Attraction and repulsion between colloids occurred over several particle diameters, indicating that pH dependent changes to colloid surface charge were not responsible for the transient colloidal crystals. The Debye length was ~10 nm in these experiments, which is two orders of magnitude smaller than the length scale over which colloids interacted during experiments. Instead, prior work has shown that colloids near charged electrodes generate toroidal electrokinetic fluid flows at their surface that can be either extensile or contractile, entraining or repelling neighboring particles . Micron sized colloids in dilute electrolytes tend to form 2D colloidal crystals under low frequency oscillatory electric fields (< 1 kHz) due to contractile EHD fluid flow around each particle . The EHD fluid flow magnitude near a colloid in a 1 mM electrolyte is predicted to be effectively invariant with solution pH over the range of 6 -8 . Conversely, prior work has shown that colloids in alkaline electrolytes (KOH, NaOH) separate due to the mismatch between anion and cation diffusivity, which generates a rectified steady electric field that induces an extensile electroosmotic (EO) flow around each particle . Here, increasing hydroxyl ion concentration increases the magnitude of the EO flow, causing increased repulsive hydrostatic forces between neighboring particles . We propose that a competition between EHD and EO flows mediates dynamics of the transient colloidal crystals. To test this mechanism, we developed a hydrodynamic scaling model for a pair of colloids interacting via mutual drag forces generated by contractile EHD flow (𝑈 𝐸𝐻𝐷 , attractive drag force) and a pH dependent extensile EO flow (𝑈 𝐸𝑂 (𝑝𝐻), repulsive drag force). The relative pair velocity of the particles (𝑈 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 (𝑝𝐻)) is