  We perform experimental investigations of osmotically driven flows in
artificial microchannels by studying the dynamics and structure of the front of
a sugar solution traveling in 200 um wide and 50-200 um deep microchannels. We
find that the sugar front travels with constant speed, and that this speed is
proportional to the concentration of the sugar solution and inversely
proportional to the depth of the channel. We propose a theoretical model,
which, in the limit of low axial flow resistance, predicts that the sugar front
indeed should travel with a constant velocity. The model also predicts an
inverse relationship between the depth of the channel and the speed and a
linear relation between the sugar concentration and the speed. We thus find
good agreement between the experimental results and the predictions of the
model. Our motivation for studying osmotically driven flows is that they are
believed to be responsible for the translocation of sugar in plants through the
phloem sieve element cells. Also, we suggest that osmotic elements can act as
integrated pumps with no movable parts in lab-on-a-chip systems.
