Evaluation of polymer rheology from drop spreading experiments

In this paper, spreading experiments on “heavy” polymer drops are performed. “Heavy” refers to large polymer drops, i.e., the radius of the drop, R, is much larger than the capillary length, κ^-1, so that the spreading is dictated by gravity. The zero-shear viscosity can be found from measurements of the time-dependent drop radius or vice versa. Viscosity values found from spreading experiments compare well with the viscosity values found from dynamic rheological experiments.     

Refinement of solid layers via wetting on homogeneous lyophilic surfaces

Surface control additives (SCAs), or surfactants, can obstruct the wetting of liquid films on lyophilic surfaces even though they lower surface tension. In this work, this unusual behavior was used to decrease the widths of printed organic and nanoparticle lines on different homogeneous surfaces. A decrease in line width accompanied a change in its cross section. The ability of a SCA to decrease the line width was correlated with contact angle but not the surface tension of the solution. Line refinement was consequently attributed to an increase in contact angle. Because there were no reports that surfactants increased contact angles, the mechanisms to increase contact angle were discussed in static and dynamic terms. First, Owens and Wendt’s theory revealed that contact angle changes depended on SCA-induced modification of polar and dispersive interfacial components. However, a definitive increase in contact angle could not be deduced from this theory alone. Second, the effect of solutal Marangoni forces induced by the SCAs on contact angle was discussed by considering the wetting behaviors of binary solvents. SCA concentration dependence of surface tension at higher than the initial SCA concentration correlated well with the ability of the SCA to decrease line width.     

Fabrication and application of superhydrophilic surfaces: a review

Extreme wetting behaviors have been the subject of numerous studies in recent decades. Superhydrophilic surfaces with water contact angle lower than 5° is one of the most exciting research areas which has attract much attention. The ultrafast drying of such surfaces can provide outstanding properties such as antifogging, evaporative cooling, self-cleaning, and others. We review here the basic strategies and recent progress in fabricating superhydrophilic surfaces. And smart surfaces combining superhydrophilic and superhydrophobic abilities are highlighted, including surfaces with stimuli reversible wettability, patterning wettability, and gradient wetting. We also provide insights into the applications of the highly wettable surfaces, especially in devising new potentials.     

Wetting of hydrophobic substrates by nanodroplets of aqueous trisiloxane and alkyl polyethoxylate surfactant solutions

Trisiloxane surfactants at low concentrations promote the complete and rapid wetting of aqueous droplets on very hydrophobic (hydrocarbon) substrates. This behavior has not been demonstrated by any other surfactant which explains why the trisiloxanes are referred to as superspreaders. Despite many experimental and theoretical investigations the mechanism of superspreading is not fully understood. Molecular dynamics simulations using all-atom force fields have been conducted to attempt to elucidate the mechanism of superspreading. Spherical nanodroplets containing approximately 10,000 water molecules in the bulk and 475 surfactant molecules at the liquid-vapor interface were placed in the vicinity of a graphite substrate and allowed to spread freely at room temperature. In the trisiloxane case the droplet was found to spread very little, although randomly removing 175 surfactant molecules lowered the final contact angle from 110^° to 80^°. In contrast, an alkyl polyethoxylate surfactant-laden droplet was found to spread significantly further, with the equilibrium contact angle reaching 55^°. Similar results for the two surfactant systems were found for cylindrical nanodroplets spreading on a self-assembled monolayer (SAM). The reasons for the lack of spreading in the trisiloxane case and the simulation challenges associated with these systems are discussed. In support of our arguments we demonstrate that the surfactant molecules of an initially uniform aqueous trisiloxane solution self-assemble into a bilayer in tens of nanoseconds on a graphite substrate. Lastly, in a final set of simulations, neat trisiloxane droplets at 450 K are found to arrange into a layered structure on a methyl-terminated SAM and to form a sand pile-shape on a hydroxyl-terminated SAM.     

Dynamics of the wetting process on dielectric barrier discharge (DBD)-treated wood surfaces

Protection and preservation of wood properties in exterior environments can only be ensured if the surface is coated with a paint or varnish. In our experiments a dielectric barrier discharge (DBD) was used as a wood surface pretreatment for improvement of the subsequent deposition of thin paint layers from solutions onto these surfaces. As the adsorption, interfacial interactions and adhesion of paints are strongly dependent on surface wettability, the dynamics of the wetting process were analyzed. The results show that the water contact angle decreases after the DBD treatment, proving a more wettable surface. Additionally, the spreading of paint solution on the DBD-treated surface is more isotropic, showing a lower tendency to elongate along the wood fiber orientation.     

