When placed over a less dense medium, a liquid layer will typically collapse downwards if it exceeds a certain size, as gravity acting on the lower liquid interface triggers a destabilizing effect called a Rayleigh-Taylor instability1,2. Of the many methods that have been developed to prevent the liquid from falling3-6, vertical shaking has proved to be efficient and has therefore been studied in detail7-13. Stabilization is the result of the dynamical averaging effect of the oscillating effective gravity. Vibrations of liquids also induce other paradoxical phenomena such as the sinking of air bubbles14-19 or the stabilization of heavy objects in columns of fluid at unexpected heights20. Here we take advantage of the excitation resonance of the supporting air layer to perform experiments with large levitating liquid layers of up to half a litre in volume and up to 20 centimetres in width. Moreover, we predict theoretically and show experimentally that vertical shaking also creates stable buoyancy positions on the lower interface of the liquid, which behave as though the gravitational force were inverted. Bodies can thus float upside down on the lower interface of levitating liquid layers. We use our model to predict the minimum excitation needed to withstand falling of such an inverted floater, which depends on its mass. Experimental observations confirm the possibility of selective falling of heavy bodies. Our findings invite us to rethink all interfacial phenomena in this exotic and counter-intuitive stable configuration.

中文翻译：

---

在漂浮的液体下漂浮

当放置在密度较小的介质上时，如果液体层超过特定尺寸，它通常会向下塌陷，因为作用在较低液体界面上的重力会触发称为瑞利-泰勒不稳定性 1,2 的不稳定效应。在已开发的许多防止液体下落的方法中 3-6，垂直摇动已被证明是有效的，因此已被详细研究 7-13。稳定是振荡有效重力的动态平均效应的结果。液体的振动还会引发其他自相矛盾的现象，例如气泡的下沉 14-19 或在意外高度的流体柱中重物稳定 20。在这里，我们利用支撑空气层的激发共振对体积高达半升、宽度高达 20 厘米的大型悬浮液层进行实验。此外，我们从理论上预测并通过实验表明，垂直振动也会在液体的下界面产生稳定的浮力位置，就像重力倒转一样。因此，物体可以倒置在悬浮液层的下界面上。我们使用我们的模型来预测承受这种倒置漂浮物坠落所需的最小激励，这取决于它的质量。实验观察证实了重体选择性下落的可能性。我们的发现邀请我们重新思考这种奇异且违反直觉的稳定配置中的所有界面现象。