Shape of ammonium chloride dendrite tips at small supersaturation

We report detailed shape measurements of the tips of three-dimensional NH4Cl dendrites grown from supersaturated aqueous solution. For growth at small supersaturation, we compare two models for the tip shape: parabolic with a fourth-order correction and a power law. Neither is ideal, but the fourth-order fit appears to provide the most robust description of both the tip shape and position for this material. For that fit, the magnitude of the fourth-order coefficient is about half of the theoretically expected value.