Controlled Directional Water‐Droplet Spreading on a High‐Adhesion Surface

Controlled directional spreading of a droplet on a smart high‐adhesion surface was made possible by simply controlling anodic oxidation. The wettability gradient of the surface was controlled from 0.14 to 3.38° mm^?1 by adjusting the anodic oxidation conditions. When a water droplet made contact with the substrate, the droplet immediately spread in the direction of the wettability gradient but did not move in other directions, such as those perpendicular to the gradient direction, even when the surface was turned upside down. The spreading behavior was mainly controlled by the wettability gradient. Surfaces with a V‐ or inverse‐V‐shaped wettability gradient were also formed by the same method, and two droplets on these surfaces spread either toward or away from one another as designed. This method could be used to oxidize many conductive substrates (e.g., copper, aluminum) to form surfaces with variously shaped wettability gradients. It has potential for application in microfluidic devices.     

Several surfaces with special wettability: Influence on spreading and motion of W/O emulsion droplets

Physical and chemical modifications were made on the surface of the aluminum sheet to change the surface properties and superhydrophobic–hydrophilic wettability gradient surface was made on the perspex surface by using microstructure-pattering technique and self-assembled-monolayer method. By using high-speed video camera system and optical tensiometer, this paper discusses the influence of special surfaces with different wettability on spreading and motion of water, oil, and W/O emulsion droplets both experimentally and theoretically. In addition, the paper also discusses the influence of the superhydrophobic–hydrophilic wettability gradient on fluidity of W/O emulsion droplets and the coalescence process of droplets. The results showed that the contact angle of W/O emulsion droplets on the modified surfaces was related to the water and oil distribution at the three-phase line. On the wettability gradient surface, the droplet moved spontaneously when the droplet was located at the junction of the gradient. A quasi-steady theoretical model was used to analyze the driving and resistant forces acting on a droplet to improve the understanding of the self-transport behavior of the droplets.     

Viscoelastic properties and interfacial tension of polystyrene–polyethylene blends

The linear viscoelastic properties of polystyrene polyethylene (PS/PE) blends have been investigated in the molten state. For concentrations of the dispersed phase equal to 30 vol %, the blends exhibited a droplet‐matrix morphology with a volume‐average diameter of 5.5 μm for a 70/30 PS/PE blend at 200 °C and 14.7 μm for a 30/70 PS/PE blend at 230 °C. Enhanced elasticity (G′) for both blends, in the terminal zone, compared to the modulus of the matrix (PS and PE, respectively) was observed. This is related to the deformation of the droplets in the matrix phase and hence to the interfacial forces between the blend components. The results for these uncompatibilized blends are shown to be in agreement with the predictions of the emulsion model of Palierne. These predictions were used to obtain the interfacial tension between PS and PE, which was found to be between 2 and 5 mN/m at 200 °C and 4 ± 1 mN/m at 230 °C. Independent interfacial tension measurements using the breaking‐thread method resulted in a value of 4.7 mN/m and 4.1 mN/m at 200 °C and 230 °C for the respective blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1359–1368, 2000     

Picoliter water contact angle measurement on polymers

Water contact angle measurement is the most common method for determining a material's wettability, and the sessile drop approach is the most frequently used. However, the method is generally limited to macroscopic measurements because the base diameter of the droplet is usually greater than 1 mm. Here we report for the first time on a dosing system to dispense smaller individual droplets with control of the position and investigate whether water contact angles determined from picoliter volume water droplets are comparable with those obtained from the conventional microliter volume water droplets. This investigation was conducted on a group of commonly used polymers. To demonstrate the higher spatial resolution of wettability that can be achieved using picoliter volume water droplets, the wettability of a radial plasma polymer gradient was mapped using a 250 microm interval grid.     

Shape-gradient composite surfaces: water droplets move uphill

The approach of water droplets self-running horizontally and uphill without any other forces was proposed by patterning the shape-gradient hydrophilic material (i.e., mica) to the hydrophobic matrix (i.e., wax or low-density polyethylene (LDPE)). The shape-gradient composite surface is the best one to drive water droplet self-running both at the high velocity and the maximal distance among four different geometrical mica/wax composite surfaces. The driving force for the water droplets self-running includes: (1) the great difference in wettability of surface materials, (2) the low contact angle hysteresis of surface materials, and (3) the space limitation of the shape-gradient transportation area. Furthermore, the average velocity and the maximal distance of the self-running were mainly determined by the gradient angle (alpha), the droplet volume, and the difference of the contact angle hysteresis. Theoretical analysis is in agreement with the experimental results.     

