Straightforward Fabrication of Double Roughness Structures on a Microcrystalline Film of a Diarylethene Derivative.

Double roughness structure mimicking the surface of a lotus leaf was prepared using a newly synthesized diarylethene having a six-membered perfluorocyclohexene ring. The cubic-shaped crystals of the open-ring isomer, with sizes of approximately 7 µm, appeared immediately following solution casting. Upon UV irradiation, each cubic crystal was covered with needle-shaped crystals of the closed-ring isomer to form double roughness structures within 1 h. This structure could bear the continuous impact of water droplets.

Friction dynamics of moisturized human skin under non-linear motion.

OBJECTIVE: Evaluating friction in human skin is important to assess its condition and the effects of skincare cosmetics. In this study, we evaluated the friction dynamics of moisturized skin to show the effects of moisturization on its mechanical properties. METHODS: Friction force was evaluated using a sinusoidal motion friction evaluation system. The skin of the upper arm of 20 subjects was rubbed using a contact probe. The water content of the stratum corneum and the softness of the skin were measured using a Corneometer and a Cutometer, respectively. RESULTS: When human skin was treated with water or 10 wt% glycerol aqueous solution, the friction coefficients increased by 0.23 ± 0.01 and 0.17 ± 0.14, respectively, and the delay times (normalized by calculating the time interval from contact with the probe to the friction response divided by the friction time for one round trip) increased by 0.048 ± 0.034 and 0.055 ± 0.024, respectively. Three different friction profiles were observed: (a) a stable pattern, in which a smooth profile was observed during the sliding process; (b) an oscillation pattern, in which significant oscillation was obtained; and (c) a stick pattern, in which the friction coefficient increased even during the deceleration process. In the case of untreated skin, the oscillation pattern was observed for the majority of subjects. The appearance rate of the stick pattern increased by 80.3% ± 29.4% after treatment with 10 wt% glycerol aqueous solution. These characteristic friction profiles can be explained by a two-step friction model consisting of two modes: (a) friction at the skin surface and (b) the delayed response due to skin deformation. CONCLUSION: Moisturizing the skin with water or 10 wt% glycerol aqueous solution increased the friction coefficient and delay time, dramatically changing the friction profile. These changes were considered to be due to the swelling and softening of the stratum corneum and the increased true contact area between the contact probe and the skin surface.

OBJECTIF: Une évaluation des effets de la friction sur la peau humaine demeure importante dans le but de juger de l'état de la peau ou de l'efficacité des produits cosmétiques pour les soins de la peau. Dans cette étude, nous avons évalué les propriétés d'une peau hydratée soumise à une friction afin d'identifier les effets de l'hydratation sur les propriétés mécaniques de la peau. MÉTHODE: Les forces de friction ont été évaluées grâce à un système d'évaluation du frottement par mouvement sinusoïdal. Une sonde de contact a été utilisée pour frotter la peau sur le haut du bras de 20 participants. La teneur en eau de la couche cornée et la souplesse de la peau ont été mesurées respectivement à l'aide d'un cornéomètre et d'un cutomètre. RÉSULTAT: Le traitement de la peau humaine avec de l'eau ou une solution de glycérol à 10% a entraîné une augmentation du coefficient de friction respectivement de 0.23 ± 0.01 et de 0.17 ± 0.14, ainsi que du délai de réaction (normalisé en divisant l'intervalle de temps entre le contact avec la sonde jusqu'à l'apparition de la réaction à la friction, par le temps de friction pour un aller-retour), de 0.048 ± 0.034 et de 0.055 ± 0.024. Trois profils de friction différents ont également été observés : (1) un modèle stable, (2) un modèle avec une grande oscillation, et (3) un modèle « collé-glissé ¼ où le coefficient de friction augmente même pendant la décélération. Lorsque la peau est sèche, le modèle oscillant a été observé chez la majorité des participants. Le taux d'apparition du modèle « collé-glissé ¼ a augmenté de 80.3 ± 29.4% dans le cas où la peau a été traitée avec une solution de glycérol à 10%. Ces profils caractéristiques de friction ont pu être expliqués à partir d'un modèle de friction composé de deux modes, (a) une friction à la surface de la peau et (b) un délai de réaction dû à la déformation de la peau. CONCLUSION: L'hydratation de la peau avec de l'eau ou une solution de glycérol à 10% a considérablement modifié le profil de friction en raison d'une augmentation du coefficient de friction et du délai de réaction. Nous avons estimé que ces changements sont relatifs au gonflement et à l'assouplissement de la couche cornée, engendrant une augmentation de la surface de contact réel entre la sonde de contact et la surface de la peau.