Wetting properties of glycerol on silicon,native SiO2, and bulk SiO2 by scanning polarization force microscopy

Scanning polarization force microscopy, a relatively new non-contact scanning probe microscopy technique, was applied in order to investigate the properties of liquid surfaces (droplets), such as: topography, microscopic contact angle θ, surface potential energy P(e), spreading coefficient S, and disjoining pressure П. Investigations were carried out on glycerol droplets deposited on surfaces of bare silicon, silicon covered with native oxide, and bulk silicon oxide. Contact angle values were determined from directly measured topography profiles of micro- and nanodroplets. Values of surface potential energy, spreading coefficient, and disjoining pressure were calculated based on a model of the dependence of contact angle on droplet height. The results of these experiments offer valuable insights into the mechanisms of wetting phenomena at the microscopic scale.     

Effect of Ti addition on the wetting and brazing of Sn0.3Ag0.7Cu filler on SiC ceramic

The wetting behavior of Sn0.3Ag0.7Cu (SAC) filler with the addition of Ti on SiC ceramic was investigated using sessile drop method. SiC/SiC was brazed by SAC-Ti filler with different Ti content at 1223 K (950°C) for 10 minutes. The wettability of SAC-Ti filler on SiC was significantly enhanced with the addition of Ti. The contact angle decreased at first and then increased with increasing Ti content. The lowest contact angle of 9° was obtained with SAC-1.5Ti (wt%) filler. When Ti content further increased to 2.0 wt%, the contact angle increased, due to the intense reaction of Ti–Sn. The reaction between Ti and SiC controlled the wetting behavior of SAC-Ti on the SiC substrate and the reaction products such as TiC and Ti₅Si₃ were formed. The wetting of SAC-Ti on SiC was reaction-controlled. Interfacial reaction products TiC and Ti₅Si₃ were observed. The wetting activation energy in spreading stage was calculated to be 129.3 kJ/mol. Completely filled SiC/SiC joints were obtained using the filler with Ti content higher than 0.5 wt%. The fillet height increased firstly then decreased with mounting Ti content. The shear strength of joints increased first with the addition of Ti then decreased with Ti content increasing to 2.0 wt%. The highest shear strength of 35.7 MPa was obtained with SAC-1.5 Ti (wt%) filler.     

Dispersion of water into oil in a rotor–stator mixer. Part 2: Effect of phase fraction

In Part 1 (Rueger and Calabrese, 2013), we monitored dilute water-in-oil dispersions in a batch Silverson L4R rotor–stator mixer to establish breakage mechanisms and develop a mechanistic basis for correlation of equilibrium mean drop size. In this study (Part 2) we consider the effect of water phase fraction under similar processing conditions, thereby requiring consideration of coalescence. Most of the work on the effect of phase fraction in stirred vessels was done with a low-viscosity continuous phase in turbulent flow with inertial subrange scaling (d > η). For that case drop size increases linearly with phase fraction, ?. In this study, viscous oils comprised the continuous phase, with water as the drop phase. The equilibrium DSD was measured in both laminar and turbulent flow conditions. The diameter of the largest drops was always less than the Kolmogorov microscale (d < η). A much greater increase (than the aforementioned linear relationship) in drop size with phase fraction was observed for ? ≤ 0.05; including cases where an oil soluble surfactant was present and where metal mixing head surfaces were rendered hydrophobic by treatment with silane functional groups. It is argued that this significantly greater dependence on ? is due to the flow field being locally laminar near the drops with coalescence rate being strongly affected by the collision efficiency, which depends on the viscosity of both phases. The presence of surfactant decreased drop size. The silane treatment decreased drop size; possibly by altering water drop interactions with mill head surfaces. Additional experiments were performed at higher phase fraction, where surfactant was required to stabilize the emulsion. The equilibrium drop size was found to plateau for 0.10 < ? < 0.50. The high phase fraction behavior is attributed to the competing rates of coalescence and breakage and their dependence on ? and drop size.     

Spreading of a droplet on a solid surface

The spontaneous spreading of small liquid droplets on solid surfaces is examined with the objective of developing closed-form expressions for the spreading dynamics, both for the case in which there is complete equilibrium spreading, that is the equilibrium contact angle is 0°, and for the case in which equilibrium spreading is incomplete. Such solutions are obtained using a simple hydrodynamic model. The results are consistent with the format of the universal Hoffman–Voinov–Tanner law (for complete spreading) and the modified Hoffman–Voinov–Tanner law for incomplete spreading. In the latter case, concurrence is found only when the dynamic contact angle is close to the equilibrium angle throughout the spreading process. © 1994 John Wiley & Sons, Inc.     

