Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Tetsuo Yamaguchi Last modified date:2018.07.02

Associate Professor / Machine elements and design engineering laboratory / Department of Mechanical Engineering / Faculty of Engineering

1. Teruo Murakami, Seido Yarimitsu, Nobuo Sakai, Kazuhiro Nakashima, Tetsuo Yamaguchi, Yoshinori Sawae, Atsushi Suzuki, Ardian Morina, Anne Neville, Tomasz Liskiewicz, Superior lubrication mechanism in poly(vinyl alcohol) hybrid gel as artificial cartilage, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 10.1177/1350650117712881, 231, 9, 1160-1170, 2017.09, With recent progress of material technologies, the wear resistance of ultra-high molecular weight polyethylene for total joint prostheses has been improved, but under severe conditions friction and wear problems have not yet been completely solved. Therefore, the application of artificial hydrogel cartilage with similar properties to natural articular cartilage is expected to solve the friction and wear problems by improvement of lubrication mechanism with superior tribological functions. In this study, reciprocating tests of four kinds of poly(vinyl alcohol) hydrogels were carried out and the biphasic finite element analysis was conducted. As artificial cartilage specimens, four kinds of poly(vinyl alcohol) hydrogels were prepared using the repeated freeze-thawing (FT) method, the cast-drying (CD) method and the hybrid method with different layered structure as FT on CD or CD on FT. In reciprocating test of ellipsoidal poly(vinyl alcohol) hydrogel specimen against flat glass plate in saline solution, four kinds of hydrogels exhibited very different frictional levels as hybrid (CD on FT) < CD < FT < hybrid (FT on CD). It is noticed that hybrid (CD on FT) gel maintained extremely low friction and showed minimal wear. The effectiveness of biphasic lubrication was evaluated by biphasic finite element analysis. The importance of the load support by fluid phase at early stage and the surface lubricity after lowering of interstitial fluid pressure in poly(vinyl alcohol) hybrid (CD on FT) gel are discussed by comparison of experiment and finite element analysis..
2. D. Nečas, Yoshinori Sawae, T. Fujisawa, Kazuhiro Nakashima, Takehiro Morita, Tetsuo Yamaguchi, M. Vrbka, I. Křupka, M. Hartl, The Influence of Proteins and Speed on Friction and Adsorption of Metal/UHMWPE Contact Pair, Biotribology, 10.1016/j.biotri.2017.03.003, 11, 51-59, 2017.09, It was reported in several studies that friction and wear of joint prosthesis are apparently influenced by the proteins contained in the synovial fluid. However, detailed mechanisms of these tribological processes have not been clarified yet. The present study aims on the effect of adsorbed protein film on frictional behaviour of metal/polyethylene contact pair. Reciprocating pin-on-plate test was conducted, while the CoCrMo pin was sliding against UHMWPE plate. The contact was lubricated by various solutions of albumin and γ-globulin solved in PBS. After the friction test, the thickness of adsorbed film was evaluated using spectroscopic ellipsometry. Structure of the adsorbed proteins was later examined by FT-IR. The results showed that at low sliding speed equal to 10 mm/s, there was a linear correlation between the friction coefficient and the thickness of the adsorbed protein film. An increase of friction was thus accompanied by the evolution of protein film. In that case, both proteins undergone substantial conformational changes, losing their original structure. On the contrary, proteins could sustain their secondary structure to some extent at higher sliding speed (50 mm/s), when different behaviour of the both proteins could be observed. This phenomenon was attributed to different structure of albumin and γ-globulin in its native state. It might be concluded that friction coefficient of metal-on-polyethylene joint prosthesis is influenced by protein content, as well as kinematic conditions, since the sliding conditions have a certain effect on both the adsorbed film formation and structure of the adsorbed molecules..
