Unusual Wetting Behavior Research Articles

Turning a Superhydrophilic Surface Weakly Hydrophilic: Topological Wetting States.

For water droplets placed on a rough or structured surface, two distinct wetting states commonly observed are either the Wenzel state (droplets wet the surface without showing air pockets beneath the droplets) or the Cassie state (droplets reside on top of the structure with air pockets trapped beneath the droplets). Herein, we show molecular dynamics (MD) simulation evidence of a previously unreported wetting behavior, i.e., the rise of multiple Wenzel states on the structured surfaces whose flat-surface counterparts are superhydrophilic (i.e., complete wetting surfaces with the hallmark of zero contact angle for water droplets). Specifically, our MD simulations show that on the structured surfaces with topology of closed-loop nanowalls/nanochannels, the water droplet can exhibit multiple Wenzel wetting states with the apparent contact angles >0°. We name these distinct multiple Wenzel states as "topological wetting states" because their existence can be attributed to the topology of the closed-loop nanowalls/nanochannels. Regardless of the shape of the closed loops, such topological wetting states can always arise due to the topological invariant (i.e., all closed loops entail the same topological genus value). This unusual wetting behavior is contrary to the conventional view (and to the prediction of the Wenzel equation), namely, a rough hydrophilic surface should have stronger hydrophilicity than its flat-surface counterpart.

Unusual Wetting Behavior of Liquid Metals on Porous Layer Formed at Surface of Iron Substrate Prepared by Oxidation-Reduction Process

Unusual wetting behavior of liquid Cu was found on a surface-oxidized iron substrate in reducing atmosphere. Liquid Cu wetted and spread very widely on the iron substrate when a droplet was attached with the substrate in Ar-10%H2 after the surface oxidation of the substrate. The oxidationreduction process fabricates a porous layer at the surface of the iron substrate. The pores in the porous iron layer are 3-dimensionally interconnected. Thus, liquid metals, which are contacted with the reduced iron samples, penetrate into these pores by capillary force to cause the unusual wetting behavior. It has been already confirmed that liquid Ag, Sn, In and Bi show this phenomenon onto surface-porous iron samples as well as liquid Cu. This unusual wetting behavior of a liquid metal has been correlated to the normal contact angle of the liquid metal on a flat iron substrate.

Effect of Iron Substrates on Reduced Porous Morphologies Adequate for Unusual Wetting

The authors have found that liquid metal droplets wetted and spread very widely on solid iron substrates in a reducing atmosphere after the surface oxidation of the specimens. This unusual wetting is caused by capillary penetration into the porous layer formed on the iron sample that is reduced after first being oxidized in air. In order to find adequate conditions for the unusual wetting behavior, the effect of the underlying iron substrate on the reduced porous morphology of the oxidized Fe layer has been investigated. Pores in the porous iron layers of all samples formed on iron substrates are 3-dimensionally inter-connected, while those of the porous iron films of some samples without underlying iron substrates were partially isolated and not entirely connected within the films. Adequate porous iron layers that show this unusual wetting behavior were obtained mainly from reduction of the oxidized iron layer formed on the surface of iron substrates rather than from reduction of the oxidized iron films formed without underlying iron substrates.

Unusual Wetting of Liquid Bismuth on a Surface-Porous Copper Substrate Fabricated by Oxidation-Reduction Process

The unusual wetting phenomenon of liquid Bi on surface-porous copper has been studied. A porous layer was prepared at the surface of a copper substrate and the wetting behavior of liquid Bi on this layer was investigated and compared with that on a flat solid copper substrate. A surface-oxidized copper substrate was reduced at 473 K in H2 to prepare a porous layer at the surface. Since the contact angle of liquid Bi on the flat solid copper substrate was gradually declining with increasing temperature, the behavior of liquid Bi on solid copper was shown to have good-wettability. In addition, the unusual wetting behavior of liquid Bi was observed on surface-porous copper substrate. This result is evidence that the pores in the porous layer are 3-dimensionally inter-connected. This usual wetting phenomenon of liquid Bi was then applied in the joining of copper wire with the surface-porous copper substrate. Bonding was successfully achieved with a minimized weld overlay at the joining point. [doi:10.2320/matertrans.MRA2007179]

