  From paints to food products, solvent evaporation is ubiquitous and
critically impacts product rheological properties. It affects Newtonian fluids
by concentrating any non-volatile components and viscoelastic materials, which
hardens up. In both of these cases, solvent evaporation leads to a change in
the sample volume, which makes any rheological measurements particularly
challenging with traditional shear geometries. Here we show that the
rheological properties of a sample experiencing `slow' evaporation can be
monitored in a time-resolved fashion by using a zero normal-force controlled
protocol in a parallel-plate geometry. Solvent evaporation from the sample
leads to a decrease of the normal force, which is compensated at all times by a
decrease of the gap height between the plates. As a result, the sample
maintains a constant contact area with the plates despite the significant
decrease of its volume. We validate the method under both oscillatory and
continuous shear by accurately monitoring the viscosity of water-glycerol
mixtures experiencing evaporation and a relative volume decrease as large as
70%. Moreover, we apply this protocol to dehydrating suspensions. Specifically,
we monitor a dispersion of charged silica nanoparticles undergoing a glass
transition induced by evaporation. While the decrease in gap height provides a
direct estimate of the increasing particle volume fraction, oscillatory and
continuous shear measurements allow us to monitor the suspension's evolving
viscoelastic properties in real-time. Overall, our study shows that a zero
normal-force protocol provides a simple approach to bulk and time-resolved
rheological characterization for systems experiencing slow volume variations.
