Cloud chambers and crystal growth: Effects of electrically enhanced diffusion on dendrite formation from neutral molecules

We present an extension of the solvability theory for free dendrite growth that includes the effects of electrically enhanced diffusion of neutral polar molecules. Our theory reveals a new instability mechanism in free dendrite growth, which arises when electrically enhanced diffusion near the dendrite tip overwhelms the stabilizing influence of surface tension. This phenomenon is closely related to the growth instability responsible for the visualization of charged particle tracks in cloud chambers, and is expected for enhanced diffusion of neutral molecules, but not for the case of ionic diffusion. Above a threshold applied potential, the crystal growth can no longer be described by the usual solvability theory, and requires a new physical mechanism to limit the growth velocity. We also describe experimental observations of the free dendrite growth of ice crystals from water vapor in supersaturated normal air. These observations demonstrate the calculated growth instability, which results in the rapid growth of branchless ice needles with a tip velocity 5-50 times the normal dendrite tip velocity. The production of clean ice needles is useful for the study of ice crystal growth from vapor, allowing the controlled growth of isolated single-crystal samples. This instability mechanism may find further application in crystal growth from a wide variety of polar molecules. [S1063-651X(99)12203-7].