Capillary filling in microchannels with wall corrugations: a comparative study of the Concus-Finn criterion by continuum, kinetic, and atomistic approaches

Photothermal and Concus Finn capillary assisted superhydrophobic fibrous network enabling instant viscous oil transport for crude oil cleanup

Rapid and effective removal of highly viscous oil spills from the sea remains a great challenge globally. Superhydrophobic materials are attractive candidates for handling oil spills, but they are restrained to recover oils with low viscosity exclusively. Herein, we report a novel polypyrrole wrapped superhydrophobic fibrous network using cross-shaped polyester fibers as starting blocks. The polypyrrole coating enables the absorbent to convert light to heat, ensuring that the viscosity of heavy oils in the proximity can be easily controlled. In the meanwhile, the special structure of the starting fibers initiates Concus Finn (CFin) capillary allowing instant oil transport in the network. When the absorbent is exposed to light oils (0-500 mPa.s), the oils can be transported instantly via CFin capillary. Interestingly, under synergistic effect of light-to-heat conversion and CFin capillary, a drawing-sticking crude oil strip (10⁵ mPa.s) is sucked instantly against gravity by the absorbent. The absorbent is successfully applied to efficiently separate both oil/water mixtures and oil/water emulsions (efficiency > 99%). Such absorbent can absorb 62.99-74.23 g/g light oils on average and up to 123.3 g/g crude oil under 0-2 sun illumination, holding a huge potential in managing oil spills.

Concus Finn Capillary driven fast viscous oil-spills removal by superhydrophobic cruciate polyester fibers

Developing functional materials integrating multi-tasking oil/water separation performances is significant but challenging for the remediation of large-scale oil spills causing pernicious environmental damages. Herein, a novel Concus Finn Capillary driven oil sorbent (OSCPF) fabricated by aligning superhydrophobic cruciate polyester fibers based on yarn spinning mechanism is designed to realize the clean-up of oil spills and various oil/water mixtures at high speeds. Instantaneous oil diffusion is achieved by abrupt Concus Finn Capillary driven oil-flows along aligned channels. This advance reduces the penetrating time for viscous crude oils by 95.00% comparing with that of non-oriented circular polyester fibers. The OSCPF possess great oil sorption capacity of 54.36-124.71 g/g and can separate oils from immiscible oil/water mixtures, including seawater, soap-water, CuCl₂-water, and KMnO₄-water, and surfactant-stabilized O/W emulsions by the way of adsorption with satisfactory separation efficiency (99.41-99.83%). Especially, the OSCPF is effectively used to enclose oil spills to prevent rapid oil diffusion and in-situ continuously collect the spillages from water surface and underwater by pumping device with recovery rates of 15,727-104,227 L·m^-2·h^-1. Considering the unique structural design, fast oil sorption speed, and low operating cost, this work provides a prospective oil remover addressing the remediation of catastrophic multi-tasking oil/water pollutions.

A numerical model of superspreading surfactants on hydrophobic surface

Many contributions significantly on experimental and mathematical studies are made to understand the mechanism of superspreading. Only few numerical methods have been proposed which solve the system of equations with soluble and insoluble surfactants. Among them, we propose a computational fluid dynamics model, based on the volume of fluid technique, with the piecewise linear interface calculation method. Interface reconstruction is applied to simulate the time evolution of the dynamics of drop spreading of surfactants on a thin water layer. We have allowed the occurrence of both the regimes relating to a series of trisiloxane (M(D′EnOH)M), sodium dodecyl sulphate, and Tergitol NP10 surfactants drop on a thin water layer with the influence of Marangoni stress. The numerical data seem consistent with those experimental for both regimes. It validates predictions for the spreading exponent in which the law of the radius of the circular area covered by the surfactant grows as tα, where 0 < α < 1. The comparison of the numerical and experimental predictions by Lee et al. [“Spreading of trisiloxanes over thin aqueous layers,” Colloid J. 71, 365–369 (2009)] is well represented in both regimes. The numerical study confirms that the spreading rates during the first stage increase as the solubility increases. This finding suggests that the model is adequate for describing the spreading of surfactants on thin fluid layers.