Enhanced liquid spreading due to porosity

A conceptual pore level model (Chem. Eng. Sci. 57 (2002) 3401) of spreading of liquid over internally wet porous particles is applied to explain reported rector level enhancement in wetting of trickle bed reactor (Ind. Eng. Chem. Res. 36 (1997) 5133). It is confirmed that a symbiotic relationship exists between internal and external wetting of porous particles whereby each enhances the other. Further, it is illustrated that liquid spreading in porous solids is driven more by porosity than by contact angle. A major implication of this phenomena is that liquid will spread more on less wettable (but porous) surfaces in comparison to more wettable (but nonporus) surfaces and a reinterpretation of experiments involving spreading of liquid over porous solids is required. On a large scenario, it is hoped that present exercise will be in important step towards explaining the complex reactor level macro phenomena by simple and conceptual pore level micro models.     

Reactive wetting and interfacial reaction mechanism of ZrC-SiC ceramic and Ag-Zr filler

In this study, the isotherm wetting and spreading behaviors of molten Ag-Zr filler on ZrC-SiC ceramic surface was investigated using a sessile drop method in vacuum. The effect of Zr content in Ag-Zr filler on the wetting behavior was studied, and the wetting mechanism was revealed in details. The results showed that the contact angle of Ag-Zr filler on the ZrC-SiC surface decreased from 140 ° to 20 ° with the Zr content varied from 0 to 8 wt.%, and the Zr was the key factor to the wetting process. The wetting dynamic analysis illustrated that the interfacial reaction controlled the wetting of Ag-Zr filler on the ZrC-SiC surface. Due to the Zr activity difference at Ag-Zr/ZrC interface, the ZrC released C into Ag-Zr filler, and reacted with Zr to form new-born ZrC with higher Zr content. The formation of the new-born ZrC promoted wetting and spreading of Ag-Zr filler on ZrC-SiC surface.     

Absorption of picoliter droplets by thin porous substrates

Absorption of picoliter (pL) droplets into porous substrates is studied experimentally and numerically. In the case of pL droplets, major phenomena involved in the interaction between droplet and porous media develop at different time scales: spreading and wetting at microseconds, absorption and wicking at milliseconds, and evaporation at seconds. Therefore, one can decouple these processes to minimize the complexity of the study. A high‐speed imaging system capable of 1 million frames per second is used to visualize individual droplets impacting, spreading, and imbibing on substrates. To simulate droplet dynamics, the governing equations for flow outside and inside porous media are proposed and solved using an in‐house developed computational fluid dynamics solver. The simulation results are in good agreement with the experimental data. The effect of drop impact velocity and fluid properties on final dot shape in the porous substrates is investigated through a series of parametric numerical studies. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1690–1703, 2017     

UV-resistant amorphous fluorinated coating for anodized titanium surfaces

In this work, a high molecular weight copolymer of tetrafluoroethylene and perfluoro-4-trifluoromethoxy-1,3-dioxole (HYFLON^® AD60) and two perfluoropolyethers (PFPEs) containing ammonium phosphate or triethoxysilane functionalities, FLUOROLINK^® F10 and FLUOROLINK^® S10 respectively, have been evaluated as protective coatings that can be easily applied on anodized titanium surfaces. Water and n-dodecane contact angle measurements have been recorded by using the sessile drop method for bare and coated surfaces in order to determine the hydrophobic and oleophobic properties of the coatings. The UV-stability of coatings have been studied by Fourier transform infrared spectroscopy (FT-IR) analyses and by observing the variation of water contact angles on coated substrates before and after UV irradiation at regular time intervals. The thickness of the fluorinated films has been measured by ellipsometry and by weight evaluation. Preliminary tests of the adhesion between films and substrate have been conducted.     

