Experimental study of soft porous lubrication

In this paper, we report an experimental study to examine the lift generation and friction reduction using soft porous media. The study is inspired by the frictionless motion of a red blood cell gliding over the highly porous endothelial glycocalyx layer that covers the inner surface of our blood vessels. The experimental setup consists of a running conveyer belt coated with a soft porous sheet and an upper fully instrumented planar board, i.e., a planing surface. Both the lubrication pressure and the sliding friction are measured. The results clearly demonstrate a huge potential for using highly compressible porous media for a new generation of soft bearings with tremendously increased pore pressure and infinite life.

Figure 1

Experimental setup for soft porous lubrication. (a) Problem schematic. A planing surface glides over a soft porous layer with a tilt angle at velocity $U$. The porous layer thicknesses under the trailing and leading edges are $h_1$ and $h_2$, respectively, and the undeformed porous layer thickness is $h_0$. For displaying purposes, this figure doesn't represent the accurate dimensional proportion in the experiments. (b) Experimental design. A fibrous porous layer is attached to a conveyor belt, and a planar board is held fixed above the porous layer serving as the planing surface. The conveyor belt is driven by two rollers connected to a DC electric motor. (c) The arrangement of the pressure sensors, $P_i$, and the load cells, $L_i$, on the planing surface.

Figure 2

(a) Representative time-averaged pore pressure distribution (measured by pressure sensors $P_i)$ and total lifting pressure distribution (measured by load cells $L_i)$ from trailing edge at $x/L=0$ to leading edge at $x/L=1$. (b) Comparison between experimental results and theoretical predictions in pore pressure generation. The belt with porous sheet attached on was running at a constant velocity $U=4.5\mathrm{m}/\mathrm{s}$. The length of the planing plate, $L=0.381\mathrm{m}$. The porous layer thickness is $h_2=h_0=19\mathrm{mm},h_1=13\mathrm{mm}$. The Brinkman parameter at the leading edge, $\alpha =h_2/K_p^{0.5}=98$. Under the running condition, the experiment was repeated three times. The error bars show that the variations in the measured pore pressure are below 5 Pa, which is close to the experimental error of the pressure sensors due to their calibrations.

Figure 3

$f_{\mathrm{air}}$ versus ${\eta }_1$ for “material P.” $f_{\mathrm{air}}$ is the pore pressure's contribution to the total lift. ${\eta }_1$ is the friction coefficient between the planing surface and the porous sheet. Under the same running conditions, each experiment in the parametric study was repeated three times. The error bars are plotted for showing its repeatability.