| Abstract: | A simple heat transfer model is presented to show that convection from the cryogen to the specimen is the limiting step in rapid freezing of small samples for electron microscopy when the Biot modulus, which measures the ratio of convective to conductive heat transfer, (hd/k) < 1. In comparison to the commonly accepted conduction-limited model, the convection-limited model predicts that cooling rates are proportional to the surface area/volume ratio, independent of the sample thermal conductivity, and inversely proportional to the product of sample density and heat capacity. Literature values of experimentally measured cooling rates fit the convection limited model. Simple analogies to predict the heat transfer coefficient as a function of cryogen properties, specimen geometry and cryogen velocity are presented. |
