that, when these metamaterials are made out of a piezoelectric nanocomposite, they can be deployed as tactile sensors with directional sensitivity, or smart energy-absorbing materials.
Rayne Zheng and Aimy Wissa [‘Coverts inspire adaptive wing design’ on September 11th, 2019] both made Compelling Presentations [see post on March 21st, 2018] that captured my attention and imagination; and kept my phone in my pocket!
Earlier this summer, when we were walking the South West Coastal Path [see ‘The Salt Path‘ on August 14th, 2019], we frequently saw kestrels hovering above the path ahead of us. It is an enthralling sight watching them use the air currents around the cliffs to soar, hang and dive for prey. Their mastery of the air looks effortless. What you cannot see from the ground is the complex motion of their wing feathers changing the shape and texture of their wing to optimise lift and drag. The base of their flight feathers are covered by small flexible feathers called ‘coverts’ or ‘tectrix’, which in flight reduce drag by providing a smooth surface for airflow. However, at low speed, such as when hovering or landing, the coverts lift up and the change the shape and texture of the wing to prevent aerodynamic stalling. In other words, the coverts help the airflow to follow the contour of the wing, or to remain attached to the wing, and thus to generate lift. Aircraft use wing flaps on their trailing edges to achieve the same effect, i.e. to generate sufficient lift at slow speeds, but birds use a more elegant and lighter solution: coverts. Coverts are deployed passively to mitigate stalls in lower speed flight, as in the picture. When I was in the US last month [see ‘When upgrading is downgrading‘ on August 21st, 2019], one of the research reports was by Professor Aimy Wissa of the Department of Mechanical Science & Engineering at the University of Illinois Urbana-Champaign, who is working on ‘Spatially distributed passively deployable structures for stall mitigation‘ in her Bio-inspired Adaptive Morphology laboratory. She is exploring how flaps could be placed over the surface of aircraft wings to deploy in a similar way to a bird’s covert feathers and provide enhanced lift at low speeds. This would be useful for drones and other unmanned air vehicles (UAVs) that need to manoeuvre in confined spaces, for instance in cityscapes.
I must admit that I had occasionally noticed the waves of fluttering small feathers across the back of a bird’s wing but, until I listened to Aimy’s presentation, I had not realised their purpose; perhaps that lack of insight is why I specialised in structural mechanics rather than fluid mechanics with the result that I was worrying about the fatigue life of the wing flaps during her talk.
We are lucky to live in a house with a great view of Liverpool cathedral [see picture in ‘Two for one‘ on January 2nd, 2019]. Hundreds of tourists visit every day and take pictures of the cathedral with their smart phones. A few even turn around and take a picture of our house! It is a modern disease: capturing pictures of a spectacle without actually looking at it and then probably never looking at the photograph. There is some small level of fulfilment in having taken the photograph; however, 120 years ago there were fewer tourists and they had no cameras. Instead, when Charles Rennie MackIntosh visited Naples on April 8th, 1891, he admired the tower of the Church of Santa Maria del Carmine and ‘took a sketch’. It must have taken him some time and concentrated effort. The level of pleasure and fulfilment from taking a sketch must have been much greater than from our modern experience of snapping a photo.
Of course, there was no Liverpool Cathedral in 1891 and ten years later, Rennie Mackintosh was disappointed that his proposals for it were not selected from the 103 submitted.
There is an excellent exhibition of Keith Haring’s work at the Tate Liverpool until November 2019. Keith Haring and I were born a couple of years apart but that’s where the similarity ends. He was an American artist who collaborated with the likes of Andy Warhol and Jean-Michel Basquiat and was influenced by Pablo Picasso, Walt Disney and Dr Seuss. He was part of the New York street culture of the 1980s and many of his early works were forms of graffiti painted in subways and on the sides of buildings. Some people think that art should challenge the way you think about the things; however, “Haring felt that the artist is ‘spokesman for a society at any given point in history’ whose visual vocabulary is determined by their perception of the world”. His work about racism, the excesses of capitalism and the misuse of religion for oppressive purposes seem as relevant today as thirty years ago.
Quotation from the one of exhibition displays with apologies to curators of the Tate exhibition, Darren Pih and Tamar Hemmes.