• 152 Downloads • Abstract Analytic solutions for two of the similarity cases identified by Johnson and Cheng (1978) for the unsteady free-convection boundary-layer flow over an impermeable vertical flat plate adjacent to a fluid saturated porous medium are given in the present paper. These are the solutions corresponding to an exponential ( e sup a 2 t sup) and a power-law ( t m) variation of the surface temperature, respectively. They represent exact solutions for doubly infinite plates and approximate solutions for semi-infinite plates. In the latter cases their validity is restricted to the so-called `conduction regime' of the flow. It is shown that in the power law case, physical solutions only exist in the range m>−1 of the temperature exponent and they can be expressed in terms of Kummer's confluent hypergeometric functions. For m ≥ 0 exponentially decaying unique solutions were found, while in the range −1.

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A block of aerogel in a person's hand Aerogel is a porous derived from a, in which the component of the gel has been replaced with a. The result is a solid with extremely low and low. Nicknames include frozen smoke, solid smoke, solid air, solid cloud, blue smoke owing to its nature and the way in the material. It feels like fragile to the touch. Aerogels can be made from a variety of chemical compounds. Aerogel was first created by in 1931, as a result of a bet with Charles Learned over who could replace the liquid in 'jellies' with gas without causing shrinkage. Aerogels are produced by extracting the liquid component of a gel through.

This allows the liquid to be slowly dried off without causing the solid matrix in the gel to collapse from, as would happen with conventional. The first aerogels were produced from. Kistler's later work involved aerogels based on, and.

Aerogels were first developed in the late 1980s. Aerogel does not have a designated material with set chemical formula but the term is used to group all the material with a certain geometric structure.

A flower is on a piece of aerogel which is suspended over a flame from a. Aerogel has excellent insulating properties, and the flower is protected from the flame. Despite their name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: the name comes from the fact that they are made from gels. Pressing softly on an aerogel typically does not leave even a minor mark; pressing more firmly will leave a permanent depression. Pressing extremely firmly will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass (a property known as ), although more modern variations do not suffer from this.

Despite the fact that it is prone to shattering, it is very strong structurally. Its impressive load bearing abilities are due to the microstructure, in which particles of average size (2–5 ) are fused together into clusters. These clusters form a three-dimensional highly structure of almost chains, with pores just under 100 nm. The average size and density of the pores can be controlled during the manufacturing process. Aerogel is a material that is 99.8% air.