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Reported data of measured slip lengths in nanostructures span several orders of magnitude, from a few nanometers to tens of micrometers. Small roughness on surfaces caused by structural defects or thermal fluctuations dramatically reduces slippage. Tiny bubbles entrapped on rough surfaces can also affect slippage. We used an asymptotic solution and a high density-ratio pseudopotential lattice Boltzmann model to systematically study the drag resistance of a rough surface with attached bubbles. As bubbles nucleate and grow, drag resistance is slightly reduced until the tri-phase contact line reaches the edges of roughness, where bubbles with small angles substantially reduce drag resistance. As bubbles grow to become a continuous gas layer on the surface, the drag resistance greatly decreases. However, the interface deformation from flat to curved shape greatly hinders liquid flow, and the vortex structures cause a wave-like fluctuation in the effective slip length. This finding sheds light on the controversies of reported large variations in the slip length of super-hydrophobic surfaces in nanostructures, e.g., carbon nanotubes. [ABSTRACT FROM AUTHOR]

The carrier diffusion length (LD) is of great relevance to the quantum efficiencies of organic-inorganic perovskites. However, so far there is no direct and noncontact measurement of the carrier diffusion length in this emerging material. Herein, we directly visualize the carrier diffusion length of organic-inorganic perovskites via spatial mapping of photoluminescence. Our results reveal that the carrier diffusion length (LD) is 6.25 μm for three-dimensional FAPbBr3 single crystals. Further, we discover that nonlinear and nonradiative effects can be neglectable during the diffusion process of photogenerated carriers in FAPbBr3 single crystals. In contrast, two-dimensional BA2PbI4 single crystals display shorter LD (2.40 μm) due to the transport barrier from their insulating organic layer. Our work simplifies the complicated operation for carrier diffusion length measurements, which is fundamentally important for technical development and scientific research on organic-inorganic perovskites. [ABSTRACT FROM AUTHOR]

The hydrodynamics in lab-on-a-chip provides an efficient and tunable platform for manipulating mesoscopic particles. Current capture-tunable technology has been mainly focused on inertial flow with little attention on a thermocapillary vortex. The boundary condition is one of the most important factors on particle manipulation in a microvortex. By integrating a photothermal waveguide with a triangular channel in lab-on-a-chip, we present a tunable microvortex array for achieving size-tunable capture. Ellipticity of the temperature field and intensity of vortices are continuously adjustable by moving the photothermal waveguide along the triangular channel, resulting in tunable particle trajectories. Particles can be trapped in a vortex center and driven out of the vortex along with external flow. The detailed theoretical results reveal that a threshold size of trapped particles can be adjustable by the channel width. We believe that the approach, the thermocapillary vortex on chip, will provide a facile way for seamless connection between photonics and microfluidics. [ABSTRACT FROM AUTHOR]