A GOOD BOUNCE
Background: Droplets already bounce off water-repellent surfaces, but scientists have engineered materials that speed up the bounce to repel water faster. (Read the Nature paper here.)
Apparently it’s all about contact time. Even the brief contact time between a surface and a droplet that bounces off it can allow water to freeze or collect on the surface and cause damage. Scientists previously thought that contact time was shortest when droplets spread and recoil symmetrically on a material before bouncing off.
In this week’s issue, Kripa Varanasi and colleagues have now shown that a textured surface can cause droplets to recoil with controlled asymmetry, and bounce away faster than was thought possible.
(Watch how it works in this video, with accompanying news story.)
There are of course important applications for things like airplanes and steam turbines, but I’m looking forward to seeing this tech in some trousers!
Design challenge: We were very impressed by the group’s initial images, which show water drops bouncing off the wings of a Morpho butterfly (the ridged wings of the Morpho cause water droplets to recoil asymmetrically), but we needed a very high resolution version of this phenomena to make a suitable cover. So Kripa’s team went to work creating some stunning high res visualizations for us to consider. We were obviously thrilled with the result (above).
The team had to be technically creative to produce the image, as the resolution of high speed cameras is limiting. How did they do it?
This from Kripa:
The impact surface was the rear wing of Morpho Deidamia Briseis (male, Leppidio insect supply).
The drops were produced by pulsing open a solenoid valve connected to a water reservoir, held approximately 10 cm above the wing by a standard lab stand. A photogate was held below the solenoid to trigger a delay module (Cognisys Stopshot), which subsequently triggered the strobe.
The strobe light source (MicroFlash Ultra, HighSpeedFlash) was positioned relative to the wing and lens at an angle of incidence that yielded a purple structural hue. Determination of this angle was made by first fixing the area of interest of the wing in the focal area of the camera, with one of the photographers then moving a hand-held light source around while the other monitored the reflected color through the camera.
Camera and lenses
Camera: Nikon D7000
Lens: Nikon Micro-Nikkor 70-180mm f/4.5-5.6 ED
Focal length: 140
Extension tube: 14mm
Teleconverter: Tamron 1.4x
The photos were shot in a dark room with the camera shutter held in “open shutter” mode so that the exposure was controlled by the duration of the flash source (~ 9 µs) and the F-number.
The photography sequence consisted of the following steps:
1 - manually open shutter
2 - send an “open” pulse to the solenoid to generate a droplet
3 - wait for the droplet to pass through the photogate and trigger the flash source
4 - manually close shutter
This was a collaborative effort between us and Jim Bales and Kyle Hounsell of the MIT Edgerton Center. Photo credits : A. T. Paxson, K. Hounsell, J. W. Bales, J. C. Bird, K. K. Varanasi.”