特異拡張濡れに適したポーラス状還元鉄形態に及ぼす下地鉄基板の影響

The authors have found that liquid metals droplet wetted and spread very widely on solid substrates of Fe in reduced atmosphere after the surface oxidation of the surfaces. This unusual wetting is caused by capillary penetration into porous layer formed on the iron sample that is reduced after oxidized in air. In order to find adequate conditions for the unusual wetting behavior, we have investigated the effect of iron substrate on reduced porous morphology of oxidized iron. Pores in the porous iron layers of all samples with iron substrates are connected 3-dimensionally each other, while those of porous iron films of some samples without substrates were isolated partially and were not connected entirely in the film. Adequate porous iron layers for the unusual wetting were obtained mainly from the condition on the reduction of oxidized iron layer formed on the surface of iron substrates rather than the reduction of oxidized iron film without iron substrates.

Polyurethanes Containing Oxetane-Derived Poly(2,2-substituted-1,3-propylene oxide) Soft Blocks: Copolymer Effect on Wetting Behavior

Hydroxy-terminated poly(2,2-substituted-1,3-propylene oxide) telechelics and co-telechelics bearing semifluorinated (R = -CH(2)OCH(2)(CF(2))(n)CF(3), n = 0, 1) and/or bromomethyl pendant groups were synthesized from the corresponding 3,3-substituted oxetanes. The new telechelics were incorporated in polyurethanes (PUs) with isophorone diisocyanate (IPDI) and 1,4-butanediol (BD) as the hard block. Surface properties were evaluated using tapping mode atomic force microscopy (TM-AFM) and dynamic contact angle (DCA) analysis. Interestingly, polyurethanes containing P(3FOx-BrOx) have higher theta(adv) and lower theta(rec) than the homo-telechelic PUs [P(3FOx) = poly(2-methyl-2-trifluoroethoxymethyl-1,3-propylene oxide; P(BrOx) = poly(2-methyl-2-bromomethyl-1,3-propylene oxide)]. For IPDI-BD(40)/P(3FOx/BrOx-1:1), theta(adv) (116 degrees) is higher and theta(rec) (32 degrees) is lower (Deltatheta, 84 degrees ) than any other homo- or co-telechelic polyurethane. The unusual wetting behavior for P(FOx/BrOx) polyurethanes is correlated with FOx-BrOx dyad content, and a reversible H-bonding mechanism is proposed to explain the results.

Surface thermodynamic properties of polyelectrolyte multilayers

Multilayer architectures of polyelectrolytes fabricated by the layer-by-layer technique (LbL) on pretreated polymeric and inorganic substrates were studied by contact angle measurements. Poly(diallyldimethylammonium chloride), PDADMAC, and poly(sodium 4-styrenesulfonate), PSS, were used as polyelectrolytes. Contact angle data were used to calculate the van der Waals and Lewis acid–base components of the surface tension of the investigated surfaces. Knowledge of these quantities provides valuable information on surface composition, coating density, and possible interactions of the surface with other substances. Unusual wetting behavior of PDADMAC layers upon prewetting of the multilayer surfaces was found and described in terms of surface tension changes. A model of polymer chain rearrangement upon wetting was proposed to explain this behavior.

還元雰囲気下における表面酸化した固体鉄上の液体金属の特異拡張濡れ

The authors found that liquid Cu droplet wetted and spread very widely on a solid substrate of Fe in a reduction atmosphere after the surface oxidation of the substrate. The mechanism of the unusual wetting behavior was investigated by using surface-oxidized Fe substrate with liquid Cu, Ag, Sn and In. It was found that under reduction atmosphere, fine pores were formed at the surface of the substrate which had been oxidized, and that the pores were connected each other continuously over the whole surface. The liquid metals penetrate into those pores by capillary force to cause the unusual wetting behavior.

