將盛有矽油的燒杯,置於垂直的震動臺上,接著調整頻率和振幅,使略低於產生法拉第波的臨界值,接著用長金屬針,在液面挑起一顆小矽油滴。有趣的事情發生了,可以發現油滴開始在液面上行走,並存活超過一天。文獻上已經記載此「行走液滴(walking droplet)」具有類量子的行為,例如:當容器大小有限,邊界效應明顯時,液滴在空間的機率會呈現離散的分佈。我們質疑此現象和類量子(即同樣邊界條件下的薛丁格方程式的穩定機率波(eigenfunctions))的相似,只是巧合,希望檢查這個類比在方形容器是否仍然可以觀察得到。如果結果是否定的,那暗示此巨觀現象背後得機制,可能其實是我們藉由關小震動臺振幅,而壓抑的法拉第波。為什麼呢?因為在震動油滴的撞擊下,原本平靜的矽油表面,有機會藉由獲致來自碰撞的些微能量,重新產生(振幅仍然小到無法用肉眼辨識,必須藉由高速攝影技術才觀察得到的)法拉第波。 我們同時也重新檢視行走液滴的繞射,確定法拉第波在該實驗是否也同樣扮演不可忽略的角色。
We study a classical macro-state that exhibits quantum-like behavior: a droplet that can move and live a long time on a vibrated vessel that contains the same liquid. We are particularly interested at the phenomenon in which the spatial density distribution of the droplet is discrete on a circular corral. Instead of making an analogy to that of a quantum particle in similar confinement geometry, we speculated and went on to prove that the underlying mechanism is in fact the Faraday waves and may exhibit a distribution dissimilar to the quantum prediction in a vessel with different geometry. The way we prove that the distribution adopts the same symmetry as the Faraday waves is by measuring the acceleration of the droplet. Note that, although we followed the proctor of tuning down the vibration amplitude to suppress the Faraday waves, the instability was resuscitated when we poked at the liquid surface to generate a droplet. This tiny amount of input energy turns out to be enough to trigger the Faraday waves, although only faintly and undetectable to bare eyes without the aid of a high-speed camera.
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