Capillary Pinning Assisted Patterning of Cell-Laden Hydrogel Microarrays in Microchips

We present a capillary pinning technique that gives complete control on the local patterning of hydrogel structures in closed microchips. The technique relies on selective trapping of liquids at predefined locations in a microchip using capillary barriers. In selective patterning, the abrupt expansion in the cross-sectional geometry of a microchannel at capillary barriers results in a confined advancement of the liquid-air meniscus. This protocol describes a detailed procedure to design and fabricate microarrays of different hydrogel types, fabricated with photopolymerization or thermogelation. The process can be subdivided into two parts. First, a PDMS microchip containing microfeatures with customized patterns is fabricated. Second, the microchip is filled with a hydrogel precursor to be cross-linked by either photopolymerization or thermogelation. The production of the microchip takes approximately 2 days, depending on the substrate selection. Preparation of the hydrogel solutions takes 1-2 h, whereas the patterning and reaction to cross-link the hydrogels is completed in a few minutes.

Dynamics of Capillary-Driven Flow in 3D Printed Open Microchannels

Microchannels have applications in microfluidic devices, patterns for micromolding, and even flexible electronic devices. Three-dimensional (3D) printing presents a promising alternative manufacturing route for these microchannels due to the technology's relative speed and the design freedom it affords its users. However, the roughness of 3D printed surfaces can significantly influence flow dynamics inside of a microchannel. In this work, open microchannels are fabricated using four different 3D printing techniques: fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering, and multi jet modeling. Microchannels printed with each technology are evaluated with respect to their surface roughness, morphology, and how conducive they are to spontaneous capillary filling. Based on this initial assessment, microchannels printed with FDM and SLA are chosen as models to study spontaneous, capillary-driven flow dynamics in 3D printed microchannels. Flow dynamics are investigated over short (∼10^-3 s), intermediate (∼1 s), and long (∼10² s) time scales. Surface roughness causes a start-stop motion down the channel due to contact line pinning, while the cross-sectional shape imparted onto the channels during the printing process is shown to reduce the expected filling velocity. A significant delay in the onset of Lucas-Washburn dynamics (a long-time equilibrium state where meniscus position advances proportionally to the square root of time) is also observed. Flow dynamics are assessed as a function of printing technology, print orientation, channel dimensions, and liquid properties. This study provides the first in-depth investigation of the effect of 3D printing on microchannel flow dynamics as well as a set of rules on how to account for these effects in practice. The extension of these effects to closed microchannels and microchannels fabricated with other 3D printing technologies is also discussed.

Wetting hysteresis induced by nanodefects

Wetting of actual surfaces involves diverse hysteretic phenomena stemming from ever-present imperfections. Here, we clarify the origin of wetting hysteresis for a liquid front advancing or receding across an isolated defect of nanometric size. Various kinds of chemical and topographical nanodefects, which represent salient features of actual heterogeneous surfaces, are investigated. The most probable wetting path across surface heterogeneities is identified by combining, within an innovative approach, microscopic classical density functional theory and the string method devised for the study of rare events. The computed rugged free-energy landscape demonstrates that hysteresis emerges as a consequence of metastable pinning of the liquid front at the defects; the barriers for thermally activated defect crossing, the pinning force, and hysteresis are quantified and related to the geometry and chemistry of the defects allowing for the occurrence of nanoscopic effects. The main result of our calculations is that even weak nanoscale defects, which are difficult to characterize in generic microfluidic experiments, can be the source of a plethora of hysteretical phenomena, including the pinning of nanobubbles.

Oscillatory regimes of capillary imbibition of viscoelastic fluids through concentric annulus

Here we report the capillary filling dynamics of a viscoelastic fluid through a concentric annulus, which offers a distinct disparity in the dynamical characteristics as compared to the classical cylindrical capillary based paradigm.