Unusual Wetting of Liquid Metals on Iron Substrate with Oxidized Surface in Reduced Atmosphere

The authors found that a liquid Cu droplet wetted and spread very widely on a solid substrate of Fe in a reduced atmosphere after the surface oxidation of the substrate. The mechanism of the unusual wetting behavior was investigated by using surface-oxidized Fe substrate with liquid Cu, Ag, Sn, and In. It was found that under a reduced atmosphere condition, fine pores were formed at the surface of the substrate which had been oxidized, and that the pores were connected to each other continuously over the whole surface. The liquid metals penetrate into these pores by capillary force to cause the unusual wetting behavior.

Transition temperatures and contact angles in the sequential-wetting scenario of n-alkanes on (salt) water

Alkanes of medium chain length show an unusual wetting behavior on (salt) water, exhibiting a sequence of two changes in the wetting state. When deposited on the water surface at low temperatures, the liquid alkane forms discrete droplets that are interconnected only by a molecularly thin film. On increase of the temperature, there occurs a sudden jump of the film thickness and, at this first-order transition, a mesoscopically thick layer of liquid alkane is formed. Heating the system further leads to a divergence of the film thickness in a continuous manner. While the long-range forces between substrate and adsorbate are responsible for the critical transition, which occurs at the temperature at which the Hamaker constant changes sign, it is primarily the short-range components of the forces that bring about the first-order transition. We calculate the Hamaker constant of the system and show how, within a modified Cahn theory, accurate predictions of the first-order transition temperatures can be obtained for n-alkanes (pentane and hexane) on water and even on brine. Furthermore, we study the variation of the contact angle as a function of temperature.

Surfactant Self-Assembly ahead of the Contact Line on a Hydrophobic Surface and Its Implications for Wetting

The carryover of surfactant ahead of contact lines on hydrophilic surfaces results in unusual wetting behavior of surfactant solutions, such as the autophobic effect. This carryover of surfactant ahead of the contact line is due to the strong interactions between the hydrophilic surface and the headgroup of ionic and nonionic surfactants. However, in the case of a nonionic surfactant and a hydrophobic surface, the possibility of self-assembly of surfactant ahead of the contact line has been neglected because of the absence of any obvious strong interactions and any unusual wetting behavior. In this paper, we provide evidence of carryover of a nonionic surfactant (octaethyleneoxide monododecyl ether, C12E8) ahead (i.e., on the solid−vapor, SV, side) of advancing and receding contact lines on hydrophobic self-assembled monolayers of octadecyltrichlorosilane. This system of a nonionic surfactant and hydrophobic surface was thought to be an unlikely system to show the presence of surfactant ahead of the contact line. However, the presence of a region with higher wettablity ahead of advancing and receding contact lines in condensation figure experiments provides direct evidence of the self-assembly and carryover of nonionic surfactant ahead of the contact line even on a hydrophobic surface. More evidence of this carryover is provided by comparing advancing and receding contact-angle measurements, along with independently determined solid−liquid (SL) and liquid−vapor (LV) surface tensions. These measurements show that the local SV surface tension increases with an increasing bulk surfactant concentration. As a result of this increase in the SV surface tension on a hydrophobic surface, the contact angle is lower than expected from the reduction in the SL and LV surface tensions alone. This phenomenon is “the autophilic effect”, in contrast with the autophobic effect that results in a larger contact angle. Our results also invalidate the assumption of no surfactant ahead of the contact line, a basis commonly used to estimate adsorption on SL interfaces from contact-angle data.

Wetting of fine wires and fibers by a liquid film

When a fine wire or fiber is coated with a liquid film, the latter breaks up spontaneously into beads. Such unusual wetting behavior is a particular manifestation of the general phenomenon of liquid column instability first studied by Plateau and Rayleigh. In this work various physical factors that influence the instability and the subsequent beading are examined in detail. In the first part hydrodynamic equations are solved to obtain the rate of growth of disturbances and the wavelength of the most rapidly growing disturbance. These quantities are governed primarily by the viscosity of the liquid and the thickness of the liquid film as compared to the wire radius. In the second part the static equilibrium configuration of the beads attained at the completion of the dewetting process is derived. The bead shape is determined by the contact angle of the liquid on wire and the volume of liquid available per bead. It is shown that even when the contact angle is zero, not more than three quarters of the wire surface is covered by the liquid beads.