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We’ve solved the mystery of
the food coloring drops that chase each other
A puzzling observation, pursued through hundreds of experiments, has led Stanford researchers to a simple yet profound discovery: Under certain circumstances, droplets of fluid will move like performers in a dance choreographed by molecular physics.
What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animated gifs. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven.
Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article.
Stanford: http://news.stanford.edu/news/2015/march/dancing-droplets-prakash-031115.html
video 1: https://www.youtube.com/watch?v=LTllH6RnHnQ
video 2: https://www.youtube.com/watch?v=h7o55tyHzxM
video 3: https://www.youtube.com/watch?v=K8Wx2PHIYGI
video 4: https://www.youtube.com/watch?v=ZMsaH6SY4CY
GIFs via freshphotons
A puzzling observation, pursued through hundreds of experiments, has led Stanford researchers to a simple yet profound discovery: Under certain circumstances, droplets of fluid will move like performers in a dance choreographed by molecular physics.
What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animated gifs. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven.
Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article.
Stanford: http://news.stanford.edu/news/2015/march/dancing-droplets-prakash-031115.html
video 1: https://www.youtube.com/watch?v=LTllH6RnHnQ
video 2: https://www.youtube.com/watch?v=h7o55tyHzxM
video 3: https://www.youtube.com/watch?v=K8Wx2PHIYGI
video 4: https://www.youtube.com/watch?v=ZMsaH6SY4CY
GIFs via freshphotons
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