Transport Of Liquid Droplets Research Articles

Electrically switched asymmetric interfaces for liquid manipulation.

External field driven fluid manipulation, in particular electric field, offers the advantages of real-time control and exceptional flexibility, rendering it highly promising for applications in microfluidic devices, liquid separation and energy catalysis. However, it is still challenging for controlled liquid transport and fine control of droplet splitting. Herein, we demonstrate a strategy to achieve direction-controlled liquid transport and fine droplet splitting on an anisotropic groove-microstructured electrode surface via an electrically switched asymmetric interface. The balance of asymmetric capillary force generated by microstructures and electro-capillary force is critical in determining directional liquid transport and fine droplet splitting. Asymmetric bubbles generated by liquid electrolysis form an asymmetric liquid-gas-solid interface and result in gradient liquid wetting behavior on the two neighboring electrode surfaces. The electric field further enhances the asymmetric wetting of a liquid droplet on the electrode surface, exhibiting electric field direction-dependent motion. Moreover, the groove-microstructured electrode surface can strengthen the liquid droplet anisotropic wetting and correspondingly refine the volume range of the splitting sub-droplet. Even unidirectional/bidirectional liquid droplet transport can be controlled in collaboration with the asymmetric groove-microstructure and electric field. Thus, this work provides a new route for liquid transport and droplet splitting, showing great potential in controllable separation, microreaction and microfluidic devices.

High-Performance Directional Water Transport Using a Two-Dimensional Periodic Janus Gradient Structure.

Numerous materials in micro- or nanoscale hierarchical structures with surface gradients serve as the enablers in directional liquid transportation. However, concurrent high-speed and long-range liquid transport is yet to be fully realized so far. Here, an overall-improved approach is achieved in both water transport distance and velocity aspects using a 2D periodic Janus gradient structure, which is inspired by the Janus-wettable desert beetle back, tapered asymmetric cacti spine, and periodic Nepenthes alata microcavity. This 2D channel can efficiently regulate the kinetics of liquid transport within its confined structure, in which the terminal potential well and periodic Janus topological structure enable sustaining water propelling through a long distance. In addition, the rapidly formed aqueous film facilitates a high initial momentum and fast transport of liquid droplets along the channel, achieving an averaged velocity of over 400mm s-1 and a maximum normalized transport distance of 23.4 for a 3µL droplet, as well as an ultralow liquid volume loss of 6.02% upon high-flux water transport. This scalable, controllable, and easy-fabricable 2D water transport system provides an insightful pathway in realizing high-performance water manipulation and possibly facilitates substantial innovative applications in multidisciplinary fields.

Wetting Dynamics of Nanoparticle Dispersions: From Fully Spreading to Non-sticking and the Deposition of Nanoparticle-Laden Surface Droplets.

Controlled transport of liquid droplets on solid surfaces is critical in many practical applications, such as self-cleaning surfaces, coating, drug delivery, and agriculture. Non-adhesive liquid drops levitate on solid surfaces; therefore, they are highly mobile and directed toward desired locations by external stimuli. Although research on liquid-repellent surfaces has proliferated, the existing methods are still limited to creating surface roughness or coating the liquid droplets. Here, we create non-contact aqueous drops on hydrophilic surfaces in an oleic environment and use them to deposit submicrometer droplets encapsulating nanoparticles on solid surfaces. A glass surface is buried under an oil phase that contains a high concentration of Span 80 surfactants, and a drop of silica nanoparticle dispersion is released on the solid surface. We study the effect of surfactant concentration in oil and nanoparticle concentration in water on wetting dynamics and report a plethora of droplet spreading regimes from fully wetting to non-wetting. We find a threshold Span 80 concentration above which surfactant assemblies are formed on the solid and prevent the direct contact of the drop with the surface. At the same time, water-in-oil emulsions are generated at the drop-oil interface. The drop moves and leaves a trace of emulsions with encapsulated nanoparticles on the solid. We demonstrate the possibility of local surface coating with hydrophilic nanoparticles in a hydrophobic medium. The developed methodology in this study is a generic approach facilitating the droplet patterning in numerous applications, from pharmaceutical polymetric carriers to the formulation of cosmetics, insecticides, and biomedical diagnoses.

