From bouncing to boxing.
From bouncing to boxing.
复制标题
从弹跳到拳击。
作者：
D. Terwagne;T. Gilet;N. Vandewalle;S. Dorbolo
当液滴放在液体浴上时，一旦液滴和液体浴之间的空气膜被排出，它最终会聚结。避免此问题的一种方法是根据正弦信号 A sin t 垂直摇动浴槽，其中 A 是振幅，是角频率，t 是时间。当浴A 2 的最大加速度大于阈值th时，持续弹跳是可能的，阈值th取决于液滴尺寸、液滴粘度和强制振荡的频率。最近，我们研究了当油浴的粘度比液滴大 100 倍左右时会发生什么。在这样的系统中，液滴的变形比熔池的变形重要得多。这些实验表明，必须考虑液滴的变形来解释弹跳。事实上，已经观察到一些共振效应，即给定频率下阈值加速度的减小：可能会激发各种变形模式。基本上，这些模式对应于瑞利勋爵发现的自然激发模式。另一方面，在恒定频率下，会激发单一变形模式。然而，以与弹跳球类似的方式观察不同的弹跳模式。将粘度为 1.5 cSt、半径为 0.94 mm 的液滴滴在以 20 Hz 频率垂直振动的 1000 cSt 油浴上。调整振幅 A，从而修改加速度 。考虑了三种加速度：重力 g 的 0.4、0.9 和 1.3 倍。请注意，由于液滴变形，当加速度低于 g 时，可能会观察到持续弹跳。为了显示弹跳模式的证据，每个连续快照的垂直切片已并置。结果如图 1 所示，对应于非线性图像画廊 http://chaos.aip.org/chaos/gallery/index.jsp 上的电影。黑色区域的上边界对应于液滴的顶部，而位于每个图底部的规则正弦曲线代表浴的运动。对于低加速度 = 0.4g，液滴以与浴液相同的频率弹跳（图 1，顶部）。对于中间加速度=0.9g，大跳跃之后是小跳跃图1，中；弹跳周期加倍。当跟踪液滴顶部时，这一点尤其明显。对于大加速度=1.3g，弹跳是混乱的并且液滴发生巨大变形：花生形状（图2），观察到小液滴的内陷、喷射和排出。液滴最终与浴合并。 T.G.和 S.D.感谢 FRIA/FNRS 的财政支持。这项工作受益于 COST P21“液滴物理”计划 ESF。
When a droplet is laid on a liquid bath, it eventually coalesces as soon as the air film located between the droplet and the bath is drained out. A way to avoid this issue is to vertically shake the bath according to a sinusoidal signal A sin t , where A is the amplitude, is the angular frequency, and t is the time. Sustained bouncing is possible when the maximum acceleration of the bath A 2 is larger than a threshold value th that depends on the droplet size, the droplet viscosity, and the frequency of the forcing oscillation. Recently, we studied what happens when the oil bath is about 100 times more viscous than the droplet. In such a system, the deformations of the droplet are much more important than those of the bath. These experiments have shown that the deformation of the droplet has to be taken into account to explain the bouncing. Indeed, some resonance effects have been observed, i.e., a decrease of the threshold acceleration at given frequencies: various deformation modes may be excited. Basically, these modes correspond to the natural excitation modes discovered by Lord Rayleigh. On the other hand, at constant frequency, a single deformation mode is excited. However, different bouncing modes are observed in an analogous way as a bouncing ball. A droplet with viscosity 1.5 cSt and radius 0.94 mm is dropped on a 1000 cSt oil bath that is vertically vibrated at a frequency of 20 Hz. The amplitude A is tuned, which modifies the acceleration . Three accelerations have been considered: 0.4, 0.9, and 1.3 times the gravity g. Note that sustained bouncing may be observed for accelerations below g thanks to the droplet deformation. In order to show evidence of the bouncing mode, vertical slices from each successive snapshot have been juxtaposed. The results are shown in Fig. 1 and correspond to the movie on the Gallery of Nonlinear Image http://chaos.aip.org/chaos/gallery/index.jsp . The upper border of the black area corresponds to the top of the droplet while the regular sinusoidal curves located at the bottom of each figure represent the motion of the bath. For a low acceleration =0.4g, the droplet bounces at the same frequency as the bath Fig. 1, top . For an intermediate acceleration =0.9g, a big jump is followed by a small one Fig. 1, middle ; the bouncing period is doubled. That is particularly visible when tracking the top of the droplet. For a large acceleration =1.3g, the bouncing is chaotic and the droplet is enormously deformed: peanut shapes Fig. 2 , invaginations, jets, and expulsions of small droplets are observed. The droplet eventually coalesces with the bath. T.G. and S.D. thank FRIA/FNRS for financial support. The work has benefited from COST P21 “Physics of droplet” program ESF .