  Viscosity is a fundamental property of liquids. It determines transport and
diffusion of particles in solution. Nonetheless, it is an open question how a
gradient of viscosity - causing a gradient in diffusivity - can lead to
viscophoretic transport, i.e., directed transport of particles and molecules in
solution. Here, we determine viscophoretic drift experimentally. We generate
steep, stable viscosity gradients in a microfluidic device and image transport
of suspended nanoparticles in these gradients using high-resolution microscopy.
We observe high viscophoretic drift velocities which significantly exceed
theoretical predictions. In addition, we demonstrate a new method for trapping
and concentrating particles by using the interplay of viscophoresis and
diffusiophoresis. We believe that a quantification of viscophoresis will
advance the understanding and application of transport processes of gradients
of viscosity occurring in very diverse fields such as cell biology,
chromatography, and membrane technology.
