The modification of soft-wall turbulence in a microchannel due to small amounts of polymer dissolved in water is experimentally studied. The microchannels are of rectangular cross-section with height ${\sim}$160 $\unicode[STIX]{x03BC}\text{m}$, width ${\sim}$1.5 mm and length ${\sim}$3 cm, with three walls made of hard polydimethylsiloxane (PDMS) gel, and one wall made of soft PDMS gel with an elasticity modulus of ${\sim}$18 kPa. Solutions of polyacrylamide of molecular weight $5\times 10^{6}$ and mass fraction up to 50 ppm, and of molecular weight $4\times 10^{4}$ and mass fraction up to 1500 ppm, are used in the experiments. In all cases, the solutions are in the dilute limit below the critical overlap concentration, and the solution viscosity does not exceed that of water by more than 10 %. Two distinct types of flow modifications are observed below and above a threshold mass fraction for the polymer, $w_{t}$, which is ${\sim}$1 ppm and 500 ppm for the solutions of polyacrylamide with molecular weights $5\times 10^{6}$ and $4\times 10^{4}$, respectively. At or below $w_{t}$, there is no change in the transition Reynolds number, but there is significant turbulence attenuation, by up to a factor of 2 in the root-mean-square velocities and a factor of 4 in the Reynolds stress. When the polymer concentration increases beyond $w_{t}$, there is a decrease in the transition Reynolds number and in the intensity of the turbulent fluctuations. The lowest transition Reynolds number is ${\sim}$35 for the solution of polyacrylamide with molecular weight $5\times 10^{6}$ and mass fraction 50 ppm (in contrast to 260–290 for pure water). The fluctuating velocities in the streamwise and cross-stream directions are lower by a factor of 5, and the Reynolds stress is lower by a factor of 10, in comparison to pure water.
EFFECTIVENESS OF SOFT WALL CAPPING IN CONSERVING RUINS
