Shapes and Fissility of Highly Charged and Rapidly Rotating Levitated Liquid Drops

Spontaneous Motion and Rotation of Acid Droplets on the Surface of a Liquid Metal

Self-propulsion of droplets is of great significance in many fields. The spontaneous horizontal motion and self-jumping of droplets have been well realized in various ways. However, there is still a lack of an effective method to enable a droplet to rotate spontaneously and steadily. In this paper, by employing an acid droplet and a liquid metal, the spontaneous and steady rotation of droplets is achieved. For an acid droplet, it may spontaneously move when it is deposited on the surface of the liquid metal. By adjusting experimental parameters to the proper range, the self-rotation of droplet happens. This phenomenon originates from the fluctuation of the droplet boundary and the collective movement of bubbles that are generated by the chemical reactions between the acid droplet and liquid metal. This rotation has a simpler implementation method and more steady rotation state. Its angular velocity is much higher than that driven by other mechanisms. Moreover, the movements of acid droplets on the liquid metal are classified according to experimental conditions. The internal flow fields, the movements and distribution of bubbles, and the fluctuation of the droplet boundary are also explored and discussed. The theoretical model describing the rotational droplet is given. Our work may deepen the understanding of the physical system transition affected by chemical reactions and provide a new way for the design of potential applications, e.g., micro- and nanodevices.

Magnetically induced Rayleigh-Taylor instability under rotation: Comparison of experimental and theoretical results

Our theoretical work has shown that rotating a Rayleigh-Taylor-unstable two-layer stratification about a vertical axis slows the development of the instability under gravity and can stabilize axisymmetric modes indefinitely. Here we compare theoretical predictions directly with our experiments on a rotating two-layer system which is made unstable by magnetic forces applied using a superconducting magnet.

Real-time observation of jumping and spinning nanodroplets

The manipulation of liquids at nanoscale dimensions is a central goal of the emergent nanofluidics field. Such endeavors extend to nanodroplets, which are ubiquitous objects involved in many technological applications. Here, we employ time-resolved electron microscopy to elucidate the formation of so-called jumping nanodroplets on a graphene surface. We flash-melt a thin gold nanostructure with a laser pulse and directly observe how the resulting nanodroplet contracts into a sphere and jumps off its substrate, a process that occurs in just a few nanoseconds. Our study provides the first experimental characterization of these morphological dynamics through real-time observation and reveals new aspects of the phenomenon. We observe that friction alters the trajectories of individual droplets. Surprisingly, this leads some droplets to adopt dumbbell-shaped geometries after they jump, suggesting that they spin with considerable angular momentum. Our experiments open up new avenues for studying and controlling the fast morphological dynamics of nanodroplets through their interaction with structured surfaces.

Spontaneous droplets gyrating via asymmetric self-splitting on heterogeneous surfaces

Droplet impacting and bouncing off solid surface plays a vital role in various biological/physiological processes and engineering applications. However, due to a lack of accurate control of force transmission, the maneuver of the droplet movement and energy conversion is rather primitive. Here we show that the translational motion of an impacting droplet can be converted to gyration, with a maximum rotational speed exceeding 7300 revolutions per minute, through heterogeneous surface wettability regulation. The gyration behavior is enabled by the synergetic effect of the asymmetric pinning forces originated from surface heterogeneity and the excess surface energy of the spreading droplet after impact. The findings open a promising avenue for delicate control of liquid motion as well as actuating of solids.

Controlling droplet impact and rebound behaviour can have applications in inkjet printing and self-cleaning. Here the authors show how a chemically-patterned surface with high-adhesive spirals surrounded by hydrophobic, low-adhesive regions leads to gyration behaviour of impacting droplets.

Analog cosmology with two-fluid systems in a strong gradient magnetic field

We propose an experiment combining fluid dynamics and strong magnetic field physics to simulate cosmological scenarios. Our proposed system consists of two immiscible, weakly magnetized fluids moved through a strong gradient magnetic field. The diamagnetic and paramagnetic forces thus generated amount to a time-dependent effective gravity, which allows us to precisely control the propagation speed of interface waves. Perturbations on the interface therefore experience a nonstationary effective metric. In what follows, we demonstrate that our proposed system is capable of simulating a variety of cosmological models. We then present a readily realizable experimental setup which will allow us to capture the essential dynamics of standard inflation, wherein interface perturbations experience a shrinking effective horizon and are shown to transition from oscillatory to frozen and squeezed regimes at horizon crossing.

Liquid-drop model for fragmentation of multiply charged mercury clusters

The fragmentation of doubly and triply charged mercury clusters is theoretically studied to analyze an experiment performed by Katakuse's group at Osaka University [T. Satoh et al., J. Mass Spectrom. Soc. Jpn. 51, 391 (2003)]. The fission barrier is calculated using a liquid-drop model proposed by Echt et al. In the decay of doubly charged clusters, the barrier height is found to take the minimum value for nearly symmetric fission. On the other hand, in the decay of triply charged clusters, the barrier is the lowest for strongly asymmetric fission. These results well explain the product size distribution observed in the experiment. The appearance size for multiply charged clusters measured in the experiment is found to be the size where the fission barrier is equal to the monomer evaporation energy. These findings provide evidence that small mercury clusters behave like van der Waals clusters in the process of fragmentation.