Large scale patterning of hydrogel microarrays using capillary pinning

Capillary barriers provide a simple and elegant means for autonomous fluid-flow control in microfluidic systems. In this work, we report on the fabrication of periodic hydrogel microarrays in closed microfluidic systems using non-fluorescent capillary barriers. This design strategy enables the fabrication of picoliter-volume patterns of photopolymerized and thermo-gelling hydrogels without any defects and distortions.

Phaseguides as tunable passive microvalves for liquid routing in complex microfluidic networks

A microfluidic passive valving platform is introduced that has full control over the stability of each valve. The concept is based on phaseguides, which are small ridges at the bottom of a channel acting as pinning barriers. It is shown that the angle between the phaseguide and the channel sidewall is a measure of the stability of the phaseguide. The relationship between the phaseguide-wall angle and the stability is characterized numerically, analytically and experimentally. Liquid routing is enabled by using multiple phaseguide with different stability values. This is demonstrated by filling complex chamber matrices. As an ultimate demonstration of control, a 400-chamber network is used as a pixel array. It is the first time that differential stability is demonstrated in the realm of passive valving. It ultimately enables microfluidic devices for massive data generation in a low-cost disposable format.

Switchable imbibition in nanoporous gold

Spontaneous imbibition enables the elegant propelling of nano-flows because of the dominance of capillarity at small length scales. The imbibition kinetics are, however, solely determined by the static host geometry, the capillarity, and the fluidity of the imbibed liquid. This makes active control particularly challenging. Here we show for aqueous electrolyte imbibition in nanoporous gold that the fluid flow can be reversibly switched on and off through electric potential control of the solid-liquid interfacial tension, that is, we can accelerate the imbibition front, stop it, and have it proceed at will. Simultaneous measurements of the mass flux and the electrical current allow us to document simple scaling laws for the imbibition kinetics, and to explore the charge transport in the metallic nanopores. Our findings demonstrate that the high electric conductivity along with the pathways for fluid/ionic transport render nanoporous gold a versatile, accurately controllable electrocapillary pump and flow sensor for minute amounts of liquids with exceptionally low operating voltages.

Thin liquid film flow over substrates with two topographical features

A multicomponent lattice Boltzmann scheme is used to investigate the surface coating of substrates with two topographical features by a gravity-driven thin liquid film. The considered topographies are U- and V-shaped grooves and mounds. For the case of substrates with two grooves, our results indicate that for each of the grooves there is a critical width such that if the groove width is larger than the critical width, the groove can be coated successfully. The critical width of each groove depends on the capillary number, the contact angle, the geometry, and the depth of that groove. The second groove critical width depends on, in addition, the geometry and the depth of the first groove; for two grooves with the same geometries and depths, it is at least equal to that of the first groove. If the second groove width lies between the critical widths, the second groove still can be coated successfully on the condition that the distance between the grooves is considered larger than a critical distance. For considered contact angles and capillary numbers our results indicate that the critical distance is a convex function of the capillary number and the contact angle. Our study also reveals similar results for the case of substrates with a mound and a groove.

Self-similarity of contact line depinning from textured surfaces

The mobility of drops on surfaces is important in many biological and industrial processes, but the phenomena governing their adhesion, which is dictated by the morphology of the three-phase contact line, remain unclear. Here we describe a technique for measuring the dynamic behaviour of the three-phase contact line at micron length scales using environmental scanning electron microscopy. We examine a superhydrophobic surface on which a drop's adhesion is governed by capillary bridges at the receding contact line. We measure the microscale receding contact angle of each bridge and show that the Gibbs criterion is satisfied at the microscale. We reveal a hitherto unknown self-similar depinning mechanism that shows how some hierarchical textures such as lotus leaves lead to reduced pinning, and counter-intuitively, how some lead to increased pinning. We develop a model to predict adhesion force and experimentally verify the model's broad applicability on both synthetic and natural textured surfaces.

