The asymmetric texture with β¼14° decreased the apparent
viscosity the most out of all the textures tested, but does not have
the largest average gap, which for the asymmetric textures is for
β¼21.7° (see Fig. 4 for gap profiles). This may be due to the
competing effects of having a larger effective gap (lower shear
stress) and the possibility of a large texture generating recirculation
zones inside the textured surface, which has been observed in
other studies [7,37–39] (higher dissipation and therefore less
reduction in shear stress). A recirculation zone would generate
laminar vortices, similar to secondary flow effects. Ewoldt et al.
[40] have shown that secondary flow effects can cause an increase
in apparent viscosity in Newtonian fluids. Therefore, a large
recirculation zone inside the textures would cause a higher viscosity
to be measured. Because the intermediate angle β¼14° has
the largest decrease in shear stress, this suggests that there is an
optimal texture configuration for decreasing the apparent viscosity
with Newtonian lubricants.
The same trends seen here are also seen with the S60 data
given in the supplemental information. However, with the S60
data, the lowest apparent viscosity was obtained with the symmetric
texture instead of with β¼14°, and the lowest apparent
viscosity with the asymmetric textures was obtained with β¼9.4°.
This again suggests that there is an optimal texture configuration
for decreasing the apparent viscosity with Newtonian lubricants