Diffusiophoresis describes the motion of colloids in an electrolyte or non-electrolyte solution where there is a concentration gradient. While most of the studies of diffusiophoresis focus on the motion of solid particles, soft objects such as drops and bubbles are also known to experience diffusiophoresis. Here, we investigate the diffusiophoresis of charged drops in an electrolyte solution both analytically and experimentally. The drop is assumed to remain spherical. An analytical solution of the diffusiophoretic velocity of drops is obtained by perturbation methods. We find that the flow inside the drop is driven by the tangential electric stress at the interface and it directly influences the diffusiophoretic speed of the drop. Using charged oil droplets, we measure the drop speed under solute concentration gradients and find good agreement with the analytical solution. Our findings have potential applications for oil recovery and drug delivery.
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