A phaseguided passive batch microfluidic mixing chamber for isothermal amplification

With a view to developing a rapid pathogen detection system utilizing isothermal nucleic acid amplification, the necessary micro-mixing step is innovatively implemented on a chip. Passive laminar flow mixing of two 6.5 μl batches differing in viscosity is performed within a microfluidic chamber. This is achieved with a novel chip space-saving phaseguide design which allows, for the first time, the complete integration of a passive mixing structure into a target chamber. Sequential filling of batches prior to mixing is demonstrated. Simulation predicts a reduction of diffusive mixing time from hours down to one minute. A simple and low-cost fabrication method is used which combines dry film resist technology and direct wafer bonding. Finally, an isothermal nucleic acid detection assay is successfully implemented where fluorescence results are measured directly from the chip after a one minute mixing sequence. In combination with our previous work, this opens up the way towards a fully integrated pathogen detection system in a lab-on-a-chip format.

Modeling the transport of nanoparticle-filled binary fluids through micropores

Understanding the transport of multicomponent fluids through porous medium is of great importance for a number of technological applications, ranging from ink jet printing and the production of textiles to enhanced oil recovery. The process of capillary filling is relatively well understood for a single-component fluid; much less attention, however, has been devoted to investigating capillary filling processes that involve multiphase fluids, and especially nanoparticle-filled fluids. Here, we examine the behavior of binary fluids containing nanoparticles that are driven by capillary forces to fill well-defined pores or microchannels. To carry out these studies, we use a hybrid computational approach that combines the lattice Boltzmann model for binary fluids with a Brownian dynamics model for the nanoparticles. This hybrid approach allows us to capture the interactions among the fluids, nanoparticles, and pore walls. We show that the nanoparticles can dynamically alter the interfacial tension between the two fluids and the contact angle at the pore walls; this, in turn, strongly affects the dynamics of the capillary filling. We demonstrate that by tailoring the wetting properties of the nanoparticles, one can effectively control the filling velocities. Our findings provide fundamental insights into the dynamics of this complex multicomponent system, as well as potential guidelines for a number of technological processes that involve capillary filling with nanoparticles in porous media.

Interplay between shape and roughness in early-stage microcapillary imbibition

Flows in microcapillaries and associated imbibition phenomena play a major role across a wide spectrum of practical applications, from oil recovery to inkjet printing and from absorption in porous materials and water transport in trees to biofluidic phenomena in biomedical devices. Early investigations of spontaneous imbibition in capillaries led to the observation of a universal scaling behavior, known as the Lucas-Washburn (LW) law. The LW allows abstraction of many real-life effects, such as the inertia of the fluid, irregularities in the wall geometry, and the finite density of the vacuum phase (gas or vapor) within the channel. Such simplifying assumptions set a constraint on the design of modern microfluidic devices, operating at ever-decreasing space and time scales, where the aforementioned simplifications go under serious question. Here, through a combined use of leading-edge experimental and simulation techniques, we unravel a novel interplay between global shape and nanoscopic roughness. This interplay significantly affects the early-stage energy budget, controlling front propagation in corrugated microchannels. We find that such a budget is governed by a two-scale phenomenon: The global geometry sets the conditions for small-scale structures to develop and propagate ahead of the main front. These small-scale structures probe the fine-scale details of the wall geometry (nanocorrugations), and the additional friction they experience slows the entire front. We speculate that such a two-scale mechanism may provide a fairly general scenario to account for extra dissipative phenomena occurring in capillary flows with nanocorrugated walls.

Spontaneous imbibition in disordered porous solids: a theoretical study of helium in silica aerogels

We present a theoretical study of spontaneous imbibition of liquid (4)He in silica aerogels focusing on the effect of porosity on the fluid dynamical behavior. We adopt a coarse-grained three-dimensional lattice-gas description like in previous studies of gas adsorption and capillary condensation and use a dynamical mean-field theory, assuming that capillary disorder predominates over permeability disorder as in recent phase-field models of spontaneous imbibition. Our results reveal a remarkable connection between imbibition and adsorption as also suggested by recent experiments. The imbibition front is always preceded by a precursor film, and the classical Lucas-Washburn √t scaling law is generally recovered, although some deviations may exist at large porosity. Moreover, the interface roughening is modified by wetting and confinement effects. Our results suggest that the interpretation of the recent experiments should be revised.

