The principle of atmospheric drying--assuming that the pores of the gel are cylindrical pores, according to Laplace's equation , the additional pressure of liquid meniscus in capillary pores can be expressed as follows: 
    ρ=-2γcosθ/rm   [13]
Where γ  is the gas-liquid interfacial energy, θ is the contact angle, rm is the pore radius. The additional pressure P acts on the liquid, thus, the compression pressure of the liquid causes the gel network to contract. According to formula, if increasing the capillary radius, or increasing the contact angle and decreasing the interfacial energy of solvent can reduce the additional pressure and reduce the shrinkage of aerogels.[6]
With respect to freeze drying,  by lowering the temperature, the gel is frozen, and then the solvent is sublimated, so that the surface tension of the gas-liquid surface disappears, so as to achieve the purpose of drying the aerogels.    
‘Comparing to the supercritical drying, both freeze drying and atmospheric drying have the advantages of simple operation and low cost, while low success rate, long dry time, easy cracking are their common defects.’[5]
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