3. Tomohiko G. Sano, Tetsuo Yamaguchi, Hirofumi Wada, Slip Morphology of Elastic Strips on Frictional Rigid Substrates, Physical Review Letters, 10.1103/PhysRevLett.118.178001, 118, 17, 2017.04, The morphology of an elastic strip subject to vertical compressive stress on a frictional rigid substrate is investigated by a combination of theory and experiment. We find a rich variety of morphologies, which - when the bending elasticity dominates over the effect of gravity - are classified into three distinct types of states: pinned, partially slipped, and completely slipped, depending on the magnitude of the vertical strain and the coefficient of static friction. We develop a theory of elastica under mixed clamped-hinged boundary conditions combined with the Coulomb-Amontons friction law and find excellent quantitative agreement with simulations and controlled physical experiments. We also discuss the effect of gravity in order to bridge the difference in the qualitative behaviors of stiff strips and flexible strings or ropes. Our study thus complements recent work on elastic rope coiling and takes a significant step towards establishing a unified understanding of how a thin elastic object interacts vertically with a solid surface..
4. Teruo Murakami, Seido Yarimitsu, Nobuo Sakai, Kazuhiro Nakashima, Tetsuo Yamaguchi, Yoshinori Sawae, Importance of adaptive multimode lubrication mechanism in natural synovial joints, Tribology International, 10.1016/j.triboint.2016.12.052, 113, 306-315, 2017.01, The superior tribological performance in natural synovial joints with low friction and minimal wear appears to be actualized by not single lubrication mode but the synergistic combination of various modes from the fluid film lubrication to boundary lubrication corresponding to the severity of rubbing conditions. We have conducted the biphasic finite element analysis and some experimental studies for articular cartilage to elucidate this ingenious lubrication mechanism from the viewpoint of the adaptive multimode lubrication mechanism. In this paper, the effectiveness of different lubrication modes such as biphasic, boundary, gel-film and hydration lubrication particularly at low speed conditions is discussed..
5. Keisuke Fukuda, Yoshifumi Shigyo, Hideaki Ariura, Seiji Omata, Takehiro Morita, Tetsuo Yamaguchi, Yoshinori Sawae, Relationship between dynamic stress field and ECM production in regenerated cartilage tissue, 27th International Symposium on Micro-NanoMechatronics and Human Science, MHS 2016 2016 International Symposium on Micro-NanoMechatronics and Human Science, MHS 2016, 10.1109/MHS.2016.7824183, 2017.01, Mechanical stresses exerted in articular cartilage during daily joint movements can stimulate the metabolism of chondrocytes in cartilage. Upregulative effects of mechanical stimuli on extracellular matrix (ECM) biosynthesis of chondrocytes have been utilized in the cartilage tissue for regenerative tissue engineering. However, dynamic functionalities of these regenerated cartilages were lacked compared with the natural arthrodial cartilage. Natural synovial joint surfaces are contacting and sliding each other. As a result, the loading condition applied to the cartilage tissue is not simple. Under the contact between surfaces, non-uniform stress distribution is exerted in the cartilage tissue. The shear stress at contact area is highly depended on the distance from the surface. Moreover, the surface region is exposed to larger shear stress compared with the deep region. We hypothesized that high stress in surface region may stimulate chondrocytes selectively and establish anisotropic structure in elaborated tissue. In this study, the relative motion between cartilage surfaces in a synovial joint is simulated by the rolling-sliding motion of the roller on the cultured chondrocyte-agarose construct. Then, we investigated effects on the ECM distribution and morphology of regenerated cartilage tissue..
6. Tetsuo Yamaguchi, Yoshinori Sawae, Shmuel M. Rubinstein, Effects of loading angles on stick–slip dynamics of soft sliders, Extreme Mechanics Letters, 10.1016/j.eml.2016.09.008, 9, 331-335, 2016.12, When soft gels move across a hard surface, stick–slip frictional sliding is mediated by propagation of adhesion and detachment fronts. Here we experimentally investigate the sliding dynamics of an extended frictional interface between soft Silicone gel and hard PMMA and identify three distinct sliding regimes. We directly visualize the interface and show that a minute manipulation of the initial loading angle results in a sharp bifurcation between the different sliding states. The phase diagram as well as universal scaling relations governing the dynamics is presented..