Nonlinear Friction Dynamics of Oil-in-Water and Water-in-Oil Emulsions on Hydrogel Surfaces.

In this study, the friction properties of emulsions in an oral environment were investigated to understand the food-texture recognition mechanisms occurring on biological surfaces. Numerous publications have suggested that the friction phenomena depend on friction conditions, such as the surface characteristics, as well as the shape and movement of contact probes. Traditional friction evaluation systems are unsuitable for mimicking the oral environment. Thus, in this study, the friction forces between two fractal agar gel substrates in an emulsion were examined using a sinusoidal motion friction evaluation system that effectively mimics the oral environment. The physical properties of the fractal agar gel, including the elasticity, hydrophilicity, and surface roughness, were analogous to those of the human tongue. Furthermore, the sinusoidal motion imitated the movements of living organisms. Depending on the samples, three friction profiles were observed. For water, the surfactant aqueous solution, and olive oil, the friction profiles of the outward and homeward processes were symmetric (stable pattern). Interestingly, for an oil-in-water (O/W) emulsion, friction behaviors with not only an asymmetric friction profile (unstable pattern I) but also a lubrication phenomenon, which temporarily decreased the friction force (unstable pattern II), were noted. The probability for the appearance of unstable patterns and adhesion force between the gel substrates increased with the oil content of the O/W emulsions. These characteristic friction phenomena were attributed to the strong adhesive force in the emulsion, which was sandwiched between the agar gel substrates. The findings obtained in this study would contribute significantly to understanding the food-texture recognition mechanisms and dynamic phenomena occurring on biological surfaces.

Locomotion of a Nonaqueous Liquid Marble Induced by Near-Infrared-Light Irradiation.

Micrometer-sized hydrophobic polyaniline (PANI) grains were synthesized via an aqueous chemical oxidative polymerization protocol in the presence of dopant carrying perfluoroalkyl or alkyl groups. The critical surface tensions of the PANIs synthesized in the presence of heptadecafluorooctanesulfonic acid and sodium dodecyl sulfate dopants were lower than that of PANI synthesized in the absence of dopant, indicating the presence of hydrophobic dopant on the grain surfaces. The PANI grains could adsorb to air-liquid interfaces, and aqueous and nonaqueous liquid marbles (LMs) were successfully fabricated using liquids with surface tensions ranging between 72.8 and 42.9 mN/m. Thermography studies confirmed that the surface temperature of the LMs increased by near-infrared light irradiation thanks to the photothermal property of the PANI, and the maximum temperatures measured for nonaqueous LMs were higher than that measured for aqueous LM. We demonstrated that transport of the LMs on a planar water surface can be achieved via Marangoni flow generated by the near-infrared light-induced temperature gradient. Numerical analyses indicated that the LMs containing liquids with lower specific heat and thermal conductivity and higher density showed longer path length per one light irradiation shot and longer decay time. This is because generated heat could efficiently transfer from the LMs to the water surface and larger inertial force could work on the LMs. The LMs could also move over the solid substrate thanks to their near-spherical shapes. Furthermore, it was also demonstrated that the inner liquids of the LMs could be released on site by an external stimulus.

Light-Driven Locomotion of Bubbles.