Dynamic behavior of droplet transport on realistic gas diffusion layer with inertial effect via a unified lattice Boltzmann method

The dynamic behavior of liquid droplets on a reconstructed real gas diffusion layer (GDL) surface with the inertial effect produced by the three dimensional (3D) flow channel is investigated using an improved pseudopotential multiphase model within the unified lattice Boltzmann model (ULBM) framework, which can realize thermodynamic consistency and tunable surface tension. The microstructure of the GDL (Toray-090) including carbon fibers and polytetrafluoroethylene (PTFE) is reconstructed by a stochastic and mixed-wettability model. The critical force formulation for the Cassie-Wenzel transition of a droplet on GDL surface is derived. The effects of inertia and contact angles on the liquid droplet transport process on a reconstructed real GDL surface with a 3D flow channel are investigated. The results show the normalized center-of-mass coordinate X may enter the channel wall area or fluctuate around the initial position. With increased inertia applied on the droplet, the normalized center-of-mass coordinate Y grows faster and the normalized center-of-mass coordinate Z decreases. It is found by the ULBM for the first time that the liquid droplet is pushed back into the GDL by inertial effect. With the increase of inertia and the decrease of contact angle of GDL, both the droplet penetration depth in GDL and the droplet invasion fraction increase. The droplet invasion fraction in GDL is up to 30%.

Design of Continuous Transport of the Droplet by the Contact-Boiling Regime.

Joule-heat-driven directional transport of liquid droplets has comprehensive engineering applications in various water and thermal management, cooling systems, and self-cleaning. Generally, the driving force for the transport of liquid droplets was always observed at an extremely high Leidenfrost temperature, which limits the potential application between liquid boiling and Leidenfrost points. In this work, we design a new strategy to directionally drive the transport of droplets by blockading the vapor cushion at a temperature much lower than the Leidenfrost point. On the surface of the microhole arrays, we observed the continuous rebound behavior of ethanol droplets at Ts = 110 °C. Employing the thermal multiphase lattice Boltzmann model, the continuous rebound behavior was reproduced, verifying that the driving force was provided by the blockaded vapor pressure in microholes. By cooperating with the Laplace pressure difference, we directionally transport ethanol and water droplets on the horizontal asymmetrical concentric microridge surface. The horizontal velocity of water is 11.25 cm/s at Ts = 180 °C, similar to the traditional ratchets at the Leidenfrost point. The design of microtextures enriches the fundamental understanding of how to drive droplets at far below the Leidenfrost point and pushes the application in nongravity-driven self-cleaning and cooling systems.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part I: Experimental Findings.

Directional transport of liquid droplets is crucial for various applications including water harvesting, anti-icing, and condensation heat transfer. Here, bouncing of water droplets with patterned superhydrophobic surfaces composed of circular equidistant grooves was studied. The directional transport of droplets toward the pole of the grooves was observed. The impact of the Weber number, initial polar distance r, and geometrical parameters of the surface on the directional droplet bouncing was experimentally explored. The nature of bouncing was switched when the Weber numbers exceeded We ≅ 20-25. The rebouncing height was slightly dependent on the initial polar coordinate of the impact point for a fixed We, whereas it grew for We > 20. The weak dependence of the droplet spreading time on the Weber number was close to the dependence predicted by the Hertz bouncing, thus evidencing the negligible influence of viscosity in the process. Change in the scaling exponent describing the dependence of the normalized spreading time on the Weber number was registered for We ≅ 25. The universal dependence of the offset distance ΔL of the droplets on the Weber number ΔL/D0 ∼ We1.5 was established. The normalized offset distance decreased with the normalized initial polar distance as ΔL/D0 ∼ (r/S)-1, where D0 and S are the droplet diameter and groove width, respectively. This research may yield more insights into droplet bouncing on patterned surfaces and offer more options in directed droplet transportation.

Directed transport of liquid droplets on vibrating substrates with asymmetric corrugations and patterned wettability: a dissipative particle dynamics study

ABSTRACTThe development of digital (droplet-based) microfluidic (DMF) devices has received significant attention due to their importance for chemical and biomedical analyses. The precise control and manipulation of liquid droplets on a solid substrate is a major requirement for DMF devices. In this study, we propose a novel strategy to generate continuous droplet motion by combining asymmetric corrugations and patterned wettability on a vibrating substrate. The time periodic oscillations of the substrate with asymmetric triangle corrugations provide the input energy to transport a droplet along patterned stripes. Using dissipative particle dynamic (DPD) simulations, we demonstrate that hydrophobic stripes on a superhydrophobic substrate create a wettability step, which effectively constrains the droplet motion along the stripe. Due to the special design of asymmetric triangle corrugations and orientation of hydrophobic stripes, the proposed strategy enables the transport of multiple droplets with different initial locations towards a single spot and coalescence into a large droplet. We further identify a power-law dependence between the droplet velocity and the period of substrate vibration and show that this function is independent of the droplet size. The proposed droplet transportation strategy can be potentially useful for the efficient manipulation of liquid droplets in DMF devices.

