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Microtextured surfaces can reduce friction in lubricated dynamic contact. However, no prior experimental study has explored the dimensionless parameter space of Reynolds number and dimensionless gap, since normal force is easier to control than the fluid film thickness. Here, we develop a custom precision-aligned setup for gap-controlled tribo-rheometry based on a rotational rheometer. The novel experimental setup allows for measurement of full film lubrication of parallel disks down to 20 μm gaps with gap precision ±3 μm, over a range of Reynolds numbers. We show for the first time in gap-controlled conditions that microtextured surfaces reduce friction. The reduction in dimensionless shear stress is nearly independent of velocity for Re <10 which matches computational results in this work. [ABSTRACT FROM AUTHOR]

We present plasmonic optical trapping of micron-sized particles in biologically relevant buffer media with varying ionic strength. The media consist of 3 cell-growth solutions and 2 buffers and are specifically chosen due to their widespread use and applicability to breast-cancer and angiogenesis studies. High-precision rheological measurements on the buffer media reveal that, in all cases excluding the 8.0 pH Stain medium, the fluids exhibit Newtonian behavior, thereby enabling straightforward measurements of optical trap stiffness from power-spectral particle displacement data. Using stiffness as a trapping performance metric, we find that for all media under consideration the plasmonic nanotweezers generate optical forces 3–4x a conventional optical trap. Further, plasmonic trap stiffness values are comparable to those of an identical water-only system, indicating that the performance of a plasmonic nanotweezer is not degraded by the biological media. These results pave the way for future biological applications utilizing plasmonic optical traps. [ABSTRACT FROM AUTHOR]

For rotational rheometers, surface tension results in a torque that should not occur in an ideal, rotationally symmetric geometry. This paper identifies and explains the effect, which is due to surface tension and contact line traction forces, and not surface rheology at the liquid-air interface. For water, we show that this torque can be more than two orders of magnitude larger than the viscous torque. In steady shear flow, the effect appears as a constant torque independent of rate, which would appear inaccurately as apparent shear thinning of water. In oscillatory tests, this may appear inaccurately as an elastic modulus. This surface tension torque is sensitive to wetting conditions and contact line asymmetries and cannot be deterministically corrected in experimental measurements. It therefore raises the lower bound of the instrument low-torque limit. The surface tension torque is reduced by maximizing rotational symmetry of the contact line, minimizing evaporation and the migration of the contact line, reducing the radial location of the contact line, and lowering the surface tension. Identifying and eliminating the surface tension torque is critical for low viscosities, intrinsic viscosities, soft materials, subdominant viscoelastic components, small gaps, and any circumstance where the low-torque limit is experimentally important. [ABSTRACT FROM AUTHOR]