Remotely controlling the movement of small objects is a challenging research topic, which can realize the transportation of materials. In this study, remote locomotion control of particle-stabilized bubbles on a planar water surface by near-infrared laser or sunlight irradiation is demonstrated. A light-induced Marangoni flow was utilized to induce the locomotion of the bubbles on water surface, and the timing and direction of the locomotion can be controlled by irradiation timing and direction on demand. The velocity, acceleration, and force of the bubbles were analyzed. It was also confirmed that the bubbles can work as light-driven towing engines to pull other objects. Furthermore, it was demonstrated that the bubbles can work as an adhesive to bond two solid substrates by application of compressive stress under water. Such remote transport of the materials, pulling of the objects by light, and controlling the release of gas on demand should open up a wide field of conceivable applications.

Crystal Growth Technique for Formation of Double Roughness Structures Mimicking Lotus Leaf.

Bio-inspired functional materials have received much attention for their potential to provide sustainable and advanced materials. The lotus effect has proven to be one of the most remarkable biomimetic effects since it was discovered by Barthlott. A superhydrophobic surface with the ability to bounce water droplets is the origin of the self-cleaning mechanism that keeps the surface clean by removing dust using water droplets moving with momentum. We have developed a crystal growth technique (CGT) of photochromic diarylethenes over the past decade, and from this, we fabricated a surface structure that closely resembles the natural lotus leaf's characteristic of controlling the Laplace pressure and clarified the importance of the double roughness structure of the surface. The bouncing ability is also discussed in terms of the characteristic size of the double roughness structure theoretically. Moreover, this work clarifies the exquisiteness of the double roughness structure of the leaf. We also show that the CGT is a versatile technique with the potential to fabricate desired structured surfaces.

Liquid Marbles in Nature: Craft of Aphids for Survival.

Some aphids that live in the leaf galls of the host plant are known to fabricate liquid marbles consisting of honeydew and wax particles as an inner liquid and a stabilizer, respectively. In this study, the liquid marbles fabricated by the galling aphids, Eriosoma moriokense, were extensively characterized with respect to size and size distribution, shape, nanomorphology, liquid/solid weight ratio, and chemical compositions. The stereo microscopy studies confirmed that the liquid marbles have a near-spherical morphology and that the number-average diameter was 368 ± 152 µm, which is 1 order of magnitude smaller than the capillary length of the honeydew. The field emission scanning electron microscopy studies indicated that micrometer-sized wax particles with fiber- and dumpling-like shapes coated the honeydew droplets, which rendered the liquid marbles hydrophobic and nonwetting. Furthermore, the highly magnified scanning electron microscopy images confirmed that the wax particles were formed with assemblage of submicrometer-sized daughter fibers. The contact angle measurements indicated that the wax was intrinsically hydrophobic and that the liquid marbles were stabilized by the wax particles in the Cassie-Baxter model. The weight ratio of the honeydew and the wax particles was determined to be 96/4, and the honeydew consisted of 19 wt % nonvolatile components and 81 wt % water. The 1H nuclear magnetic resonance, Fourier transform infrared spectroscopy, and mass spectroscopy studies confirmed that the wax mainly consisted of triglycerides and that the honeydew mainly consisted of saccharides (glucose and fructose) and ribitol. The atomic force microscopy studies confirmed that honeydew is sticky in nature.

Photochromic Crystalline Systems Mimicking Bio-Functions.

Photoresponsive crystalline systems mimicking bio-functions are prepared using photochromic diarylethenes. Upon UV irradiation of the diarylethene crystal, the photogenerated closed-ring isomers self-aggregate to form needle-shaped crystals on the surface. The rough surface shows the superhydrophobic lotus effect. In addition, the rose-petal effects of wetting, the anti-reflective moth-eye effect, and a double-roughness structure mimicking the surface of a lotus leaf are observed by controlling the heating procedures, UV irradiation processes, and molecular structural modification. By changing the molecular structure, a superhydrophilic surface mimicking a snail shell can be generated. We also find the crystal of a diarylethene derivative that shows a photosalient effect. The effect is observed partly due to the hollow structure of the crystal. It is demonstrated that a photo-response similar to the response of impatiens plant to stimulation is observed by packing small beads in the hollow. These photoresponsive functions are unique, and they demonstrate a macroscopic response by means of microscopic molecular movement induced by light. In the future, such a molecular assembly system will be a promising candidate for fabricating photoresponsive architectures and soft robots.