Unidirectional Self-Driving Liquid Droplet Transport on a Monolayer Graphene-Covered Textured Substrate.

Controllable directional transport of liquid droplets on a functionalized surface has been a challenge in the field of microfluidics because it does not require energy supply, and the physical mechanism of such self-driving transport exhibits extraordinary contribution to fundamental understanding of some biological processes and the design of microfluidic apparatus. In this paper, we report a novel design of a surface microstructure that can realize unidirectional self-driving liquid mercury (Hg) droplet transport on a graphene-covered copper (Cu) substrate with a three-dimensional surface microstructure. We have demonstrated that a liquid Hg droplet spontaneously propagates on a grooved Cu substrate covered by a monolayer graphene without any external force fields. Classical molecular dynamics results provide a profound insight on the self-driving process of Hg droplets. It shows that the Hg droplet undergoes acceleration, deceleration, and return stages successively from the narrow to wide ends of the gradient groove. Intriguingly, Hg droplets can move continuously and unidirectionally on the three-dimensional graphene-covered surface microstructure when they accumulate enough kinetic energy from the gradient groove to break the energy barrier at the step junctions between the two neighboring unit cells. The design of the zigzag textured surface covered by a monolayer graphene artfully uses the facts; (1) the monolayer graphene can effectively reduce the droplet pinning on the textured surface, (2) the hydrophobic graphene layer reduces the friction between Hg droplets and the substrate, and (3) the textured surface can permeably interact with the droplets through the monolayer graphene to achieve a continuous self-driving process. The findings reported here open a door to explore the graphene-covered functional surface to directional transport of liquid droplets and provide an in-depth understanding of the self-driving mechanism for liquid droplets on graphene-covered textured substrates.

Biological and Engineered Topological Droplet Rectifiers.

The power of the directional and spontaneous transport of liquid droplets is revealed through ubiquitous biological processes and numerous practical applications, where droplets are rectified to achieve preferential functions. Despite extensive progress, the fundamental understanding and the ability to exploit new strategies to rectify droplet transport remain elusive. Here, the latest progress in the fundamental understanding as well as the development of engineered droplet rectifiers that impart superior performance in a wide variety of working conditions, ranging from low temperature, ambient temperature, to high temperature, is discussed. For the first time, a phase diagram is formulated that naturally connects the droplet dynamics, including droplet formation modes, length scales, and phase states, with environmental conditions. Parallel approaches are then taken to discuss the basic physical mechanisms underlying biological droplet rectifiers, and a variety of strategies and manufacturing routes for the development of robust artificial droplet rectifiers. Finally, perspectives on how to create novel man-made rectifiers with functionalities beyond natural counterparts are presented.

Dynamic Spreading and Transport of Liquid Droplets over Asymmetric Sawtooth Surface Structures

Dynamic Spreading and Transport of Liquid Droplets over Asymmetric Sawtooth Surface Structures

High-speed transport of liquid droplets in magnetic tubular microactuators.

Magnetic field-induced droplet actuation has attracted substantial research interest in recent years. However, current magnetic-controlled liquids depend primarily on magnetic particles added to a droplet, which serves as the actuator on an open surface. These liquids inevitably suffer from droplet splitting with the magnetic particles or disengaging with the magnet, possibly leading to sample contamination, which severely limits their transport speed and practical applications. Here, we report a simple and additive-free method to fabricate magnetic tubular microactuators for manipulating liquid droplets by magnetism-induced asymmetric deformation, which generates an adjustable capillary force to propel liquids. These magnetic tubular microactuators can drive various liquid droplets with controllable velocity and direction. A speed of 10 cm s-1 can be achieved, representing the highest speed of liquid motion driven by an external stimulus-induced capillary force in a closed tube found so far.

Contact dynamic modeling of a liquid droplet between two approaching porous materials

A computational fluid dynamics model based on a finite difference solution to mass and momentum conservation equations (Navier–Stokes equations) for a liquid droplet transport between two porous or nonporous contacting surfaces (CSs) is developed. The CS dynamic (equation of motion) and the spread of the incompressible liquid available on the primary surface for transfer are coupled with the Navier–Stokes equations. The topologies of the spread dynamic between and inside both surfaces (primary and CSs) are compared with experimental data. The amount of mass being transferred into the CS, predicted by the model, is also compared to the experimental measurements. The impact of the initial velocity on the spread topology and mass transfer into the pores is addressed. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2346–2353, 2014

Self-Cleaning Poly(dimethylsiloxane) Film with Functional Micro/Nano Hierarchical Structures