Wall Effects on Spheres Settling Through Non-Newtonian Fluid Media in Cylindrical Tubes

Experimental study is performed to understand and quantify the wall and eccentric retardation effects on spheres settling in shear thinning and shear-thinning viscoelastic fluids over a wide range of diameter ratios (0.02 < λ < 0.9). The four-parameter Carreau viscosity equation has been chosen to represent the apparent viscosity-shear rate of polyacrylamide solutions. Two new wall factor corrections are presented with excellent agreement compared to experimental data.

The terminal settling velocity of a sphere in bounded fluid is significantly reduced by the presence of confining boundaries, named wall retardation effect that decreases due to the shear-thinning behavior of power law fluids, which is weaken further by the elastic effect of viscoelastic fluids. The wall factors of spheres settling in viscoelastic fluids increase at low ξ up to 50, followed by a horizontal confidence region (0.7 ≤ f ≤ 1) at high ξ. In this region, the wall factor is mainly dominated by fluids’ elasticity, which is more distinguished for small spheres. As the settling spheres approach to the wall (b/R → 1), the neighboring wall exert more intensive retardation that reduce the terminal settling velocity greatly when b/R > 0.6 in pure shear-thinning fluids, and the extra retardation effect of nearby wall increases at high concentration due to the enhanced non-Newtonian property. In contrast, the eccentric effect on settling velocity in viscoelastic fluids is cut down greatly by the fluid's elasticity, which is negligible.     

Droplet spreading on microstriped surfaces

When a droplet of fluid is deposited on a surface with chemical and/or topological patterns, its static shape is highly dependent on the 2D distribution of the patterns. In the case of chemical stripes, three distinct spreading regimes have been observed as a function of wettability contrast between the two kind of stripes. For low wettability contrast, the droplet spreads with the same [corrected] velocity normal and parallel to the stripes [corrected] and the macroscopic contact angle is close to Cassie's contact angle. When the wettability contrast is intermediate/high, the resulting shape of the droplets is elongated. In the intermediate wettability contrast regime, an ideal situation shows stick and slip behavior of the contact line, during which the contact line jumps from one stripe to another. For a high wettability contrast, the confinement of the fluid between two chemical stripes leads to a 2D spreading.     

Electrochemical Detection of Droplets in Microfluidic Devices: Simultaneous Determination of Velocity,Size and Content

Microfluidic devices were designed to electrochemically detect in a two‐phase flow the velocity, size and content of aqueous droplets containing redox species. The principle of these determinations is based on the analysis of a unique chronoamperometric response recorded during the passage of a droplet over channel microelectrodes. Two configurations of electrochemical cell with different geometries were investigated both theoretically and experimentally. Velocity and size of droplets, as well as internal recirculating convection within droplets, were evaluated from chronoamperometric curves by specific transition times depending on the cell configuration. In addition, the droplet content was probed from the Faradaic current controlled by mass transport and by internal hydrodynamic regime. For droplet velocity and size, experimental data were systematically compared to optical measurements. All the results demonstrated the high performance of the electrochemical detection reached under these conditions. They successfully validate the concept of self‐consistent electrochemical detections of aqueous droplets within microchannels for the simultaneous determination of their velocity, size and content.     

Influence of bulk elasticity and interfacial tension on the deformation of gelled water-in-oil emulsion droplets: an AFM study

We used atomic force microscopy (AFM) to study the deformation and wetting behavior of large (50-250 microm) emulsion droplets upon mechanical loading with a colloidal glass probe. Our droplets were obtained from water-in-oil emulsions. By adding gelatin to the water prior to emulsification, also droplets with a bulk elasticity were prepared. Systematic variations of surfactant and gelatin concentrations were made, to investigate their effect on the deformation and wetting behavior of the droplets and to identify the contributions of interfacial tension, bulk elasticity, and expelled water. The AFM experiments were performed in force--distance mode and showed on approach a repulsive regime which in many cases was terminated by a jump-in of the probe. In the case of pure water (i.e. gelatin-free) droplets, the repulsive part of the curve showed a good linearity, thus allowing the extraction of an effective droplet spring constant. This quantity was found to decrease on raising the surfactant concentration from below the critical micelle concentration (cmc) to well above the cmc, and its numerical values were found to correspond remarkably well to literature values for the interfacial tension. Our findings indicate that, on gelatin increase inside the droplets, the bulk elasticity gradually becomes dominant and the droplets' stiffness does not depend anymore on surfactant concentration. Also the stability of the droplet interface against wetting, as measured by the force at which the jump-in instability occurs, was enhanced by gelatin. For gelatin concentrations of > or =15 wt %, the droplets were found to behave like purely elastic bodies. Both gelatin and surfactant contribute positively to the stability against interface breakup.     

