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In an externally imposed electrolyte (salt) concentration gradient, chargedcolloids drift at speeds of order one micrometre per second. This phenomenon isknown as diffusiophoresis. In systems with multiple salts and 'crossed' saltgradients, a nonlocal component of the electric field associated with acirculating (solenoidal) ion current can arise. This is in addition to theconventional local component that depends only on the local salt gradients.Here we report experimental observations verifying the existence of thisnonlocal contribution. To our knowledge this is the first observation ofnonlocal diffusiophoresis. The current develops quasi-instantaneously on thetime scale of salt diffusion. Therefore, in systems with multiple salts andcrossed salt gradients, one can expect a nonlocal contribution todiffusiophoresis which is dependent on the geometry of the system as a wholeand appears as a kind of instantaneous 'action-at-a-distance' effect. Theinterpretation is aided by a magnetostatic analogy. Our experiments arefacilitated by a judicious particle-dependent choice of salt (potassiumacetate) for which the two local contributions to diffusiophoresis almostcancel, effectively eliminating conventional diffusiophoresis. This enables usto clearly identify the novel, nonlocal effect and may be useful in othercontexts, for example in sorting particle mixtures.

Colloidal diffusiophoresis in crossed electrolyte gradients:  experimental demonstration of an 'action at a distance' effect predicted by  the Nernst-Planck equations