  The evaporation of drops of water placed at the center of long poly(methyl
methacrylate) microfluidic channels with a rectangular cross section of 0.38
mm2 is studied by simultaneously monitoring the shapes of two samples, one is
in a 300 mT magnetic field, the other is in no field. A magnetic enhancement of
the evaporation rate of up to 140 % is observed, which can be understood by
treating the ortho and para nuclear isomers of water vapor as quasi-independent
gasses with an ortho:para ratio in fresh vapor close to 2:3. It would take much
longer than the 2 - 4 h duration of an experiment in the channel, for the ratio
to approach the 3:1 equilibrium value. Magnetic field influences evaporation
rate by equalizing the isomeric populations in the vapor phase. The atmosphere
in the channel is saturated with water vapor yet the evaporation rate far
exceeds that in open beakers.
