Drop impact on superheated surfaces: short-time dynamics and transition to contact

When a volatile drop impacts on a superheated solid, air drainage and vapour generation conspire to create an intermediate gas layer that delays or even prevents contact between the liquid and the solid. In this article, we use high-speed synchronized reflection interference and total internal reflection imaging to measure the short-time dynamics of the intermediate gas film and to probe the transition between levitation and contact. We observe that the substrate temperature strongly affects the vertical position of the liquid–gas interface and that the dynamic Leidenfrost transition is influenced by both air and vapour drainage (i.e. gas drainage), and evaporation, the latter giving rise to hitherto unreported vertical oscillations of the gas film that can trigger liquid–solid contact. We first derive scaling relations for the height of the gas film trapped under the drop's centreline, called the dimple height, and the minimum film thickness at short times. The former is set by a competition between gas drainage and liquid inertia, similarly as for isothermal impacts, while the latter strongly depends on the vapour production. The gas pressure, at the location where the minimum thickness is reached, is determined by liquid inertia and vapour production and ultimately balanced by the increasing interfacial curvature, determining the minimum thickness. We show that, in the low impact velocity limit, the transient stability of the draining gas film remarkably makes dynamic levitation less demanding than static levitation. We characterize the vertical gas film oscillations by measuring their frequency and monitoring their occurrence in the parameter space spanned by surface temperature and impact velocity. Finally, we model the occurrence of these oscillations and account for their frequency through a hydrodynamic mechanism.

A Methodology for Evaluating the Progression of Damage in a Glass Fibre Reinforced Polymer Laminate Subjected to Vertical Weight Drop Impacts

This study describes a methodology that allows evaluating the behavior of a glass fibre reinforced polymer (GFRP) laminate impacted by a vertical weight drop, analyzing the damage that occurred inside. The purpose of the designers was, by means of characterization tests of the curing processes, evaluation of the cohesion of a particular laminate, application of vertical tests by weight drops and with the use of the readings of an accelerometer in a single direction, know the trend of how intralaminar breaks in the matrix and interlaminar breaks between layers occur. It is proposed to establish the behavior of the laminate before the tests by analyzing curing times, for after the tests by observations with penetrating fluorescent inks. This allows researchers to know the response of the laminate to the loads imposed on the applied structure. For the tests, prepreg material cured outside the autoclave in an oven was used and qualitative quantification of the damage by observing sections of the tested material infiltrated with penetrating fluorescent ink exposed to ultraviolet light.

Towards an Acoustic Simulation of a Water Drop Impacting in a Water Pool

The sound which is produced when a water drop impacts into a water pool is a prominent example for acoustics produced by multiphase flow. In this work the feasibility of numerical methods for simulating this challenging test case is evaluated. First the multiphase flow needs to produce the correct physical mechanisms, e.g. the bubble entrapment. For this an in-house block-structured finite-volume solver with the volume-of-fluid method is used. For the curvature computation a standard finite difference method within the continuum surface force model is employed, including some necessary improvements. A high resolution in space and time is essential and therefore the method is parallelized by domain decomposition. The acoustic part is simulated with the linearized Euler equations which are valid in each phase but need to be adapted in the interface region. The results are compared with numerical and experimental data. It is shown, that the methods are suitable for simple test cases. A coupled drop impact test case corresponds with equivalent experiments until the drop detachment. The acoustic pressure shows a significant rise in the vicinity of the bubble detachment within both phases. However, an oscillation of the cavity bottom can not be observed in the multiphase neither in the acoustic outputs of the airborne signal.

Temperature-regulated adhesion of impacting drops on nano/microtextured monostable superrepellent surfaces

When a drop impacts a monostable superrepellent surface, the solid–liquid wetted region could be regulated by varying the temperature of the surface. We construct connections between the wetting state transition on the microscale and the condensation in the nanotextures.

Analysis and Design of a Novel Concept Gasket to Improve the Reliability of the Balanced Armature Receiver Used in Earphones

Currently, balanced armature (BA) receivers are frequently used in earphones, owing to their small size and superior sound quality. However, the reliability of BA receiver earphones has become a considerable challenge, as they easily fail when subjected to external forces, especially during drop impacts. In addition, the original gasket cannot protect the BA receiver well. Therefore, this article focuses on improving the reliability of BA receiver earphones by designing a novel concept for the gasket. Based on a simplified model and analysis methods, the maximum von Mises stress on the armature with different drop directions and the maximum von Mises stress point must first be determined. The gasket was divided into two parts, one for linking and the other for shock absorption. This article focused on the design of the shock absorption structure. A novel concept gasket was proposed, and the analysis results showed that the gasket improved the shock absorption performance. For demonstrating the validity of the shock absorption performance of the novel concept gasket, three confirmatory experiments were performed: the drop impact test, X-ray photography, and sound performance, which included the sound pressure level and total harmonic distortion. The analysis results were experimentally verified.

Synchronized Multiple Drop Impacts into a Deep Pool

Drop impacts (onto dry or wet surfaces or into deep pools) are important in a wide range of applications, and, consequently, many studies, both experimental and numerical, are available in the literature. However, such works are focused either on statistical analyses of drop populations or on single drops. The literature is heavily lacking in information about the mutual interactions between a few drops during the impact. This work describes a computational fluid dynamics (CFD) study on the impact of two, three, and four synchronized drops into a deep pool. The two-phase finite-volume solver interFoam of the open source CFD package OpenFOAM® was used. After validation with respect to high speed videos, to confirm the performance of the solver in this field, impact conditions and aspects that would have been difficult to obtain and to study in experiments were investigated: namely, the energy conversion during the crater evolution, the effect of varying drop interspace and surface tension, and multiple drop impacts. The results show the very significant effect of these aspects. This implies that an extension of the results of single-drop, distilled-water laboratory experiments to real applications may not be reliable.