Whilst electrohydrodynamic (EHD) flow actuation of dielectric fluids has been widely demonstrated, the fundamental mechanisms responsible for their behaviour is not well understood. By highlighting key distinguishing features of the various EHD mechanisms discussed in the literature, and proposing a more general mechanism based on Maxwell (electric) pressure gradients that arise due to induced polarization, we suggest that it is possible to identify the dominant EHD mechanisms that are responsible for an observed flow. We demonstrate this for a class of low conductivity dielectric fluids - Electro-Conjugate Fluids (ECFs) - that have recently been shown to exhibit EHD flow phenomena when subjected to non-uniform fields of low intensities. Careful inspection of the salient attributes of the flow, at least at low field strengths (<1 kV/cm) - for example, the absence of a threshold voltage for the onset of flow, the quadratic scaling of the flow velocity with the applied voltage, and flow from the high to the low field region - eliminate the possibility of mechanisms based on space charge. Instead, we suggest that flow can be attributed to the existence of a Maxwell pressure gradient. This is further corroborated by good agreement between our experimental results and theoretical analysis.
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