Anomalous Friction between Agar Gels under Accelerated Motion.

Understanding the friction phenomena on a gel surface under accelerated conditions is important for the designing of functional materials. However, there are few reports on friction under such conditions. In the present study, the effects of velocity, normal force, and gel hardness on the friction force were evaluated between two agar gels under sinusoidal motion. We found a friction phenomenon with an extremely low friction coefficient on the gel surfaces: the friction coefficient became less than 0.02 when sliding velocity increased. In addition, the profile of the friction coefficient was different between outward and homeward processes in the reciprocating sliding motion. In the outward direction, the low friction coefficient was maintained even if the sliding velocity decreased. On the other hand, the friction coefficient increased with sliding velocity in the homeward direction. This characteristic friction profile is caused by a long relaxation time on the gel surfaces. When the gel substrate is rubbed for a shorter time than the relaxation time, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of a thick liquid film can induce a super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing functional materials but also for understanding nonequilibrium phenomena in soft biological systems.

Fractal Surfaces of Molecular Crystals Mimicking Lotus Leaf with Phototunable Double Roughness Structures.

Double roughness structure, the origin of the lotus effect of natural lotus leaf, was successfully reproduced on a diarylethene microcrystalline surface. Static superwater-repellency and dynamic water-drop-bouncing were observed on the surface, in the manner of natural lotus leaves. Double roughness structure was essential for water-drop-bouncing. This ability was not observed on a single roughness microcrystalline surface showing the lotus effect of the same diarylethene derivative. The double roughness structure was reversibly controlled by alternating irradiation with UV and visible light.

Penetration Behavior of a Water Droplet into a Cylindrical Hydrophobic Pore.

Understanding the dynamics with which a water droplet penetrates a pore is important because of its relationship with transfer phenomena in plants and animals. Using a high-speed camera, we observe the penetration processes of a water droplet into a cylindrical pore on a silicone substrate. The force on the water droplet is generated by dropping the substrate plus water droplet from a height of several centimeters onto an acrylic resin substrate. The penetration characteristics depend on pore size Dp, height of release of a drop h, and the viscosity of the droplet liquid and are classified into the following patterns: spreading, penetration, and breaking. During the process of relaxation to the steady state, various interesting deformation or oscillation phenomena occur. Based on high-speed images, we estimate the interfacial energy ΔG during the intermediate states and find an energy barrier ΔG = 1 × 10(-7) J when Dp = 1.0 mm and h = 15 mm for the spreading pattern and ΔG = 0.7 × 10(-7) J when Dp = 1.0 mm and h = 10 mm for the penetration pattern. Finally, based on a theoretical model considering the driving and suppression factors, we explain the experimentally obtained phase diagram including the separation, penetration, and breaking patterns.

Theoretical Explanation of the Lotus Effect: Superhydrophobic Property Changes by Removal of Nanostructures from the Surface of a Lotus Leaf.

Theoretical study is presented on the wetting behaviors of water droplets over a lotus leaf. Experimental results are interpreted to clarify the trade-offs among the potential energy change, the local pinning energy, and the adhesion energy. The theoretical parameters, calculated from the experimental results, are used to qualitatively explain the relations among surface fractal dimension, surface morphology, and dynamic wetting behaviors. The surface of a lotus leaf, which shows the superhydrophobic lotus effect, was dipped in ethanol to remove the plant waxes. As a result, the lotus effect is lost. The contact angle of a water drop decreased dramatically from 161° of the original surface to 122°. The water droplet was pinned on the surface. From the fractal analysis, the fractal region of the original surface was divided into two regions: a smaller-sized roughness region of 0.3-1.7 µm with D of 1.48 and a region of 1.7-19 µm with D of 1.36. By dipping the leaf in ethanol, the former fractal region, characterized by wax tubes, was lost, and only the latter large fractal region remained. The lotus effect is attributed to a surface structure that is covered with needle-shaped wax tubes, and the remaining surface allows invasion of the water droplet and enlarges the interaction with water.

