Fig. 4: Evaluation of the drag reduction performance of the sanded PTFE foam with pneumatic active plastron stabilization.

a Results of the open jetting drag reduction test. The PTFE foam (with and without gas supply) was compared against the acrylic control in terms of the measured drag force. b The open jetting test repeated with an albumin-fouled PTFE sample, which showed generally inferior drag reduction capabilities after protein fouling. A pressurized gas supply was able to restore the drag reducing function of the material. c Schematic diagram of the setup for the closed loop drag reduction test. The test sample was connected in series to a centrifugal pump, water reservoir and flowmeter. Pressure sensors immediately upstream and downstream of the test sample measured the pressure drop across the sample. d Pressure drop measured across the test samples with varying gas pressures under different pump speeds. The red symbols indicate data points corresponding to the Wenzel state (left) or the bubbling regime (right). (e) Drag reduction with varying Reynolds numbers, comparing the pressure drop between the Cassie-Baxter (while supplied with 3 kPa of N2 gas) and Wenzel states. f Normalized ΔR monitored over 120 min under different sample conditions (nonfouled vs in situ fouling; with gas supply vs without gas supply).