Manipulation of droplets by dynamically controlled wetting gradients

The reversible transportation of droplets was realized by spatiotemporal control of the wetting gradient. The surface wetting was reversibly regulated by using electrochemical reactions of the ferrocenyl (Fc) alkanethiol monolayer, and the wetting gradient was generated by the application of the in-plane bias voltage to the substrate. The back-and-forth motion of the wetting boundary, where the surface changed from wetting to repulsive, sequentially caused a droplet unidirectional spreading and shrinking on the surface. These unidirectional deformations resulted in the net transport of the droplet in an inchwormlike manner. The droplet moved backward when the direction of the in-plane bias voltage was reversed.     

Wetting and self-cleaning properties of artificial superhydrophobic surfaces

The wetting and the self-cleaning properties (the latter is often called the "Lotus-Effect") of three types of superhydrophobic surfaces have been investigated: silicon wafer specimens with different regular arrays of spikes hydrophobized by chemical treatment, replicates of water-repellent leaves of plants, and commercially available metal foils which were additionally hydrophobized by means of a fluorinated agent. Water droplets rolled off easily from those silicon samples which had a microstructure consisting of rather slender spikes with narrow pitches. Such samples could be cleaned almost completely from artificial particulate contaminations by a fog consisting of water droplets (diameter range, 8-20 microm). Some metal foils and some replicates had two levels of roughening. Because of this, a complete removal of all particles was not possible using artificial fog. However, water drops with some amount of kinetic impact energy were able to clean these surfaces perfectly. A substrate where pronounced structures in the range below 5 microm were lacking could not be cleaned by means of fog because this treatment resulted in a continuous water film on the samples.     

Nanoporosity-driven superhydrophilicity: a means to create multifunctional antifogging coatings

Multifunctional nanoporous thin films have been fabricated from layer-by-layer assembled silica nanoparticles and a polycation. The resultant multilayer films were found to exhibit both antifogging and antireflection properties. The antifogging properties are a direct result of the development of superhydrophilic wetting characteristics (water droplet contact angle <5 degrees within 0.5 s or less). The nearly instantaneous sheetlike wetting promoted by the superhydrophilic multilayer prevents light scattering water droplets from forming on a surface. The low refractive index of the multilayer film (as low as 1.22) resulting from the presence of nanopores was found to impart excellent antireflection properties. Glass slides coated on both sides with a nanoporous multilayer film exhibited transmission levels as high as 99.8%. Stable superhydrophilic wetting characteristics were obtained only after a critical number of bilayers were deposited onto a surface. The assembly conditions (solution pH and nanoparticle concentration), as well as the choice of nanoparticle size, were found to strongly influence film properties. It is suggested that the superhydrophilic behavior is driven by the rapid infiltration of water into a 3D nanoporous network created under specific assembly conditions.     

Liquid nanodroplets spreading on chemically patterned surfaces

Controlling the spatial distribution of liquid droplets on surfaces via surface energy patterning can be used to deliver material to specified regions via selective liquid/solid wetting. Although studies of the equilibrium shape of liquid droplets on heterogeneous substrates exist, much less is known about the corresponding wetting kinetics. Here we present large-scale atomistic simulations of liquid nanodroplets spreading on chemically patterned surfaces. Results are presented for lines of polymer liquid (droplets) on substrates consisting of alternating strips of wetting (equilibrium contact angle theta0 = 0 degrees) and nonwetting (theta0 approximately 90 degrees) material. Droplet spreading is compared for different wavelength lambda of the pattern and strength of surface interaction on the wetting strips. For small lambda, droplets partially spread on both the wetting and nonwetting regions of the substrate to attain a finite contact angle less than 90 degrees. In this case, the extent of spreading depends on the interaction strength in the wetting regions. A transition is observed such that, for large lambda, the droplet spreads only on the wetting region of the substrate by pulling material from nonwetting regions. In most cases, a precursor film spreads on the wetting portion of the substrate at a rate strongly dependent on the width of the wetting region.     

Wetting films on chemically patterned surfaces

The behavior of thin wetting films on chemically patterned surfaces was investigated. The patterning was performed by means of imprinting of micro-grid on methylated glass surface with UV-light (λ=184.8 nm). Thus imprinted image of the grid contained hydrophilic cells and hydrophobic bars on the glass surface. For this aim three different patterns of grids were utilized with small, medium and large size of cells. The experiment showed that the drainage of the wetting aqueous films was not affected by the type of surface patterning. However, after film rupturing in the cases of small and medium cells of the patterned grid the liquid from the wetting film underwent fast self-organization in form of regularly ordered droplets covering completely the cells of the grid. The droplets reduced significantly their size upon time due to evaporation. In the cases of the largest cell grid, a wet spot on the place of the imprinted grid was formed after film rupturing. This wet spot disassembled slowly in time. In addition, formation of a periodical zigzag three-phase contact line (TPCL) was observed. This is a first study from the planned series of studies on this topic.     

