We report a first application of vertical small-angle X-ray scattering to investigate the drying process of a colloidal suspension by overcoming gravity related restrictions. From the observation of the drying behavior of charge-stabilized colloidal silica in situ, we find the solidification of the colloidal particles exhibits an initial ordering, followed by a sudden aggregation when they overcome an electrostatic energy barrier. The aggregation can be driven not only by capillary pressure but also by thermal motion of the particles. The dominating contribution is determined by the magnitude of the energy barrier at the transition, which significantly decreases during drying due to an increased ionic strength.