We investigate the motion of a gas-liquid interface in a test tube induced by a large acceleration via impulsive force. We conduct simple experiments in which the tube partially filled with a liquid falls under gravity and hits a rigid floor. A curved gasliquid interface inside the tube reverses and eventually forms a so-called focused jet. In our experiments, there arises either vibration of the interface or an increment in the velocity of the liquid jet, accompanied by the onset of cavitation in the liquid column. These phenomena cannot be explained by a considering pressure impulse in a classical potential flow analysis, which does not account for finite speeds of sound or phase changes. Here we model such water-hammer events as a result of the one-dimensional propagation of a pressure wave and its interaction with boundaries through acoustic impedance mismatching. The method of characteristics is applied to describe pressurewave interactions and the subsequent cavitation. The model proposed is found to be able to capture the time-dependent characteristics of the liquid jet.

Solvent exchange is a simple solution-based process to produce surface nanodroplets over a large area. The final size of the droplets is determined by both the flow and solution conditions for a given substrate. In this work, we investigate the growth dynamics of surface nanodroplets during solvent exchange by using total internal reflection fluorescence microscopy (TIRF). The results show that during the solvent exchange, the formation of surface nanodroplets advanced on the surface in the direction of the flow. The time for the number density and surface coverage of the droplets to reach their respective plateau values is determined by the flow rate. From the observed evolution of the droplet volume and of the size of individual growing droplets, we are able to determine that the growth time of the droplets scales with the Peclet number Pe with a power law ∝Pe-1/2. This is consistent with Taylor-Aris dispersion, shedding light on the diffusive growth dynamics during the solvent exchange. Further, the spatial rearrangement of the droplets during coalescence demonstrates a preference in position shift based on size inequality, namely, the coalesced droplet resides closer to the larger of the two parent droplets. These findings provide a valuable insight toward controlling droplet size and spatial distribution.

Biphasic chemical reactions compartmentalized in small droplets offer advantages, such as stream-lined procedures for chemical analysis, enhanced chemical reaction efficiency and high specificity of conversion. In this work, we experimentally...

Striking the top of a liquid-filled bottle can shatter the bottom. An intuitive interpretation of this event might label an impulsive force as the culprit in this fracturing phenomenon. However, highspeed photography reveals the formation and collapse of tiny bubbles near the bottom before fracture. This observation indicates that the damaging phenomenon of cavitation is at fault. Cavitation is well known for causing damage in various applications including pipes and ship propellers, making accurate prediction of cavitation onset vital in several industries. However, the conventional cavitation number as a function of velocity incorrectly predicts the cavitation onset caused by acceleration. This unexplained discrepancy leads to the derivation of an alternative dimensionless term from the equation of motion, predicting cavitation as a function of acceleration and fluid depth rather than velocity. Two independent research groups in different countries have tested this theory; separate series of experiments confirm that an alternative cavitation number, presented in this paper, defines the universal criteria for the onset of acceleration-induced cavitation. . More commonly, sudden valve closing inside of a water pipe can cause a loud hammering sound due to the collapse of cavitation, resulting in damage on the inner walls (4). The effects of blast and impact that cause traumatic brain injury from cavitation have also been investigated (5, 6). To avoid cavitation-induced damage (7-9), it is crucial to predict the onset of cavitation.Cavitation onset in a high-speed flow [e.g., flow around propellers and in pumps (10)] can be characterized using a variant of the Euler number known as the cavitation number (11,12). The cavitation number is typically of the formwhere pr is the reference pressure, pv is the liquid vapor pressure, ρ is the liquid density, and v is the local velocity (13-16). This cavitation number is a ratio of the pressure difference to the pressure drop due to the fluid momentum. The large momentum of the fluid dominates when C 1, inducing cavitation. However, when C 1, the pressure at all locations is above the threshold for bubble formation, making cavitation unlikely. Practically, several advanced coefficients have been proposed to estimate cavitation inception for various geometries (16).However, the conventional cavitation number (C ) could incorrectly predict the cavitation onset in a liquid accelerated in a short amount of time. For example, the cavitation event that breaks the bottle in Fig. 1A has a maximum velocity of v ≈ 2 m/s, which yields the cavitation number C ∼ O(10 2 ). For the falling tube case in Fig. 1B a similar calculation leads to C ∼ O(10 2 ) (Table S1), whereas the conventional cavitation number requires C 1 (16). Thus, a new cavitation number is required to define the physics of cavitation onset.In the past, researchers have conducted similar experiments with a bullet-piston device to predict the tensile strength of a liquid, including pure water (17). They reported that the ...

We investigate the impact and penetration of a solid sphere passing through gelatine at various impact speeds up to 143.2 m s −1 . Tests were performed with several concentrations of gelatine. Impacts for low elastic Froude number Fr e , a ratio between inertia and gelatine elasticity, resulted in rebound. Higher Fr e values resulted in penetration, forming cavities with prominent surface textures. The overall shape of the cavities resembles those observed in water-entry experiments, yet they appear in a different order with respect to increasing inertia: rebound, quasi-seal, deep-seal, shallow-seal and surface-seal. Remarkably, similar to the We-Bo phase diagram in water-entry experiments, the elastic Froude number Fr e and elastic Grashof number Gr e (a ratio between gravity and gelatine elasticity) classify all five different phenomena into distinguishable regimes. We find that Fr e can be a good indicator to describe the cavity length H, particularly in the shallow-seal regime. Finally, the evolution of cavity shape, pinch-off depth, and lower cavity radius are investigated for different Fr e values.