Wetting behavior of water droplets on hydrophobic microtextures of comparable size

The wetting behavior of water droplets on periodically structured hydrophobic surfaces was investigated. The effect of structure geometry, roughness, and relative pore fraction on the contact angles was investigated experimentally for droplets of size comparable to the size of the structures. It was found that surface geometry may induce a transition from groove-filling and Wenzel-like behavior to nonfilling of surface grooves and consequential Cassie-Baxter behavior. Numerical calculations of the free energy of these systems suggest that the equilibrium behavior is in line with the experimental observations. The observations may serve as guidelines for the design of surfaces with the desired wetting behavior.     

Flow-focusing generation of monodisperse water droplets wrapped by ionic liquid on microfluidic chips: from plug to sphere

Generating droplets via microfluidic chips is a promising technology in microanalysis and microsynthesis. To realize room-temperature ionic liquid (IL)-water two-phase studies in microscale, a water-immiscible IL was employed as the continuous phase for the first time to wrap water droplets (either plugs or spheres) on flow-focusing microfluidic chips. The IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), could wet both hydrophilic and hydrophobic channel surfaces because of its dual role of hydrophilicity/hydrophobicity and extremely high viscosity, thus offering the possibility of wrapping water droplets in totally hydrophilic (THI), moderately hydrophilic (MHI), and hydrophobic (HO) channels. The droplet shape could be tuned from plug to sphere, with the volume from 6.3 nL to 65 pL, by adding an orifice in the focusing region, rendering the hydrophilic channel surface hydrophobic, and suppressing the Uw/UIL ratio below 1.0. Three different breakup processes were defined and clarified, in which the sub-steady breakup and steady breakup were essential for the formation of plugs and spheric droplets, respectively. The influences of channel hydrophilicity/hydrophobicity on droplet formation were carefully studied by evaluating the wetting abilities of water and IL on different surfaces. The superiority of IL over water in wetting hydrophobic surface led to the tendency of forming small, spheric aqueous droplets in the hydrophobic channel. This IL-favored droplet-based system represented a high efficiency in water/IL extraction, in which rhodamine 6G was extracted from aqueous droplets to [BMIM][PF6] in the hydrophobic orifice-included (HO-OI) channel in 0.51 s.     

Engineering the surface properties of microfluidic stickers

We introduce a simple and effective method to tailor the wetting and adhesion properties of thiolene-based microfluidic devices. This one-step lithographic scheme combines most of the advantages offered by the current methods employed to pattern microchannels: (i) the channel walls can be modified in situ or ex situ, (ii) their wettability can be varied in a continuous manner, (iii) heterogeneous patterning can be easily accomplished, with contact-angle contrasts extending from 0 to 90° for pure water, (iv) the surface modification has proven to be highly stable upon aging and heating. We first characterize the wetting properties of the modified surfaces. We then provide the details of two complementary methods to achieve surface patterning. Finally, we demonstrate the two methods with three examples of applications: the capillary guiding of fluids, the production of double emulsions, and the culture of cells on adhesive micropatterns.     

Statics of polymer droplets on deformable surfaces

The equilibrium properties of polymer droplets on a soft deformable surface are investigated by molecular dynamics simulations of a bead-spring model. The surface consists of a polymer brush with irreversibly end-tethered linear homopolymer chains onto a flat solid substrate. We tune the softness of the surface by varying the grafting density. Droplets are comprised of bead-spring polymers of various chain lengths. First, both systems, brush and polymer liquid, are studied independently in order to determine their static and dynamic properties. In particular, using a numerical implementation of an AFM experiment, we measure the shear modulus of the brush surface and compare the results to theoretical predictions. Then, we study the wetting behavior of polymer droplets with different surface/drop compatibility and on substrates that differ in softness. Density profiles reveal, under certain conditions, the formation of a wetting ridge beneath the three-phase contact line. Cap-shaped droplets and cylindrical droplets are also compared to estimate the effect of the line tension with respect to the droplet size. Finally, the results of the simulations are compared to a phenomenological free-energy calculation that accounts for the surface tensions and the compliance of the soft substrate. Depending on the surface/drop compatibility, surface softness, and drop size, a transition between two regimes is observed: from one where the drop surface energy balances the adhesion with the surface, which is the classical Young-Dupre? wetting regime, to another one where a coupling occurs between adhesion, droplet and surface elastic energies.     