Molecular transport and flow past hard and soft surfaces: computer simulation of model systems

The equilibrium and flow of polymer films and drops past a surface are characterized by the interface and surface tensions, viscosity, slip length and hydrodynamic boundary position. These parameters of the continuum description are extracted from molecular simulations of coarse-grained models. Hard, corrugated substrates are modelled by a Lennard-Jones solid while polymer brushes are studied as prototypes of soft, deformable surfaces. Four observations are discussed. (i) If the surface becomes strongly attractive or is coated with a brush, the Navier boundary condition fails to describe the effect of the surface independently of the strength and type of the flow. This failure stems from the formation of a boundary layer with an effective, higher viscosity. (ii) In the case of brush-coated surfaces, flow induces a cyclic, tumbling motion of the tethered chain molecules. Their collective motion gives rise to an inversion of the flow in the vicinity of the grafting surfaces and leads to strong, non-Gaussian fluctuations of the molecular orientations. The flow past a polymer brush cannot be described by Brinkman's equation. (iii) The hydrodynamic boundary condition is an important parameter for predicting the motion of polymer droplets on a surface under the influence of an external force. Their steady-state velocity is dictated by a balance between the power that is provided by the external force and the dissipation. If there is slippage at the liquid-solid interface, the friction at the solid-liquid interface and the viscous dissipation of the flow inside the drop will be the dominant dissipation mechanisms; dissipation at the three-phase contact line appears to be less important on a hard surface. (iv) On a soft, deformable substrate like a polymer brush, we observe a lifting-up of the three-phase contact line. Controlling the grafting density and the incompatibility between the brush and the polymer liquid we can independently tune the softness of the surface and the contact angle and thereby identify the parameters for maximizing the deformation at the three-phase contact.

Phaseguides: a paradigm shift in microfluidic priming and emptying

Phaseguide technology gives complete control over filling and emptying of any type of microfluidic structures, independent of the chamber and channel geometry. The technique is based on a step-wise advancement of the liquid-air interface using the meniscus pinning effect. In this paper, the main effects and parameters underlying the phaseguiding principle are discussed and a demonstration is given of its potential for dead angle filling, spatially controlled phaseguide overflow and sequential phaseguide overflow, all accumulating in a passive valving approach. Phaseguides represent a new direction in microfluidic design thinking that will prove a leap forward towards more simple, flexible and reliable microfluidic systems.

Method for wettability characterization based on contact line pinning

We demonstrate an efficient and reliable method for wettability characterization by determining the contact angle theta which a liquid-vapor interface makes with a solid wall. The purpose is to overcome the difficulties, related to the curvature of the liquid-vapor interface, which make measurements of theta rather uncertain, especially on the micro- and nanoscale. The method employs a specially designed slitlike channel in contact with a reservoir whereby the wettability of one of the slit walls is to be examined whereas the other (auxiliary) wall is separated by half into a lyophilic and a lyophobic part so as to pin the incoming fluid and fix the one end of the liquid-vapor interface. In the present work, the physical background of the method is elucidated theoretically while the method's applicability is demonstrated by molecular-dynamics simulation of a typical Lennard-Jones fluid, in contact with an atomistic wall. The wettability of the latter, as described by the corresponding contact angle theta, is accurately determined by variation of the liquid-wall interaction in a very broad interval.

Capillary filling in microchannels patterned by posts

We investigate the capillary filling of three-dimensional microchannels with surfaces patterned by posts of square cross section. We show that pinning on the edges of the posts suppresses and can halt capillary filling. We stress the importance of the channel walls in controlling whether filling can occur. In particular for channels higher than the distance between adjacent posts, filling occurs for contact angles less than a threshold angle of approximately 55 degrees , independent of the height of the channel.