22 research outputs found
Near-wall nanovelocimetry based on Total Internal Reflection Fluorescence with continuous tracking
The goal of this work is to make progress in the domain of near-wall
velocimetry. The technique we use is based on the tracking of nanoparticles in
an evanescent field, close to a wall, a technique called TIRF (Total Internal
Reflection Fluorescence)-based velocimetry. At variance with the methods
developed in the literature, we permanently keep track of the light emitted by
each particle during the time the measurements of their positions ('altitudes')
and speeds are performed. By performing the Langevin simulation, we quantified
effect of biases such as Brownian motion, heterogeneities induced by the walls,
statistical biases, photo bleaching, polydispersivity and limited depth of
field. Using this method, we obtained slip length on hydrophilic surfaces of 15 nm for sucrose solution, and 910 nm for water; On hydrophobic
surface, 325 nm for sucrose solution, and 559 nm for water. The
errors (based on 95% confidence intervals) are significantly smaller than the
state-of-the-art, but more importantly, the method demonstrates for the first
time a capacity to measure slippage with a satisfactory accuracy, while
providing a local information on the flow structure with a nanometric
resolution. Our study confirms the discrepancy already pointed out in the
literature between numerical and experimental slip length estimates. With the
progress conveyed by the present work, TIRF based technique with continuous
tracking can be considered as a quantitative method for investigating flow
properties close to walls, providing both global and local information on the
flow.Comment: 24 pages, 13 figure
Two types of Cassie-to-Wenzel wetting transitions on superhydrophobic surfaces during drop impact.
Underwater restoration and retention of gases on superhydrophobic surfaces for drag reduction.
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Underwater restoration and retention of gases on superhydrophobic surfaces for drag reduction.
Superhydrophobic (SHPo) surfaces have shown promise for passive drag reduction because their surface structures can hold a lubricating gas film between the solid surface and the liquid in contact with it. However, the types of SHPo surfaces that would produce any meaningful amount of reduction get wet under liquid pressure or at surface defects, both of which are unavoidable in the real world. In this Letter, we solve the above problem by (1) discovering surface structures that allow the restoration of a gas blanket from a wetted state while fully immersed underwater and (2) devising a self-controlled gas-generation mechanism that maintains the SHPo condition under high liquid pressures (tested up to 7 atm) as well as in the presence of surface defects, thus removing a fundamental barrier against the implementation of SHPo surfaces for drag reduction
The effects of surface wettability on the fog and dew moisture harvesting performance on tubular surfaces
bird's-eye view, view on roof, looking west southwest to surrounding town, with prayer courtyard in foreground, June 198
Superhydrophobic drag reduction in laminar flows: a critical review
A gas in between micro- or nanostructures on a submerged superhydrophobic (SHPo) surface allows the liquid on the structures to flow with an effective slip. If large enough, this slippage may entail a drag reduction appreciable for many flow systems. However, the large discrepancies among the slippage levels reported in the literature have led to a widespread misunderstanding on the drag-reducing ability of SHPo surfaces. Today we know that the amount of slip, generally quantified with a slip length, is mainly determined by the structural features of SHPo surfaces, such as the pitch, solid fraction, and pattern type, and further affected by secondary factors, such as the state of the liquid–gas interface. Reviewing the experimental data of laminar flows in the literature comprehensively and comparing them with the theoretical predictions, we provide a global picture of the liquid slip on structured surfaces to assist in rational design of SHPo surfaces for drag reduction. Because the trapped gas, called plastron, vanishes along with its slippage effect in most application conditions, lastly we discuss the recent efforts to prevent its loss. This review is limited to laminar flows, for which the SHPo drag reduction is reasonably well understood
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Structured surfaces for a giant liquid slip.
We study experimentally how two key geometric parameters (pitch and gas fraction) of textured hydrophobic surfaces affect liquid slip. The two are independently controlled on precisely fabricated microstructures of posts and grates, and the slip length of water on each sample is measured using a rheometer system. The slip length increases linearly with the pitch but dramatically with the gas fraction above 90%, the latter trend being more pronounced on posts than on grates. Once the surfaces are designed for very large slips (>20 microm), however, further increase is not obtained in regular practice because the meniscus loses its stability. By developing near-perfect samples that delay the transition from a dewetted (Cassie) to a wetted (Wenzel) state until near the theoretical limit, we achieve giant slip lengths, as large as 185 microm
Recommended from our members
Structured surfaces for a giant liquid slip.
We study experimentally how two key geometric parameters (pitch and gas fraction) of textured hydrophobic surfaces affect liquid slip. The two are independently controlled on precisely fabricated microstructures of posts and grates, and the slip length of water on each sample is measured using a rheometer system. The slip length increases linearly with the pitch but dramatically with the gas fraction above 90%, the latter trend being more pronounced on posts than on grates. Once the surfaces are designed for very large slips (>20 microm), however, further increase is not obtained in regular practice because the meniscus loses its stability. By developing near-perfect samples that delay the transition from a dewetted (Cassie) to a wetted (Wenzel) state until near the theoretical limit, we achieve giant slip lengths, as large as 185 microm
Mesoporous Highly-Deformable Composite Polymer for a Gapless Triboelectric Nanogenerator via a One-Step Metal Oxidation Process
The oxidation of metal microparticles (MPs) in a polymer film yields a mesoporous highly-deformable composite polymer for enhancing performance and creating a gapless structure of triboelectric nanogenerators (TENGs). This is a one-step scalable synthesis for developing large-scale, cost-effective, and light-weight mesoporous polymer composites. We demonstrate mesoporous aluminum oxide (Al2O3) polydimethylsiloxane (PDMS) composites with a nano-flake structure on the surface of Al2O3 MPs in pores. The porosity of mesoporous Al2O3-PDMS films reaches 71.35% as the concentration of Al MPs increases to 15%. As a result, the film capacitance is enhanced 1.8 times, and TENG output performance is 6.67-times greater at 33.3 kPa and 4 Hz. The pressure sensitivity of 6.71 V/kPa and 0.18 μA/kPa is determined under the pressure range of 5.5–33.3 kPa. Based on these structures, we apply mesoporous Al2O3-PDMS film to a gapless TENG structure and obtain a linear pressure sensitivity of 1.00 V/kPa and 0.02 μA/kPa, respectively. Finally, we demonstrate self-powered safety cushion sensors for monitoring human sitting position by using gapless TENGs, which are developed with a large-scale and highly-deformable mesoporous Al2O3-PDMS film with dimensions of 6 × 5 pixels (33 × 27 cm2)