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Traditional explanations for the efficacy of mineral oil in successful flash cooling of a protein crystal correctly point to the removal of excess liquid around the crystal to prevent external ice formation. Based on the physics of the well known Leidenfrost phenomenon, an additional role that mineral oil possibly plays in aiding vitrification is suggested: that of improving cooling rates for a protein crystal plunged into liquid nitrogen. The full potential of liquid heat transfer when using liquid nitrogen is not realized in conventional cooling techniques due to film boiling that occurs around larger protein crystals. However, a thin layer of an insulating material, such as a mineral oil, around the protein crystal can prevent this vapor film from forming by raising the Leidenfrost temperature. Surface cooling then occurs in the more efficient nucleate boiling regime where liquid nitrogen is in contact with the crystal throughout the quench. Using bare and coated thermocouples, the validity of a predictive Leidenfrost temperature equation for use in liquid-nitrogen plunge cooling of protein crystals is demonstrated.

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