Seungju Moon, Mazen Al-Ghoul
Applying external electric fields offers a powerful means of directing reaction-diffusion crystallization in soft media. Here, we investigate how a radial DC field modulates the dendritic morphogenesis of benzoic acid crystallized in a gelatin matrix through the reaction of diffusing H+ and electrophoretically redistributed benzoate (BZ-) ions. Optical imaging, fractal-dimension analysis, and scanning electron microscopy (SEM) show reproducible, polarity-dependent changes in dendritic branching and crystal habit. Positive central bias is associated with denser, more highly branched aggregates and higher fractal dimensions, whereas negative central bias produces sparser morphologies or, at the highest negative potentials, suppresses formation of the central fractal. These trends are consistent with field-dependent redistribution of the reactive ionic concentration profiles. However, local pH, benzoate activity, and supersaturation were not measured directly, and the proposed redistribution of supersaturation therefore remains a mechanistically consistent interpretation rather than a unique experimental determination. PXRD patterns remain consistent with the reported benzoic acid structure under all field conditions, with no additional resolved crystalline phase detected within the sensitivity of the measurements. SEM further shows that positive bias is associated with an increased occurrence of laterally interlocking rectangular platelet-like crystallites compared with the predominantly needle-like crystallites observed at zero and negative bias. Their greater lateral impingement may contribute to increased areal coverage and fractal dimension, although this contribution could not be separated from simultaneous changes in transport, nucleation, and growth. Together, these findings demonstrate reproducible electric-field tuning of reaction-diffusion crystallization through the coupling of drift-diffusion transport with field-associated changes in crystal growth. Given the demonstrated utility of benzoic acid fractals as scaffolds for directing metal-organic framework growth, electric-field control offers a further route for designing crystalline architectures in soft condensed media.