The air cushion firms up
(appeared in Aug2018)

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The gossamer-thin aerogel is set to gain strength and utility, says S.Ananthanarayanan.

The aerogel is like a jelly, but more delicate. It is delicate because it is a jelly where the water part has been replaced by just air. A gel, like jelly, is a liquid in its bulk, with a framework of firm, internal linkages that gives it an extent of rigidity. Now, if we replace the liquid portion with air, then most of the weight disappears while the air-filled, porous, usually transparent framework, the aerogel, remains. This lightweight structure has value and use for many purposes. The structure, however, in all aerogels created so far, is brittle and cannot be worked upon. On top of this, aerogels are expensive to produce.

Guoqing Zu, Kazuyoshi Kanamori, Ayaka Maeno, Hironori Kaji and Kazuki Nakanishi, from Kyoto University report in the journal, Angewandte Chemie, a technique to create low cost, customisable aerogels which have greater strength, are elastic and can be rolled, twisted or cut. These aerogels also have a special property of repelling water and are efficient insulators of heat. Mixing graphene in the structure also gives the possibility of electrical conductivity. The electrical properties can be controlled by manipulating the aerogel, which could lead to more applications.

The molecules of a normal liquid are free and in constant motion throughout the body of the liquid. Gels are liquids too, but the molecules connect to one another. They do not form crystals, as in solids, but they form chains and ‘cross links’, which provide a three dimensional, honey-comb structure. As the proportion of the liquid material that participates in cross linking is small, the bulk of the material stays fluid and the gel is usually unstable and ‘jelly-like’. The cross linking usually collapses when the gel is heated and the collapse is usually irreversible.

The aerogel derives from a gel. Once the gel has formed and there is a framework, the liquid in the bulk is gently evacuated and replaced with a gas. The liquid is removed by a process called supercritical drying. This is where the liquid is warmed and the pressure is reduced, gradually, so that the liquid stays liquid a little longer than it should, or gets ‘supercritical’. Evaporation can then be regulated so that the framework does not collapse. This kind of drying could be regarded as the opposite of boiling the liquid, which leads to evaporation faster than just drying.

Once the liquid has been removed from a gel, in this way, what remains, in the first place, is a lightweight frame. And then, in the second place, air, or gas, which forms its bulk, is confined to small compartments. This second property makes aerogels powerful insulators.

The way a normal gas conducts heat is that the molecules of the gas gain energy when they are in contact with a hot surface. These energetic molecules then communicate, by collisions, with other molecules, so that energy is passed on to the other parts of the container of the gas. In the case of insulators, the fibres of the insulating material confine the air to small pockets, whose dimensions compare with the average distance that gas molecules move before they collide with other molecules. This interferes with passing on energy from one side of an insulator to another.

In the case of an aerogel, although there is no material for support and the structure is fragile, the dimensions of the compartments formed are small indeed. Aerogels are thus excellent insulators both of heat as well as of sound, which also propagates by energy transfer across molecules of gases.

The aerogel was first made, in 1931, from silica gels. Aerogels from different materials, like the oxides of aluminium, chromium, iron and tin, carbon, organic polymers, semiconductor nanostructures, gold and copper have since been developed. As for the cost of supercritical drying, other methods, like drying without reducing the pressure or freeze drying have been used. In freeze drying, the gel is chilled so that the water content freezes. The pressure is then reduced, so that the ice sublimes, or evaporates without melting.

The paper in Angewandte Chemie says these methods have, however, not been able to overcome fragility, which limits the applications where aerogels can be used. Additives to increase strength are found to reduce the porosity and increase the weight, which takes away from the character of aerogel. There have been half-way solutions, the paper says, but either poor scope for bending or other processing has limited the success of aerogels developed so far.

The Kyoto University team reports that the impasse now appears to be over. They report that a class of polymers, or chain molecules, that they have been working with show “excellent mechanical properties, such as superflexiblity and processability and multifunctionality combining thermal insulation, selective absorption, and strain sensing properties.” The team has that the chain molecules of the aerogels can be made directly using a selected material, called VDMMS, or along with related material, VMDMS. Varying the proportions then results in a range of pore sizes, from 20-100 nm to 2-20 microns, which is a thousand times larger.

The paper describes the kind of cross linking that takes place in the process, which leads to great elasticity of the structure. This is a departure from the opposite, which is rigidity and hence fragility of traditional aerogels. The aerogels are thus capable of being bent, twisted and cut to different shapes. The aerogels with the finer pore size are more effective insulators than conventional materials like polyurethane foam or mats of glass wool, the paper says.

Another feature of many aerogels is that the material of the structure, once formed, is treated to repel water, for stability. The aerogels created by the Kyoto group also repels water and displays selective absorption when exposed to a mixture of water and other liquids, like oils. This property, along with flexibility, would enable using aerogels for separation of mixtures. The aerogel would selectively absorb the oil, which can then be squeezed out, and the process can be repeated till all the oil has been removed.

Aerogels that conduct electricity have also been developed by introducing slivers of graphene into the aerogel structure. These slivers are in better contact when the aerogel is under pressure. Pressure can thus be used as a means of controlling the current passed, with applications in electronics or for use as touchpads, or conversely, as a pressure gauge.

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