7. Tetsuo Yamaguchi, Yoshimi Tanaka, Risa Shimazaki, Solvent effects on fracture of chemically crosslinked gels, Soft Matter, 10.1039/C6SM01645F, 12, 8135-8142, 2016.10, We have investigated how the fracture behavior of a polyacrylamide hydrogel is affected by different types of solvents poured into its crack tips. We obtained the following results: first, when water (good solvent or reaction solvent for the polyacrylamide gel) is poured, the fracture energy Gamma becomes smaller than that measured in air for small crack velocities. Second, when good solvents other than water are poured, Gamma is enhanced for a large V region, but this effect is not observed for smaller V; Gamma(V) in good solvents converges to that in water as V approaches 0. Third, when ethanol (poor solvent for polyacrylamide) is poured, stick–slip-like crack propagation appears in the entire V range, and Gamma calculated from the time-average of the oscillating tearing forces is larger than that in air or in other solvents. We discuss the results on the basis of diffusion dynamics around the crack tips of the gel..
8. Tetsuo Yamaguchi, Akira Akamine, Yoshinori Sawae, Tribology inspired by Gecko feet, Toraibarojisuto/Journal of Japanese Society of Tribologists, 61, 4, 243-248, 2016.01.
9. Lei Zhang, Yoshinori Sawae, Tetsuo Yamaguchi, Teruo Murakami, Hong Yang, Effect of radiation dose on depth-dependent oxidation and wear of shelf-aged gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE), Tribology International, 10.1016/j.triboint.2014.12.011, 89, 78-85, 2015.09, The effect of radiation dose on the depth-dependent oxidation and wear of shelf-aged gamma-irradiated UHMWPE was investigated in this paper. FTIR, micro-indentation, pin-on-plate wear tests and SEM imaging were carried out at three representative regions (surface, subsurface and center) for each sample. The experimental results show that when the oxidation index (OI) <1, the wear rate is clearly affected by the radiation dose (crosslinking density). When 13 - except for the 1000 kGy specimen - the wear resistance is severely deteriorated and the relationship with the radiation dose is difficult to predict. Results suggest that higher irradiation (above 200 kGy) is capable of lowering the oxidative degradation of UHMWPE..
10. Teruo Murakami, Nobuo Sakai, Tetsuo Yamaguchi, Seido Yarimitsu, Kazuhiro Nakashima, Yoshinori Sawae, Atsushi Suzuki, Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage, Tribology International, 10.1016/j.triboint.2014.12.013, 89, 19-26, 2015.09, To extend the durability of artificial joints, biomimetic artificial hydrogel cartilage is proposed as a way of improving the lubrication mechanism in artificial joints. The application of hydrogels with properties similar to those of articular cartilage can be expected to duplicate the superior load-carrying capacity and lubricating ability of natural synovial joints. Frictional behaviors with three kinds of poly(vinyl alcohol) (PVA) hydrogels with high water content were examined in reciprocating tests. Interstitial fluid pressure, von Mises stress and fluid flow were compared in biphasic finite element analysis, and frictional behavior was evaluated in terms of biphasic lubrication and surface lubricity. Hybrid gel prepared by a combination of cast-drying and freeze-thawing methods showed superior low friction..
11. Tetsuo Yamaguchi, Lei Zhang, Yoshinori Sawae, Teruo Murakami, Hon Yang, Effect of radiation dose on depth-dependent oxidation and wear of shelf-aged gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE), Tribology International, 10.1016/j.triboint.2014.12.011, 89, 78-85, 2015.05, The effect of radiation dose on the depth-dependent oxidation and wear of shelf-aged gamma-irradiated UHMWPE was investigated in this paper..
12. Tetsuo Yamaguchi, Teruo Murakami, Yoshinori Sawae, Kazuhiro Nakashima, Seido Yarimitsu, Nobuo Sakai, Atsushi Suzuki, Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage, Tribology International, 10.1016/j.triboint.2014.12.013, 89, 19-26, 2015.05, To extend the durability of artificial joints, biomimetic artificial hydrogel cartilage is proposed as a way of improving the lubrication mechanism in artificial joints..
