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Soap bubbles as a research template for advanced ultra-thin materials

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Posted December 22, 2014
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A thin layer of dried soap-gelatin solution, holding the weight of a metal pellet. Image courtesy of the researchers.

How about making some soap bubbles, anyone? As simple as it may seem the soap bubbles present a lot of thought for scientific minds who are developing new methods to harness the extraordinary potential of such medium.

The nonlinear properties of the transparent thin liquid film (the thickness of which is down to a micron, regardless of its chemical composition) are manifested by the formation of narrow light passages which emerge when a light beam of even a small-power (mW) flux is introduced into an ultra-thin sheet of soapy material.

The nature of formation of such ‘tracks’ is associated with properties of film deformation: even a slight pressure of light radiation is enough to bend and stretch a transparent layer. 

E series of soap-gelatin bubbles 3-5 cm in diameter. The lifetime - more than a week.

A series of soap-gelatin bubbles 3-5 cm in diameter. The lifetime – more than a week. Image courtesy of the researchers.

Iridescent light tracks forming in the surface of soapy bubble often resemble a trapped lightning. These structures with width of 10-20 μm sometimes can get tens of centimeters long. And soapy films are not only an object of simple fascination as they attract a lot of scientific and practical interest. But to study these optical features is difficult enough, because they spontaneously relocate, change their way, and their relatively low brightness require a highly sensitive high-speed equipment.

It is possible to slow down the rate of changes of light channels inside the films, if films a prepared from high viscosity liquid solutions, say the Russian scientists describing their research at the website fian-inform.ru. To accomplish this, the researchers investigated water-based films containing special chemical compounds – surfactants – which modify the surface tension of the liquid. As they note, a behavior of thin film of aqueous gelatin exhibited probably the most interesting viscosity properties. Using this material, the light tracks can be observed in the liquid form, but they disappear as soon as the film dries and hardens. 

The lifetime of the liquid phase of such a film can be increased by changing its chemical composition by adding a specially selected soap and glycerin to an aqueous gelatin solution. To decrease the drying duration, the film can be placed in a sealed volume.

A mirror-like reflection from the frozen soap-gelatin film, diameter 57 mm, varying surface curvature.

A mirror-like reflection from the frozen soap-gelatin film, diameter 57 mm, varying surface curvature. Image courtesy of the researchers.

The unusual properties of soap-gelatin film with the addition of glycerol allowed to slow down the movement of spatial light channels, otherwise known as solitons. The team said, this was the first time when such method allowed them to clarify the nature of vibrations of these optical structures. Another interesting property useful to investigate the formation of light channels was their ability to retain their shape in hermetic volumes, where thin films keep their shape and optical properties for a long time (a year and even more).

The authors of the study have shown that the narrow tracks can be formed in such films using a broadband white light as well. During experiments, for the first time a single track formed in the soap-gelatin film under the influence of light pressure allowed to demonstrate the influence of modulation of light flux used for the excitation of the light channel itself. 

Deflection of the dried gelatin-soap films (diameter 57 mm ) under the action of 10 g liquid poured on it (left) and a steel ball with a diameter of 8 mm (right)

Deflection of the dried gelatin-soap films (diameter 57 mm ) under the action of 10 g liquid poured on it (left) and a steel ball with a diameter of 8 mm (right). Image courtesy of the researchers.

The properties of aqueous gelatin films are interesting not only in its liquid form but in open air as well, as the water gradually evaporates. As you already know, regular soap bubbles burst while drying. Meanwhile, transparent bubbles made of soap-gelatin film do not burst even after drying completely, and may persist for weeks in their original form. This opens up great possibilities of their use in optics and mechanics, the scientists say. 

Deflection of the soap-gelatin film (diameter 57 mm) under the action of a portion of a liquid epoxy resin used to produce a solidified optical lens. Image courtesy of the researchers.

Deflection of the soap-gelatin film (diameter 57 mm) under the action of a portion of a liquid epoxy resin used to produce a solidified optical lens. Image courtesy of the researchers.

In contrast to the well-known polymerized soap films which dry and let the air out approximately in a day, the gelatin bubbles remain elastic even without water and can preserve their shape during entire day. The experiments confirmed the hypothesis stating that the thin elastic gelatin films exhibit only a weak air-permeability. 

Epoxy lens with diameter of 16 mm and a focal length of 5 mm, obtained from the film shown in image above.

Epoxy lens with diameter of 16 mm and a focal length of 5 mm, obtained from the film shown in image above. Image courtesy of the researchers.

In the future, many other aspects of thin liquid-based films may pose scientific interest, including the research of thermal and electrical properties, experiments to test the mechanical strength of the dried film containing a range of possible additives, as well as the ability of film to be processed further in order for them to contact and adhere to different surfaces. A possibility to obtain long, thin filaments and fibers having a strength of the spider web from the soap-gelatin solution while freezing and shaping it simultaneously is another interesting field of research.

Computer screen, the view through the above-mentioned epoxy lens. Image courtesy of the researchers.

Computer screen, the view through the above-mentioned epoxy lens. Image courtesy of the researchers.

Theorists have predicted that particularly high ratios of weight and mechanical strength could be demonstrated by very thin monolayer graphene films. But as long as the graphene film of this size are not available, the experiments with aqueous soap-gelatin alternatives exhibiting quite similar properties can advance the development of research methods that could be equally applicable in both areas.

Epoxy lens with diameter of 16 mm and a focal length of about 5 mm easily ignites carbon paper by focusing the sunlight. You can download the video here. Image courtesy of the researchers.

Epoxy lens with diameter of 16 mm and a focal length of about 5 mm easily ignites carbon paper by focusing the sunlight. You can download the video from here. Image courtesy of the researchers.

Source: FIAN-INFORM (in Russian), written by Alius Noreika

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