Surface modification of droplet polymeric microfluidic devices for the stable and continuous generation of aqueous droplets

Droplet microfluidics performed in poly(methyl methacrylate) (PMMA) microfluidic devices resulted in significant wall wetting by water droplets formed in a liquid-liquid segmented flow when using a hydrophobic carrier fluid such as perfluorotripropylamine (FC-3283). This wall wetting led to water droplets with nonuniform sizes that were often trapped on the wall surfaces, leading to unstable and poorly controlled liquid-liquid segmented flow. To circumvent this problem, we developed a two-step procedure to hydrophobically modify the surfaces of PMMA and other thermoplastic materials commonly used to make microfluidic devices. The surface-modification route involved the introduction of hydroxyl groups by oxygen plasma treatment of the polymer surface followed by a solution-phase reaction with heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane dissolved in fluorocarbon solvent FC-3283. This procedure was found to be useful for the modification of PMMA and other thermoplastic surfaces, including polycyclic olefin copolymer (COC) and polycarbonate (PC). Angle-resolved X-ray photoelectron spectroscopy indicated that the fluorination of these polymers took place with high surface selectivity. This procedure was used to modify the surface of a PMMA droplet microfluidic device (DMFD) and was shown to be useful in reducing the wetting problem during the generation of aqueous droplets in a perfluorotripropylamine (FC-3283) carrier fluid and could generate stable segmented flows for hours of operation. In the case of PMMA DMFD, oxygen plasma treatment was carried out after the PMMA cover plate was thermally fusion bonded to the PMMA microfluidic chip. Because the appended chemistry to the channel wall created a hydrophobic surface, it will accommodate the use of other carrier fluids that are hydrophobic as well, such as hexadecane or mineral oils.     

氨气腐蚀法制备黏附性能可控的超疏水铜表面

采用简单便捷的方法制备出了具有不同黏附性能的超疏水表面. 通过控制氨气对金属铜表面的腐蚀时间, 分别制备了具有微米球及微米棒状结构的表面. 利用低表面能氟硅烷(FAS)修饰后, 2种表面均表现出超疏水特性(接触角均大于150°), 然而其黏附性能却截然相反. 具有微球结构的表面呈现出高黏附特性, 而具有微米棒状结构的表面则显示出低黏附特性. 研究发现, 表面不同的微观结构导致了液滴在其表面上分别处于Cassie-impregnating wetting态及Cassie态, 从而呈现出了不同的黏附性能.     

Wetting transitions

When a liquid droplet is put onto a surface, two situations distinguishable by the contact angle may result. If the contact angle is zero, the droplet spreads across the surface, a situation referred to as complete wetting. If the contact angle is between zero and 180°, the droplet does not spread, a situation called partial wetting. A wetting transition is a surface phase transition from partial to complete wetting. The wetting transition is generally first-order (discontinuous), implying a discontinuity in the first derivative of the surface free energy. As a consequence, at the transition a discontinuous jump in film thickness occurs from a molecularly thin to a thick film. We show here that the first-order nature of the transition can lead to the observation of metastable surface states and an accompanying hysteresis. The second part of this review deals with the exceptions to the first-order nature of the wetting transition. Two different types of continuous or critical wetting transitions have been reported, for which a discontinuity in a higher derivative of the surface free energy occurs. This consequently leads to a continuous divergence of the film thickness. The first type is long-range critical wetting, due to the long-range van der Waals forces. We show that this transition is preceded by the usual first-order wetting transition, which, however, is not achieved completely. This leads to the existence of a new intermediate wetting state, in which droplets coexist with a mesoscopic film: frustrated complete wetting. The film thickness diverges continuously from this mesoscopic film to a thick film. The second type of continuous transition is short-range critical wetting, for which the layer thickness diverges continuously all the way from a microscopic to a macroscopically thick film. This transition is interesting, as renormalization-group studies predict non-universal behaviour for the critical exponents characterizing the wetting transition. The experimental results, however, show mean field behaviour, the reason for which remains unclear.     