13. Tetsuo Yamaguchi, Superior lubricity in articular cartilage and artificial hydrogel cartilage, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 10.1177/1350650114530273, 228, 10, 1099-1111, 2014.04, In healthy natural synovial joints, the extremely low friction and minimum wear are maintained by their superior loadcarrying capacity and lubricating ability. This superior lubricating performance appears to be actualized not by single lubrication mode but by synergistic combination of multimode mechanisms such as fluid film, biphasic, hydration, gel film and/or boundary lubrication. On the contrary, in most artificial joints composed of ultra-high molecular weight polyethylene against metal or ceramic-mating material, boundary and/or mixed lubrication modes prevail and thus local direct contact brings down high friction and high-wear problems. To extend the durability of artificial joint, the reduction in friction and wear by improvement in lubrication mechanism is required as an effective design solution. In this paper, at the start, the mechanism of superior lubricity for articular cartilage is examined from the viewpoints of biphasic and boundary lubrication mechanism. Subsequently, the proposal of biomimetic artificial hydrogel cartilage is put forward
to improve the lubricating modes in artificial joints. The tribological behaviours in two kinds of poly(vinyl alcohol) hydrogels are compared with that of natural cartilage. The importance in lubrication mechanism in artificial hydrogel cartilage is discussed..
14. Atsuko Namiki, Tetsuo Yamaguchi, Ikuro Sumita, Takehito Suzuki, Satoshi Ide, Earthquake model experiments in a viscoelastic fluid: A scaling of decreasing magnitudes of earthquakes with depth, Journal of Geophysical Research: Solid Earth, 10.1002/2014JB011135, 119, 2014.03, We performed shear deformation experiments using quasi-Maxwell fluids. We found that, depending on the strain rates, the same material generates earthquakes associated with the elastic rebound and deforms viscously. Around the threshold, elastic rebound releases a certain fraction of the interseismic displacement, but the other fraction remains as a result of the viscous relaxation. We applied our experimental results to a subduction zone, in which the upper part of the hanging wall behaves as an elastic layer and generates seismicity, while the deeper part behaves as a viscous fluid and subducts with the slab. Our experimental results suggest that, around the boundary of the elastic and viscous layers, seismicity can occur, but only some part of the interseismic displacements is released. The experimentally obtained threshold of the seismic activity is determined by the combination of the subduction velocity vs, the viscosity of the hanging wall η, the fault length W, and the adhesive stress σa, vsη/(Wσa)>1. This threshold suggests that if the viscosity of the hanging wall decreases with depth, the maximum size of the earthquakes also decreases with depth, and, finally, seismicity disappears. This hypothesis is consistent with the observed fact that slow earthquakes, characterized by their small magnitudes, are observed at the downdip limit of the seismogenic zone..
15. Tetsuo Yamaguchi, Rapid Swelling and and Pattern Formation in Hydrogel Particles, Journal of the Society of Rheology, Japan, 42, 2, 129-133, 2014.02, We report on a novel wrinkle formation process of water absorbing hydrogel particles. When the hydrogel particles called Super Absorbent Polymer (SAP) are dropped onto water which is held in a cylindrical container, the particles rapidly swell at the air-water interface to form a sheet-like structure. Once the sheet-like structure reaches the wall of the container by its continuous in-plane expansion, it starts to buckle with a characteristic pattern of wrinkles. We applied a simple model to estimate the wavelength of the wrinkle, which was in agreement with experimental values. We also discussed the effect of buoyant force on the wrinkle formation. .
16. Ryuji Suzuki, Tetsuo Yamaguchi, Masao Doi, Frictional Property of Hydrogels Prepared under Electric Fields, Journal of The Physical Society of Japan,, 82, 12, 124803-1-124803-5, 2013.11, We studied frictional property of hydrogels prepared under electric fields. Pre-gel solution was filled between two electrodes and voltage was applied during crosslinking reactions. After removing the gel from the electrodes, frictional testing for the gel surfaces prepared on both cathode and anode was performed against a silicon dioxide coated surface. For the gels containing two types of monomers, an electrolyte and a neutral, friction coefficient for the gel surface prepared on the anode was much smaller than that for the cathode as well as that without electric voltage. In contrast, no significant difference was seen between the two surfaces for the single-component gels. Our results suggest that this simple technique of applying electric voltage during gelation is effective for controlling and reducing friction of hydrogels..
17. Tetsuo Yamaguchi, Hiroyuki Muroo, Yutaka Sumino, Masao Doi, Asymmetry-symmetry transition of double-sided adhesive tapes, Physical Review E, 10.1103/PhysRevE.85.061802, 85, 061802-1-6, 2012.06, We report on the debonding process of a double-sided adhesive tape sandwiched between two glass plates.