Elasticity-driven droplet movement on a microbeam with gradient stiffness: a biomimetic self-propelling mechanism

Directional movement of liquid droplets is of significance not only for certain physiological processes in nature but also for design of some microfluidic devices. In this study, we report a novel way to drive directional movement of liquid droplets on a microbeam with a varying or gradient stiffness. We use the energy method to theoretically analyze the interaction between a droplet and the elastic microbeam. The system tends to have the minimum potential energy when the droplet moves to the softer end of the beam. Therefore, a gradient change of the bending stiffness may be utilized to help the directional motion of droplets. Similarly, one can also drive droplets to move in a designed direction by varying the cross sectional geometry of the beam. Finally, some possible applications of this self-propelling mechanism are suggested.     

CFD Simulation of Droplet Formation in a Wide-Type Microfluidic T-Junction

Droplet formation in a wide-type microfluidic T-junction was studied using the computational fluid dynamics (CFD) method. Two distinct regimes of droplet formation were confirmed: dripping and jetting; and, at both regimes, droplet size decreases with an increase in capillary number. CFD simulation demonstrated that droplet formation in the T-junction can be divided into three steps: droplet emergence and growing up; separation with the disperse phase; and detachment from the channel wall. The wettability of the channel wall significantly affects the process of droplet detachment from the channel wall; also, the simulation clearly showed that droplets can be formed only when the continuous phase fluid preferentially wets the channel wall, that is, its contact angle on the wall is smaller than 90°. Finally, the CFD study verified that the disperse phase flow rate can significantly affect the droplet size as well as the mechanism of droplet formation.     

The head-on colliding process of binary liquid droplets at low velocity: high-speed photography experiments and modeling

The experimental and theoretical studies are reported in this paper for the head-on collisions of a liquid droplet with another of the same fluid resting on a solid substrate. The droplet on the hydrophobic polydimethylsiloxane (PDMS) substrate remains in a shape of an approximately spherical segment and is isometric to an incoming droplet. The colliding process of the binary droplets was recorded with high-speed photography. Head-on collisions saw four different types of response in our experiments: complete rebound, coalescence, partial rebound with conglutination, and coalescence accompanied by conglutination. For a complete rebound, both droplets exhibited remarkable elasticity and the contact time of the two colliding droplets was found to be in the range of 10–20 ms. With both droplets approximately considered as elastic bodies, Hertz contact theory was introduced to estimate the contact time for the complete rebound case. The estimated result was found to be on the same order of magnitude as the experimental data, which indicates that the present model is reasonable.     

Control over the Wettability of Colloidal Crystal Films by Assembly Temperature

Summary: A facile method to fabricate colloidal crystal films with tunable wettability from an amphiphilic material polystyrene‐block‐poly(methyl methacrylate)‐block‐poly(acrylic acid) is presented. The wettability of the film can be tuned from superhydrophilic (CA, 0°) to superhydrophobic (CA, 150.2°) by varying the assembly temperature, while the position of the photonic bandgap of the colloidal crystal films remains virtually unchanged. The method could open new application fields of colloidal crystals in diverse environments.

The relationship of assembly temperature with water CA (inset is the water droplet profile of the relative water CA).     

Low-frequency dilational elasticity of the nematic 4'-pentyl-4-biphenylcarbonitrile (5CB)/water interface

Axisymmetric oscillating pendant drop shape analysis has been used to study the interfacial rheology of the liquid crystal 4'-pentyl-4-biphenylcarbonitrile (5CB) in water with homeotropic anchoring. Nearly spherical 5CB droplets were subjected to low frequency (1-5 mHz) volume oscillations, and the increase in tension with surface dilation was used to calculate the complex modulus. The droplet interface response is completely elastic, with no relaxations occurring on the experimental time scale. This surprising result is attributed to droplet storage of elastic energy in the form of distorted orientational distributions within the bulk (Frank elasticity) and on the surface (anchoring elasticity).     

