  In an externally imposed electrolyte (salt) concentration gradient, charged
colloids drift at speeds of order one micrometre per second. This phenomenon is
known as diffusiophoresis. In systems with multiple salts and 'crossed' salt
gradients, a nonlocal component of the electric field associated with a
circulating (solenoidal) ion current can arise. This is in addition to the
conventional local component that depends only on the local salt gradients.
Here we report experimental observations verifying the existence of this
nonlocal contribution. To our knowledge this is the first observation of
nonlocal diffusiophoresis. The current develops quasi-instantaneously on the
time scale of salt diffusion. Therefore, in systems with multiple salts and
crossed salt gradients, one can expect a nonlocal contribution to
diffusiophoresis which is dependent on the geometry of the system as a whole
and appears as a kind of instantaneous 'action-at-a-distance' effect. The
interpretation is aided by a magnetostatic analogy. Our experiments are
facilitated by a judicious particle-dependent choice of salt (potassium
acetate) for which the two local contributions to diffusiophoresis almost
cancel, effectively eliminating conventional diffusiophoresis. This enables us
to clearly identify the novel, nonlocal effect and may be useful in other
contexts, for example in sorting particle mixtures.