When the glass plates are separated from each other at a constant rate, a highly asymmetric extension of top and
bottom adhesive layers and bending of the inner film are observed first. As the separation proceeds, the elongation
of both layers becomes symmetric, and the inner film becomes flat again. When this happens, there appears a
local maximum in the force-displacement curve. We explain this asymmetry-symmetry transition and discuss
the role of the bimodal force-displacement relation of each adhesive layer.We also discuss the effect of the inner
film thickness and the separation rate on the debonding behavior, which causes undesirable early detachment of
the double-sided adhesive tape in a certain condition..
18. Yutaka Sumino, Hiroki Shibayama, Tetsuo Yamaguchi, Tadashi Kajiya, Masao Doi, Failure of film formation of viscoelastic fluid: Dynamics of viscoelastic fluid in a partially filled horizontally rotating cylinder, Physical Review E, 10.1103/PhysRevE.85.046307, 85, 046307-1-6, 2012.04, The dynamics of a viscoelastic Maxwell fluid is studied in a partially filled cylinder rotating around a horizontal
axis. At low rotational velocity, the fluid behaves in the same manner as a viscous fluid. A thin fluid film is pulled
up from the edge of a fluid bump at the bottom of the cylinder, and it covers the inner wall of the cylinder
completely. As a result, a steady state is the coexistence of the film and the bump of the fluid. When the rotational
velocity of the cylinder is increased, the film formation fails and the bump of fluid rolls steadily at the bottom
of the cylinder. This failure of film formation has never been observed in the case of a viscous fluid. At higher
rotational velocity, the bump of the fluid starts to oscillate at the bottom of the cylinder. Then, the fluid bump
again rolls steadily with a further increase in the rotational velocity. The failure of film formation is explained in
terms of the elastic behavior of the viscoelastic fluid near the boundary between the film and the bump regions.
The theoretical prediction shows good agreement with the experimental results.We further estimate the condition
for which a viscoelastic fluid displays dynamically nonwetting behavior; i.e., the absence of fluid film at any
value of rotational velocity..
19. When soft viscoelastic materials are detached from or slid against a hard substrate, complex deformation behavior such as cavitation, fibrillation, and Schallamach waves is often observed in such materials. This behavior is important not only in terms of science but also in a practical or industrial sense. In this paper, we introduce our studies on adhesion and friction of pressure-sensitive adhesives and polymer gels. .
20. Tetsuo Yamaguchi, Masatoshi Morishita, Masao Doi, Takane Hori, Hide Sakaguchi, Jean-Paul Ampuero, Gutenberg-Richter’s law in sliding friction of gels, Journal of Geophysical Research, Solid Earth, 10.1029/2011JB008415, 116, B12306-1-8, 2011.12, We report on experimental studies of spatio-temporally heterogeneous stick-slip
motions in the sliding friction between a hard polymethyl methacrylate (PMMA, plexiglass)
block and a soft poly-dimethyl siloxane (PDMS, silicone) gel plate. We perform
experiments on two PDMS gels with different viscoelastic properties. For the less viscous
gel, large and rapid events are preceded by an alternation of active and less active periods.
For the more viscous gel, successive slow slip events take place continuously. The
probability distributions of the force drop, a quantity analogous to seismic moment, obey a
power law similar to Gutenberg-Richter’s empirical law for the frequency-size statistics of
earthquakes, and the exponents of the power law vary with the plate velocity and the
viscosity of the gel. We propose a simple model to explain the dependence of the power law
exponent on the plate velocity, which agrees with experimental results..
21. When soft and sticky materials are slid against a substrate, specific types of sliding behavior are observed. In this study, we conducted sliding friction experiments using a soft and sticky gel-sheet. We found that the system shows regular-irregular transition of the sliding behavior. We also found that the distribution of the size of slip events at small pull velocities obeys a power-law, which is similar to Gutenberg-Richter law for earthquakes..
