Dynamics and morphology of droplet penetrating a soap film

This work reports experimental observation and theoretical explanation of the dynamics and morphology of a droplet passing through a soap film. During the process, the film undergoes four sequential responses: (1) film deformation upon droplet impact; (2) drop–film detachment; (3) coalescence of the film shell with the drop; (4) peel-off of the film shell. Physical models and the corresponding analytical expressions are developed to reveal the underlying physics for the observed four responses. It is identified that the film is an elongated catenoid under continuous stretch by the droplet, and that they separate at the fixed height of 5.8 times of the droplet radius while the detach point is located at the centre of the height. After separation, the droplet is wrapped with a film shell, which is then punctured by the ring tip of the converging surface wave at the impacting Weber number range of [45, 225]. The film shell then coalesces with the droplet, falls off with a fixed velocity and is eventually ejected as a bubble leaving the droplet with a transplanted surface of the soap solution.

Tents: a paradigm of lightness and sustainability in vernacular architecture and in Frei Otto's work

PurposeThe aim of this paper is to explore the relationship between vernacular architecture and Frei Otto's work, searching for shared principles and specific singularities, and testing whether lightness and sustainability can be identified as a common goal.Design/methodology/approachThe study is focused on tents and yurts, as archetypal examples of traditional architecture, and membrane structures and gridshells, as two types of light structures developed by Frei Otto. A comparative analysis of their behavior, form, elements, types, materials and strength has been carried out.FindingsThe survey carried out shows that Frei Otto's innovative tents and gridshells were not based on form imitation of vernacular architecture, but rather on a thorough understanding of physical form-generating processes, driving specific materials to optimal form, like his experiments with soap film models to generate tensioned minimal surfaces or his experiments with hanging chain net models to generate compressive antifunicular lattice shells.Originality/valueThis paper highlights how Frei Otto's endeavor to get the maximum with the minimum, to achieve a lot from a little, is also a key target of lightness and sustainability, and an essential feature of vernacular architecture.

Soap Film Visualization of a 10 cm-Span Flapping Wing

Flapping wing micro-air-vehicles (FWMAVs) animate the small-space dexterous flight, hovering, and energy-saving characteristics of birds and insects, and are believed to have enlightenment for the development of bionic flight in the future. When designing FWMAVs, detailed unsteady aerodynamic information is required. Besides the computational fluid mechanics (CFD) technology study, the flow visualization is also needed to assist this research. This article innovatively used soap film visualization with high-speed photography to record two kinds of the 2D flow fields laterally and longitudinally, respectively, generated by a flapping wing of 10 cm span. Different from the qualitative comparison of soap film imaging with the conventional smoke tracing method, the subsequent processing of the soap film images was demonstrated. This work explains how to quantify the soap film imaging into lift and thrust forces, and the corresponding results are compared with the wind tunnel force measurement data preliminarily.

Soap film catastrophes

Earlier systematic experimental studies of bursting soap films by McEntee & Mysels (J. Phys. Chem., vol. 73, 1969, pp. 3018–3028) revealed the existence of a precursor shock wave preceding the expanding hole in a punctured film, with a disturbed region of shrinking film material in between known as the ‘aureole’. In the present work we report and interpret new phenomena associated with the aureole – the formation of folds on the surface of soap films. In search of the theoretical explanation of the experimentally identified conditions under which the folds appear, we establish that they correspond to catastrophes of collapsing soap films.