Induced detachment of coalescing droplets on superhydrophobic surfaces

Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on the superhydrophobic surface is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid (VOF) method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spread length of two coalesced droplets along their original center is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, and droplet size on contact time and restitution coefficient are studied and compared to the experimental results. Finally, the wetting and the self-cleaning properties of superhydrophobic surfaces have been investigated. It has been found that impinging water drops with very small amount of kinetic impact energy were able to thoroughly clean these surfaces.     

Dewetting of polystyrene thin films on poly(ethylene glycol)-modified surfaces as a simple approach for patterning proteins

A simple technique for patterning proteins utilizing dewetted polystyrene (PS) droplets is demonstrated. A polystyrene thin film was spin coated on a poly(ethylene glycol) (PEG) silane-modified surface. As the PS film dewets from the surface, upon annealing, to form droplets, the PEG-silane-modified surface is exposed, which retains its capability to resist protein adsorption, and the PS droplets allow the selective adsorption of proteins. In contrast to the undewetted flat PS film, the droplet surface had a greater amount of adsorbed proteins. Atomic force microscopy scans reveal that the roughness of the droplet surface is higher, and a multilayer of proteins results on the droplet surface. Moreover, micro- and nanoscale droplet patterns can easily be achieved by tuning the thickness of PS thin films. Because dewetting approaches for generating ordered dewetting droplets have been successfully generated by others, those approaches could be easily combined with this technique to fabricate ordered protein patterns.     

Chemical and Physical Analysis of Acetate-Oxide Sol-Gel Processing Routes for the Y-Ba-Cu-O System

The formation of three sols by fluorine-free aqueous and non-aqueous processes were analyzed and modified to vary the chemical properties of the sols (inks) to suit a variety of deposition processes such as dip-coating and ink-jet coating/printing. Ink-jet printing requires high wetting angles; choosing the right complexing agents to modify the ink allows the formation of droplets with high wetting angles on the surface. Dip-coating and ink-jet coating require low wetting angles; additives added to the sols reduce wetting angles to 10^∘ and allow complete coverage of the substrate surface. The deposition theories and requirements are briefly discussed, as are some initial tests with the printing and converting of the developed superconducting inks.     

仿生表面液固界面摩擦力的动态调控

仿生超疏水材料在自清洁、防雾抗冰、油水分离、集水等领域有着重要应用;而在不同疏水状态之间的转换将大大促进仿生超疏水材料在智能技术领域的应用.利用软印刷技术将玫瑰花表面微观结构转印到聚氨酯弹性体PU膜表面,利用机械应力实现表面微结构的动态实时调控,实现了表面微观结构在各向同性与各向异性之间的可逆转换;利用毛细管投影传感技...     

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.     

High- and low-adhesive superhydrophobicity on the liquid flame spray-coated board and paper: structural effects on surface wetting and transition between the low- and high-adhesive states

Surface wetting is an important and relevant phenomenon in several different fields. Scientists have introduced a large number of applications where special surface wetting could be exploited. Here, we study wetting phenomena on high- and low-adhesive superhydrophobic liquid flame spray (LFS)-generated TiO₂ coatings on paper and pigment-coated board substrates using water–ethanol solution as a probe liquid. Submicrometer-scale air gaps, which exist on superhydrophobic surfaces below the liquid droplets, were more stable with the ethanol increment than the larger-scale micrometric air gaps. With the droplet ethanol concentration of 15 wt%, static contact angle as high as 155?±?2° was measured on the LFS–TiO₂-coated board. Transition from the low-adhesive wetting state to the high-adhesive state was demonstrated on the LFS–TiO₂-coated paper. The LFS method enables efficient roll-to-roll production of surfaces with special wetting properties on economically viable board and paper substrate materials.     

Controllable preparation of nanoparticle-coated chitosan microspheres in a co-axial microfluidic device

In this work, we describe a novel and simple microfluidic method for fabricating nanoparticle-coated chitosan microspheres. Uniform droplets of aqueous chitosan solution were dispersed into an oil phase containing partially hydrophilic nanoparticles via a co-axial microfluidic device. Recirculating flow in the continuous phase in the area between drops enhanced mixing and allowed the nanoparticles to coat the surface of the droplets as they passed through the channel. The chitosan droplets were then crosslinked with glutaraldehyde and nanoparticle-coated microspheres were obtained. SEM characterization shows that the microspheres are monodispersed with uniform nanoparticle distribution on the surface. The dispersity, size and composition of the microspheres could all easily be controlled by changing the microfluidic flow parameters and three different types of nanoparticles were successfully used to synthesize hybrid microspheres to demonstrate the method's versatility.