Evaporation-triggered directional transport of asymmetrically confined droplets.

HYPOTHESIS: When a liquid droplet is confined between two non-parallel hydrophobic surfaces with dihedral angle α, its behavior is largely influenced by the asymmetric confinement. During evaporation, the droplet morphology under confinement will continuously evolve, leading to the directional transport of the droplet towards the cusp. EXPERIMENTS AND SIMULATIONS: During the evaporation process, droplets at different initial locations l0 from the cusp were experimentally observed to transport towards the cusp. A series of simulations using Surface Evolver were performed to obtain the three-dimensional morphologies of the confined droplets. Force and energy analyses were conducted to unveil the mechanisms dominating the evaporation-triggered actuation and transport. FINDINGS: The asymmetrically confined droplet of volume V would drift towards an equilibrium location of le from the cusp with the lowest energy. Its directional motion results from the consecutively decreasing le, which is scaled as le~α-1V13 during evaporation. Herein, the creeping and slipping modes of transport could be characterized as the quasi-stable and unstable self-relaxation processes of droplet from the stretched regime to the equilibrium regime, respectively. Our findings on the intrinsic mechanism of droplet actuation shed light on a novel approach to manipulating the confined droplet behaviors in a passive and decisive fashion.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures.

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 107. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Evaporation of squeezed water droplets between two parallel hydrophobic/superhydrophobic surfaces.

HYPOTHESIS: A liquid droplet is apt to be deformed within a compact space in various applications. The morphological change of a droplet and vapor accumulation in the confined space between two parallel surfaces with different gaps and surface wettability are expected to significantly affect the evaporation dynamics of the squeezed droplet therein. EXPERIMENTS: Here the evaporation dynamics of a squeezed droplet between two parallel hydrophobic/superhydrophobic surfaces are experimentally explored. By reducing the surface gap from 1000 µm to 400 µm, the evolution of contact angle, contact radius and volume of the evaporating droplet are measured. A diffusion-driven model based on a two-parameter ellipsoidal segment geometry is developed to predict the morphology and volume evolution of a squeezed droplet during evaporation. FINDINGS: Evaporation dynamics of a squeezed water droplet via the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, or the mixed mode are experimentally observed. Confirmed by our ellipsoidal segment model, the evaporation of the squeezed droplet is significantly depressed with the decreasing surface gap, which is primarily attributed to vapor enrichment in a more confined geometry. A linear scaling law between droplet volume and evaporation time is unveiled, which is verified by a simplified cylindrical model.

Coalescence-Induced Swift Jumping of Nanodroplets on Curved Surfaces.

In this work, we use molecular dynamics simulations to investigate coalescence-induced jumping of nanodroplets on curved surfaces with different wettabilities. On a curved surface, a liquid bridge will first form between two coalescing droplets as on a flat surface. However, contrary to symmetry-breaking-induced jumping on a flat surface, coalescing droplets would jump earlier than the liquid bridge gets into contact with the curved surface. Such an early symmetry breaking is induced by the opposite motion of coalescing droplets along the lateral direction on the curved surface. We find that surface curvature can effectively facilitate the jumping of coalescing nanodroplets via enhanced symmetry breaking. The energy conversion efficiency is improved from ∼0.15% on a flat surface to ∼2.9% on a curved surface, which is about 20 times enhancement. In addition, we conducted an energy scaling analysis by considering the lumped effects of both viscous dissipation and contact line friction on the jumping behaviors. We conclude that curvature-enhanced jumping in the nanoscale can be ascribed to the mitigated contact line dissipation Ecl, whereas viscous dissipation Evis is maintained almost at the same level. Therefore, we unveil a scaling law between the energy conversion efficiency η on surfaces with different curvatures and the product of contact line length and contact time. Interestingly, the increasing surface curvature could allow the occurrence of coalescence-induced jumping on a less superhydrophobic surface. Hence, a phase map of coalescence-induced jumping in terms of surface curvature ratio and surface wettability is presented. Essentially, the paradigm of curved surfaces in the nanoscale used in this study is characteristic of the topography of micro/nanostructured surfaces, on which coalescence-induced droplet jumping has been experimentally observed. This work justifies the critical role of nanoroughness in boosting coalescence-induced jumping of nanodroplets and sheds light on the passive control of nanodroplets jumping on functional surfaces.

Continuous size-based separation of microparticles in a microchannel with symmetric sharp corner structures.

A new microchannel with a series of symmetric sharp corner structures is reported for passive size-dependent particle separation. Micro particles of different sizes can be completely separated based on the combination of the inertial lift force and the centrifugal force induced by the sharp corner structures in the microchannel. At appropriate flow rate and Reynolds number, the centrifugal force effect on large particles, induced by the sharp corner structures, is stronger than that on small particles; hence after passing a series of symmetric sharp corner structures, large particles are focused to the center of the microchannel, while small particles are focused at two particle streams near the two side walls of the microchannel. Particles of different sizes can then be completely separated. Particle separation with this device was demonstrated using 7.32 µm and 15.5 µm micro particles. Experiments show that in comparison with the prior multi-orifice flow fractionation microchannel and multistage-multiorifice flow fractionation microchannel, this device can completely separate two-size particles with narrower particle stream band and larger separation distance between particle streams. In addition, it requires no sheath flow and complex multi-stage separation structures, avoiding the dilution of analyte sample and complex operations. The device has potentials to be used for continuous, complete particle separation in a variety of lab-on-a-chip and biomedical applications.