Effect of wetting difference across junction on dynamics of drops impacting on the junction of dual‐textured surfaces

The effect of wetting difference across the junction of smooth and groove‐textured portions of a dual‐textured surface on the various stages in the dynamics of liquid drops impacting on the junction is reported. Two dual‐textured surfaces with substantially different wetting difference across their junction, Δθ_e = 65° and 25°, made of intrinsically hydrophilic stainless steel and intrinsically hydrophobic poly‐di‐methyl‐siloxane (PDMS) are considered. The effect of wetting difference across the junction is dominantly seen on the net drop drift velocity and subsequent bulk drop movement, ξ. ξ on the PDMS dual‐textured surface is less than that on the stainless dual‐textured surface due to the lower wetting difference across the junction of the former. The variation of ξ with impact velocity shows a contrasting trend between the stainless steel and PDMS dual‐textured surfaces due to the difference in critical impact velocity corresponding to drop impregnation on their groove‐textured portions. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4109–4118, 2016     

Dynamic wetting of glass fibre and polymer fibre

To achieve high strength and chemical resistance in glass-reinforced plastics, it has been found essential to have a good bond between matrix and reinforcement. To obtain this bond it is necessary that good wetting of reinforcement by matrix must occur at some stage in the production cycle. Modern process developments have reduced the time during which wetting can occur to a few seconds in continuous sheeting manufacture. The wetting stage of this process has been investigated by the observation of dynamic contact angles. The rate of wetting has been found to be controlled by liquid flow and not surface tension at high processing speeds. Breakdown of this flow and intermittent wetting have been observed directly. When low energy surfaces are wetted by water, a different limiting factor is found. It is observed that a maximum dynamic contact angle is reached at relatively low wetting rates. It is suggested that these maximum angles offer a new means of assessing Zisman's ‘critical surface tension’ (γ_c) and by an extension of the technique may enable values of spreading pressure (π) to be measured.     

Effect of contact time parameter on wettability and interface phenomena of B4C substrate by Ni–Cr–Si–Fe–B–C filler alloy

In the present research, the wettability of boron carbide ceramic by BNi-1 filler alloy at various contact times from 10 to 40 min has been studied. The results of sessile drop wetting tests showed that the BNi-1 filler alloy could spread well on the B₄C surface at 10–40 min. With the increase of the contact time from the lowest time (10 min) to the highest time (40 min), the contact angle stably reduced, showing the enhancement of the spreading. However, by the increase of the contact time from 30 to 40 min, a slight change was observed in the wetting angle (from 21° to 19°). Overall, the appropriate spreading behavior of BNi-1 filler alloy on the B₄C substrate can be attributed to the tendency of nickel for the reaction with B₄C along with the simultaneous availability of silicon and chromium in the composition of this alloy. The maximum wetting angle of 48° was attained for the specimen with 10 min contact time and the minimum angle of 19° was achieved for the specimen with 40 min contact time. Due to the results, different compounds such as Ni₄B₃, CrB₂, CrB, SiC, and Ni₂Si have been observed at the system's interface. Moreover, the higher contact times can lead to the intensification of the system's interactions which can subsequently result in the higher penetration of the elements, the reacted area enlargement, and the formation of diverse microstructures and phases. The wetting experiments' results confirmed the spreading ratio calculations.     

聚碳酸酯液液分离器的流体力学模拟和中试试验

对聚碳酸酯混合液,通过液液间歇沉降实验得到分散相液滴的Sauter平均粒径为330 μm。采用Mixture多相流模型对聚碳酸酯液液静态分离器进行了模拟和分析。对平行板、波纹板和斜板进行了比较,得出斜板更适合聚碳酸酯液液分离。考察了斜板长、板间距、倾斜角度、入口流量对液液分离效果的影响,得出对于水油相体积比为0.6的物料,最优的聚结板参数为斜板倾角20°、板长0.8 m、板间距40 mm、筒内最大流速5.3×10^-3 m·s^-1。设计了聚碳酸酯液液分离器,进行了中试试验,水油两相出口浓度接近各自的溶解度值,筒内最大流速为3.9×10^-3 m·s^-1,结果表明该分离器起到了很好的液液分离效果,可以取代原有的碟片离心机设备,降低了设备和操作费用。     

Evaluation of the Performances of Hydrophobic Woven Meshes versus Liquids having Different Surface Tension in the Fuel‐Water Separation Applications

Abstract

In this study a new methodology has been defined for a deep understanding of the wettability of several woven meshes against liquids having very different wetting behaviour, such as water and jet fuels. At first, we refined the conventional Wilhelmy dynamic contact angle (DCA) technique using several reference solutions having surface tension values between 20–70 mN/m. In this way we covered the range of all the liquids involved in the real world applications. Successively, the study of the woven meshes was completed by the measure of the liquid intrusion pressure (LIP) with the same reference solutions used for DCA measurements.

Correlations between DCA and LIP were finally discussed.