Theoretical explanation of the photoswitchable superhydrophobicity of diarylethene microcrystalline surfaces.

Two types of superhydrophobic surfaces which show lotus and petal effects were induced on photochromic diarylethene microcrystalline surfaces by UV and visible light irradiation and temperature control. On the surfaces showing the lotus effect, a low-adhesion superhydrophobic property is attributed to the surface structure being covered with densely standing needle-shaped crystals of the closed-ring isomer. On surfaces showing the petal effect, a high-adhesion superhydrophobic surface consists of fine needle-shaped crystals with high density together with a few rod-shaped crystals, where an invasion phenomenon occurs between these rod-shaped crystals. Furthermore, the different superhydrophobic properties of the surfaces are theoretically explained using multipillar surface models.

Biomolecular motor modulates mechanical property of microtubule.

The microtubule (MT) is the stiffest cytoskeletal filamentous protein that takes part in a wide range of cellular activities where its mechanical property plays a crucially significant role. How a single biological entity plays multiple roles in cell has been a mystery for long time. Over the recent years, it has been known that modulation of the mechanical property of MT by different cellular agents is the key to performing manifold in vivo activities by MT. Studying the mechanical property of MT thus has been a prerequisite in understanding how MT plays such diversified in vivo roles. However, the anisotropic structure of MT has been an impediment in obtaining a precise description of the mechanical property of MT along its longitudinal and lateral directions that requires employment of distinct experimental approach and has not been demonstrated yet. In this work, we have developed an experimental system that enabled us to investigate the effect of tensile stress on MT. By using our newly developed system, (1) we have determined the Young's modulus of MT considering its deformation under applied tensile stress and (2) a new role of MT associated motor protein kinesin in modulating the mechanical property of MT was revealed for the first time. Decrease in Young's modulus of MT with the increase in interaction with kinesin suggests that kinesin has a softening effect on MT and thereby can modulate the rigidity of MT. This work will be an aid in understanding the modulation of mechanical property of MTs by MT associated proteins and might also help obtain a clear insight of the endurance and mechanical instability of MTs under applied stress.

Friction Dynamics of Human Skin Treated with Oil under Nonlinear Motion.

Moisturization causes physiological changes that improve the barrier function of human skin and mechanical changes, including skin friction characteristics. This study evaluated petrolatum- or silicone oil-treated human skin to determine the effect of moisturizing on the friction dynamics. The friction force on the human skin was measured using a contact probe with a sinusoidal motion. The contact probe was used to rub the skin of the upper arm of 20 subjects. The water content of the stratum corneum, softness, and barrier function of the skin were measured using a corneometer, cutometer, and tewameter, respectively. Both oils reduce the frictional force on the human skin. Simultaneously, silicone oil also reduced the delay time δ, which is the standardized time difference between the frictional force response to contact probe movement. Three typical friction patterns were also discovered, which were significantly changed by the treatment with oil. These changes were attributed to the lubrication effect and elimination of adhesion at the true contact point between the skin and the contact probe.

Photoinduced self-epitaxial crystal growth of a diarylethene derivative with antireflection moth-eye and superhydrophobic lotus effects.