This paper reports a novel single-step wafer-level fabrication of superhydrophobic micro/nano dual-scale (MNDS) poly(dimethylsiloxane) (PDMS) films. The MNDS PDMS films were replicated directly from an ultralow-surface-energy silicon substrate at high temperature without any surfactant coating, achieving high precision. An improved deep reactive ion etching (DRIE) process with enhanced passivation steps was proposed to easily realize the ultralow-surface-energy MNDS silicon substrate and also utilized as a post-treatment process to strengthen the hydrophobicity of the MNDS PDMS film. The chemical modification of this enhanced passivation step to the surface energy has been studied by density functional theory, which is also the first investigation of C4F8 plasma treatment at molecular level by using first-principle calculations. From the results of a systematic study on the effect of key process parameters (i.e., baking temperature and time) on PDMS replication, insight into the interaction of hierarchical multiscale structures of polymeric materials during the micro/nano integrated fabrication process is experimentally obtained for the first time. Finite element simulation has been employed to illustrate this new phenomenon. Additionally, hierarchical PDMS pyramid arrays and V-shaped grooves have been developed and are intended for applications as functional structures for a light-absorption coating layer and directional transport of liquid droplets, respectively. This stable, self-cleaning PDMS film with functional micro/nano hierarchical structures, which is fabricated through a wafer-level single-step fabrication process using a reusable silicon mold, shows attractive potential for future applications in micro/nanodevices, especially in micro/nanofluidics.

Omniphobic “RF Paper” Produced by Silanization of Paper with Fluoroalkyltrichlorosilanes

The fabrication and properties of “fluoroalkylated paper” (“RF paper”) by vapor‐phase silanization of paper with fluoroalkyl trichlorosilanes is reported. RF paper is both hydrophobic and oleophobic: it repels water (θappH2O>140°), organic liquids with surface tensions as low as 28 mN m‐1, aqueous solutions containing ionic and non‐ionic surfactants, and complex liquids such as blood (which contains salts, surfactants, and biological material such as cells, proteins, and lipids). The propensity of the paper to resist wetting by liquids with a wide range of surface tensions correlates with the length and degree of fluorination of the organosilane (with a few exceptions in the case of methyl trichlorosilane‐treated paper), and with the roughness of the paper. RF paper maintains the high permeability to gases and mechanical flexibility of the untreated paper, and can be folded into functional shapes (e.g., microtiter plates and liquid‐filled gas sensors). When impregnated with a perfluorinated oil, RF paper forms a “slippery” surface (paper slippery liquid‐infused porous surface, or “paper SLIPS“) capable of repelling liquids with surface tensions as low as 15 mN m‐1. The foldability of the paper SLIPS allows the fabrication of channels and flow switches to guide the transport of liquid droplets.

Lattice Boltzmann simulations of liquid droplets development and interaction in a gas channel of a proton exchange membrane fuel cell

Liquid water transport in gas channels may influence and even prevent gas supply in a proton exchange membrane (PEM) fuel cell, so significantly affect cell operation. A two-phase two-dimensional lattice Boltzmann model is employed in this paper to simulate the development and interaction of two liquid droplets in a gas channel of a PEM fuel cell, focusing mainly on parametric effects, including the gas flow velocity, initial droplet distance, different micropore combinations, and the gas diffusion layer (GDL) surface wetting properties, on liquid droplet transport behaviors. Results confirm that an increased gas stream velocity and liquid pore distance may prevent liquid droplet interaction and enhance liquid water removal. Numerical simulations further indicate that different pore size combinations may promote droplet interaction, particularly with a large pore in front of a small one. On the contrary, a more hydrophobic GDL surface can decrease liquid droplet interaction, benefit liquid water removal, and consequently improve PEM fuel cell water management.

Bioinspired Colloidal Photonic Crystals with Controllable Wettability

Because of the combinatorial advantage of their unique light manipulation properties and potential applications in novel optical devices, colloidal photonic crystals (PCs), the periodic arrangement of monodispersed latex spheres, have attracted interest from researchers. In particular, colloidal PCs exhibit structural colors based on interference effects within their periodic structures. The wavelength of these colors lies in the visible range, making them particularly attractive for a variety of applications. Colloidal PCs are extensively used in templating, catalysis, and chromatographic separations. Inspired by biological PCs with both structural color and specific wettability, researchers have fabricated colloidal PCs with controllable wettability as described in this Account. The wettability can be adjusted by the intrinsic roughness of colloidal crystals in combination with the tunable chemical composition of latex surfaces. Changes in the chemical composition of the latex surface under external stimuli, such as light, electricity, and heat, can reversibly control the wettability of PCs. Furthermore, the hierarchical structure of latex particles can effectively alter the water adhesive force of superhydrophobic colloidal PCs. Patterned PCs with a variety of wettabilities can be assembled using inkjet printing from well-designed latex suspensions. By combining their structural color and specific wettability, we also exemplify some of the promising applications of colloidal PCs as templates for the construction of hierarchical structures, as indicators for controllable transport of liquid droplets, and as color-based sensors for the monitoring changes in their environment. These findings offer innovative insights into the design of novel colloidal PCs and will be of great importance for further applications of these materials.