An Organic Crystal with High Elasticity at an Ultra‐Low Temperature (77 K) and Shapeability at High Temperatures

Organic single crystals with elastic bending capability and potential applications in flexible devices and sensors have been elucidated. Exploring the temperature compatibility of elasticity is essential for defining application boundaries of elastic materials. However, related studies have rarely been reported for elastic organic crystals. Now, an organic crystal displays elasticity even in liquid nitrogen (77 K). The elasticity can be maintained below ca. 150 °C. At higher temperatures, the heat setting property enables us to make various shapes of crystalline fibers based on this single kind of crystal. Through detailed crystallographic analyses and contrast experiments, the mechanisms behind the unusual low‐temperature elasticity and high‐temperature heat setting are disclosed.     

Colloid probe AFM investigation of the influence of cross-linking on the interaction behavior and nano-rheology of colloidal droplets

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.     

Layered droplet microstructures in sheared emulsions: finite-size effects

We investigate the influence of confinement on the steady state microstructure of emulsions sheared between parallel plates, in a regime where the average droplet dimension is comparable to the gap width between the confining walls. Utilizing droplet velocimetry, we find that the droplets can organize into discrete layers under the influence of shear. The number of layers decreases from two (at relatively higher shear rates) to one (at lower shear rates), as the drops grow slightly larger due to coalescence. We argue that the layering and overall composition profile may be controlled by the interplay of droplet collisions (which can cause separation of droplet centers in the velocity gradient direction), droplet migration toward the centerline (due to wall effects), and droplet packing constraints. We also study the effects of mixture composition on droplet microstructure, and summarize these results in the form of a morphology diagram in the parameter space of mass fraction and shear rate. We find that formation of strings of the suspended phase (reported earlier by our group in flow-visualization studies on confined emulsions) is observed over a broad composition window. We also find a stable (nontransient) morphology wherein the droplets are arranged in highly ordered pearl-necklace chain structures.     

Elastic modulus of a polymer nanodroplet: theory and experiment

We redevelop a theoretical model that, in conjunction with atomic force microscopy (AFM), can be used as a noninvasive method for determination of the elastic modulus of a polymer nanodroplet residing on a flat, rigid substrate. The model is a continuum theory that combines surface and elasticity theories for prediction of the droplet's elastic modulus, given experimental measurement of its adsorbed height. Utilization of AFM-measured heights for relevant droplets reported in the literature and from our own experiments illustrated the following: the significance of both surface and elasticity effects in determining a polymer droplet's spreading behavior; the extent of a continuum theory's validity as one approaches the nanoscale; and a droplet size effect on the elastic modulus.     

Mechanical properties of gradient copolymers of styrene and n‐butyl acrylate

The mechanical properties of linear and V‐shaped compositional gradient copolymer of styrene and n‐butyl acrylate with composition of around 55 wt % styrene were investigated by comparing with their block copolymer counterparts. Compared with their block copolymer counterparts, the gradient copolymers showed lower elastic modulus, much larger elongation at break, and similar ultimate tensile strength at room temperature. This performance could be ascribed to that the local moduli continuously change from the hardest nanodomains to the softest nanodomains in the gradient copolymer, which alleviates the stress concentration during tensile test. Compared with the V‐shaped gradient (VG) copolymer, the linear gradient copolymer showed much higher elastic modulus but lower elongation at break. The mechanical properties of the gradient copolymers were more sensitive to the change in temperature from 9 °C to 75 °C. With recovery temperature increased from 10 °C to 60 °C, the strain recovery of VG copolymer would change steadily from 40% to 99%. However, the elastic recovery of linear and triblock copolymer was poor even at 60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 860–868     

UV grafting of methacrylic acid and acrylic acid on high‐density polyethylene in different solvents and the wettability of grafted high‐density polyethylene. II. Wettability

The wettability of high‐density polyethylene grafted with methacrylic acid is strongly influenced by the nature of the grafting solvent. Here, the wettability is expressed by the water contact angle and absorbency. The initial (10‐s) contact angle of polyethylene (PE) grafted in acetone/water solution decreased rapidly with the extent of grafting at low grafting levels and then remained independent of the grafting level at about 50°. When a water droplet was left on the surface for a longer time, its contact angle decreased to a very low value in the period of about 10 min. For the PE samples grafted in dichloromethane, petroleum ether, cyclohexane, and chloroform, there was only a small decrease (10°) in the contact angle of water from that observed on pure PE, even when the extent of grafting was very large. The PE films grafted in these organic solvents also took a much longer time than PE films grafted in acetone/ water solution to obtain equilibrium water absorbency. The water absorbency of PE films grafted in 30% acetone/water solution was about twice that of PE films grafted in the other solvents at the same extent of grafting. These results suggested that for the solvents other than acetone/water, the grafted layer is partially buried below the surface of PE. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 263–270, 2004