The dynamics of planar interface motion and the formation and reorganization of interface instabilities into an array of cells and dendrites have been investigated. These dynamics are significantly influenced by the presence of fluid flow in bulk samples. Two sets of experiments using a transparent metal analog system in which the convection effects are minimized have been performed and analyzed: solidification in thin samples on ground and solidification in bulk samples under microgravity conditions. Observations on the morphological evolution of a three-dimensional interface in a diffusion controlled regime, carried out on the International Space Station (ISS), have been directly correlated with ground-based results in thin samples and with theoretical predictions.;To carry out precise analysis of the flight and ground-based directional solidification experiments, determination of the succinonitrile-water liquidus in the succinonitrile-rich region up to 1.0 wt% water was performed. The liquidus was found to be linear in this region with a slope of -8.8 K/wt%.;The analysis of microgravity results on the dynamics of planar interface motion shows that a thermal lag is present and significant for a series of bulk samples solidified in a diffusive growth condition. After an offset corresponding to the observed thermal lag is included, the Warren-Langer model accurately predicts the planar interface dynamics observed during the initial transient. In contrast, the ground-based thin sample results deviate significantly from the predictions of the model due to the dominance of the contact angle effect leading to solute trapping at the walls. This effect renders the ground-based thin-sample experiments ineffective for quantitative measurements even during the initial transient.;The steady-state spacing measurements obtained from the bulk microgravity experiments and the ground-based thin sample experiments were compared with the minimum spacing predictions of the Hunt-Lu model. The measurements obtained from the bulk microgravity experiments are shown to be in qualitative agreement with the predicted spacing changes as the velocity is increased. However, these bulk experimental spacing measurements were found to be smaller than the minimum spacings predicted by the Hunt-Lu model.