Effect of an Encapsulated Bubble in Inhibiting Droplet Sliding

The transport of liquid droplets on surfaces carrying reactants offers advantages in the creation of fluidic devices crucial for life science applications. In a majority of situations, a selection of these droplets on a surface, rather than all of them, will need to be moved at any one time. It is a formidable challenge to deliver the motive energy source only to specific droplets while leaving the others unmoved. Here, we describe an alternative novel solution of momentarily pinning specific droplets to the surface while allowing the rest to be moved. We demonstrate this concept via the injection of a sizable bubble that is attached to a PTFE surface within a droplet. This then affects the contact line of the droplet, pinning it despite the introduction of an incline that will normally result in sliding. The use of bubbles offers easy release of pinning at will by simple rupture using mechanical means.

Adaptive Chip Cooling Using Electrowetting on Coplanar Control Electrodes

As a result of the increasing speed, expanding functionality, and decreasing size of microprocessors and integrated chips, heat dissipation power density has been rapidly growing over the past two decades. Advanced cooling techniques to meet this rising demand are crucial. Recently, electrowetting transport of liquid droplets has drawn significant interest in thermal management because of its digital programmable and reconfigurable characteristics. With a two-dimensional array of control electrodes patterned on a surface, cooling droplets can be transported along a programmable path to a hot spot without the need for mechanical moving parts. For adaptive cooling of electronic devices using electrowetting, we designed novel coplanar control electrodes, in which the electrode units were used interchangeably as the activating and the ground electrodes. In this article, we report electrowetting actuation of liquid droplets along specially designed control electrodes in an open-plate configuration. At the chip level, one water droplet of ∼14 μL transmitted by our actuation mechanism can rapidly cool down a hot spot of 27 W/cm2 by as much as 30°C. Furthermore, in a parallel-plate configuration with carbon nanotubes (CNTs) grown on the cover plate, our test results indicate that multiscale roughness can lead to superhydrophobicity and electrowetting transport over the superhydrophobic surface can reduce the pressure loss significantly.

Reversible electrowetting on silanized silicon nitride

Electrowetting is a phenomenon that has received increasing attention in recent years. It offers much promise for applications in lab-on-a-chip devices, liquid lenses, and optical displays. The contact angle can be controlled electrically facilitating the transport of liquid droplets by electrocapillary forces. This paper focuses on the requirements of the dielectric layers in electrowetting on dielectric; firstly, to provide the required dielectric qualities, that is, sufficient dielectric strength and high capacitance and, secondly, to satisfy the surface wetting characteristics, that is, sufficiently high hydrophobicity and low contact angle hysteresis. We present experimental work that, firstly, compares the dielectric characteristics of several insulating layers and secondly, proposes silanization as a way to form hydrophobic layers for electrowetting. The relation between electrowetting and dielectric breakdown was tested by means of measuring the contact angle of a water droplet placed between two electrodes on an insulated surface. The leakage current and resistance between these electrodes were measured as a function of applied voltage for the following dielectric coatings: silicon dioxide, silicon nitride and Parylene C. We compare the contact angle response of electrowetting on octadecyltrichlorosilane (OTS) monolayers and plasma-enhanced chemical vapour deposition Teflon-like fluoropolymer layers. The contact angle hysteresis of the OTS monolayers is clearly lower and this results in reversible electrowetting, that is, the hydrophobicity is sufficient to avoid contact line pinning and dewetting occurs on removal of an applied potential.

Optical levitation and transport of microdroplets: Proof of concept

A new technique for microfluidic transport of liquid droplets in “lab-on-a-chip” (LOC) applications is described in which droplets are levitated above or between solid planar surfaces through the use of thermocapillarity, the variation in a liquid’s surface tension with temperature. Levitated liquid droplets are not in contact with solid surfaces and so may be transported from point-to-point along arbitrary paths with little friction. In addition, the lack of liquid-solid contact virtually eliminates the potential for sample-to-sample contamination. The new technique therefore addresses three issues associated with existing LOC devices employing microchannels: droplet pathway constraints, transport speed, and sample-to-sample contamination.