22. Masatoshi Morishita, Masaru Kobayashi, Tetsuo Yamaguchi, Masao Doi, Observation of spatio-temporal structure in stick–slip motion of an adhesive gel sheet, Journal of Physics: Condensed Matter, 10.1088/0953-8984/22/36/365104, 22, 365104-1-6, 2010.08, We studied the sliding friction between an adhesive gel sheet and a glass substrate. In this system, the probability distribution of the force drop obeys a power law similar to that found in earthquakes and granular systems. We observed the motion of the slip regions at the frictional interfaces and obtained the spatial distributions of shear strain by image analysis. The frictional force evaluated by the image analysis is in good agreement with the actual force measured by a load cell. This indicates that the present method provides a powerful tool to study the spatio-temporal structure in the heterogeneous stick–slip motions in sliding friction..
23. Masakazu Takata, Tetsuo Yamaguchi, Masao Doi, Friction Control of a Gel by Electric Field in Ionic Surfactant Solution, Journal of the Physical Society of Japan, 10.1143/JPSJ.79.063602, 79, 063602-1-3, 2010.06, We observed that the frictional force between acrylamide gel and silicon dioxide substrate is reduced upon application of electric voltage if the gel is swollen with anionic surfactant (sodium dodecyl sulfate). The effect was not seen in cationic surfactant. We conjecture that the reduction is due to formation of a lubrication layer by electric field at the gel/solid interface..
24. Julia Nase, Costantino Creton, Osvanny Ramos, Lars Sonnenberg, Tetsuo Yamaguchi, Anke Lindner, Measurement of the receding contact angle at the interface between a viscoelastic material and a rigid surface, Soft Matter, 10.1039/C001687J, 6, 2685-2691, 2010.03, We present the first quantitative measurement of the receding contact angle between a soft viscoelastic material and a solid surface during the debonding of the material. We developed a new method of observation, based on the technique pioneered by Yamaguchi et al. (T. Yamaguchi, K. Koike, and M. Doi, Euro. Phys. Lett., 2007, 77, 64002). In a probe -tack geometry, we obtained a 3D-like view of the debonding pattern as seen through the adhesive/probe interface, providing thus an excellent view of the details of the contact geometry near the interface. Combining visual and force-displacement information in a probe tack test, we identified three different categories of mechanisms and contact geometries as a function of the material parameters. For very weakly crosslinked viscoelastic materials around the gel point, air fingers form in the bulk of the layer. For higher degrees of crosslinking, the air fingers form and propagate at the interface, with a large deformation of the bulk of the layer. A well defined and reproducible receding contact angle φ < 90° depending on the viscoelastic properties of the layer was observed at the leading edge of the moving finger. Finally for well crosslinked samples the failure occurs by interfacial crack propagation with little deformation of the layer; the contact angle at the leading edge then was close to 90° as expected from elastic fracture mechanics..
25. Masakazu Takata, Tetsuo Yamaguchi, Masao Doi, Electric Field Effect on the Sliding Friction of a Charged Gel, Journal of the Physical Society of Japan, 10.1143/JPSJ.78.084602, 78, 084602-1-5, 2009.07, Sliding friction of a charged gel on a substrate was investigated by applying electric voltage between gel and the substrate. It is found that the sliding friction increases markedly with the increase of the applied voltage; the friction coefficient increased by a factor of about 10 when 70V voltage is applied. This voltage dependence of frictional force is explained by using the adhesion friction model proposed by Schallamach..
26. Tetsuo Yamaguchi, Satoshi Ohmata, Masao Doi, Regular to chaotic transition of stick-slip motion in sliding friction of an adhesive gel-sheet, Journal of Physics: Condensed Matter, 10.1088/0953-8984/21/20/205105, 21, 205105-1-7, 2009.04, Spatio-temporal pattern of the stick–slip motion of a gel-sheet pulled on a glass substrate is observed. The sliding takes place via the propagation of the wave of detachment (Schallamach wave). At large pull velocity, the detached region is a stripe which moves regularly with constant speed and the frictional force shows a periodic time dependence. As the pull velocity is decreased, the detached region is separated into bubbles which move around irregularly. In the irregular state, the frictional force shows chaotic time dependence and the statistics of the event of the force drop obeys a power law similar to the Gutenberg–Richter law known in
earthquakes. In the regular region, the detachment wave is analyzed theoretically and the velocity and lengths are obtained as a function of the pull velocity. The transition from the regular to chaotic behavior is shown to be related to the spontaneous wetting of the gel..