When a droplet of water impacts a hydrophobic surface, the drop is often observed to bounce. However, for about 10 years it has been known that the addition of very small quantities (∼100 ppm) of a flexible polymer such as poly-(ethylene oxide) can completely prevent rebound. This effect has for some time been explained in terms of the stretching of polymer chains by a velocity gradient in the fluid, resulting in a transient increase in the so-called “extensional viscosity.” Here we show, by measuring the fluid velocity inside the impacting drop, that the extensional viscosity plays no role in the antirebound phenomenon. Using fluorescently labeled λ DNA we demonstrate that the observed effect is due to the stretching of polymer molecules as the droplet edge sweeps the substrate, retarding the movement of the receding contact line.
The essence of turning physics into technology relies on one verb: control. Whether it is controlling electricity with magnetism, magnetism with electricity, or controlling spins, currents, or fields, control is the necessary ingredient. Through the shape memory effect, it now becomes possible to control mechanical effects with electromagnetic stimuli.
At present, it is known that almost any Ni-Mn-based Heusler alloy will show a martensitic transition at an appropriate off-stoichiometric composition. In addition, these materials show other interesting functional properties such as magnetoresistance or magnetocaloric effects. The complex behavior displayed by these materials is, to a large extent, a consequence of the strong coupling between magnetism and structure that arises from the martensitic transition as the sample is cooled. Now, in a paper published in Physical Review Letters, Mao Ye and Akio Kimura at Hiroshima University, together with collaborators at Tohoku University, the National Institute for Materials Science, and Tohoku Gakuin University, all in Japan, study the case of Ni2Mn1+xSn1-x alloys to elucidate the role of excess Mn ions in driving the martensitic transition.