We identified the mechanism of the formation of needle-shaped microcrystals on which the contact angle of a water droplet exceeds 170° [Nishikawa, N. et al. Langmuir, 2012, 28, 17817-17824]. The standing needle-shaped crystal of the closed-ring isomer of a diarylethene 3c grew at a much lower temperature than the eutectic temperature by irradiation of UV light on the thin films of the open-ring isomer 3o, due to the epitaxial growth of the 013 plane of 3c over the 110 plane of the crystal lattice of 3o in the subphase. Therefore, the new crystal-growth mechanism triggered by the photoisomerization does not require special inorganic single-crystal substrates and may be called self-epitaxial crystal growth. The needle-shaped crystals appeared well-ordered and stood inclined at an angle of about 60° to the surface. Consequently, the photo-induced rough surface shows not only the superhydrophobic lotus effect, but also the antireflection moth-eye effect, and these effects were switchable by alternate irradiation with UV and visible light.

Does the aquatic invertebrate nipple array prevent bubble adhesion? An experiment using nanopillar sheets.

The nipple array is a submicrometre-scale structure found on the cuticle surfaces of various invertebrate taxa. Corneal nipples are an antiglare surface in nocturnal insects, but the functional significance of the nipple array has not been experimentally investigated for aquatic organisms. Using nanopillar sheets as a mimetic model of the nipple array, we demonstrated that significantly fewer bubbles adhered to the nanopillar surface versus a flat surface when the sheets were hydrophilic. Many more bubbles adhered to the hydrophobic surface than the hydrophilic surfaces. Bubbles on the body surface may cause buoyancy problems, movement interference and water flow occlusion. Here, bubble repellence is proposed as a function of the hydrophilic nipple array in aquatic invertebrates and its properties are considered based on bubble adhesion energy.

Analysis of Rowing Force of the Water Strider Middle Leg by Direct Measurement Using a Bio-Appropriating Probe and by Indirect Measurement Using Image Analysis.

Rowing force of the middle leg of a water strider is one of the important factors affecting water repellency and applications in biomimetics, biomechanics, and biology. However, many previous studies have been based on estimated leg rowing force and lack some credibility. Therefore, we tried to measure leg rowing force directly by a force transducer. In this article, we report the rowing force of water striders obtained by direct and indirect measurements. In the direct measurement, water striders were set onto a sensor system and the rowing force of a middle leg of the set water striders was directly measured using a bio-appropriating probe (BAP), a kind of hook. In the indirect measurement, water striders were not fixed and the rowing force of locomoting water striders was evaluated by image analysis using a high-speed camera. As a result, we determined the rowing force by the direct measurement to be 955 µN, while the rowing force by the indirect measurement was 493 µN. We considered that the indirect measurement might lack some credibility because half the propellant energy was lost in the indirect force measurement due to various other factors.

Anomalous spreading with Marangoni flow on agar gel surfaces.

We have experimentally observed anomalous spreading of aqueous alcohol solutions on flat and rough fractal agar gel surfaces. On flat agar gel surfaces, extremely fast spreading [θ(D)(t) ∝ t(-0.92)] that differs from Tanner's law [θ(D)(t) ∝ t(-0.3)] was observed when the liquid contained over 9 wt % of 1-propanol in which strong Marangoni flow was observed as a fluctuation on the liquid surface. However, on fractal gel surfaces, different spreading dynamics [θ(D)(t) ∝ t(-0.58)] were observed, although Marangoni flow still occurred. We found the surface-dependent spreading can be discussed in terms of competition between Marangoni flow and the pinning effect due to surface roughness.

Photoinduced formation of superhydrophobic surface on which contact angle of a water droplet exceeds 170° by reversible topographical changes on a diarylethene microcrystalline surface.

A superhydrophobic surface on which the contact angle of a water droplet exceeds 170° was reversibly produced by alternate irradiation with UV and visible light. Superhydrophobicity is due to the formation of densely generated submicrometer sized needle-shaped crystals (less than 0.2-0.3 µm diameter and 2.2-2.5 µm long) at 30 °C, which is much lower than the eutectic temperature of either isomers of the diarylethene. Below the eutectic temperature, the generated crystals were much smaller than those generated above the eutectic temperature. These smaller crystals more effectively enhanced the superhydrophobicity.