Masashi Mizukami

A new physical model for resonance shear measurement of confined liquids between solid surfaces

This paper describes a new physical model for resonance shear measurement. The resonance shear method developed by us provides a tool for investigating the rheological and tribological properties of liquids confined between two surfaces as a function of the surface distance from micrometer to zero (contact) with nanometer level resolution with high sensitivity and stability. The properties of the confined liquid can be quantitatively studied by analyzing the resonance curve using a physical model. However, the quantitative analysis using the previously developed model was applicable only for the condition of the relatively low liquid viscosity (below approximately 100 Pa s). A new physical model described in this paper enabled us to continuously analyze the properties of confined liquids at all distances, which was not possible by the previous model. It became possible to calculate the movement of a lower surface and the shear rate applied on the confined sample using the parameters obtained from the resonance curves.

Structuring of Interfacial Water on Silica Surface in Cyclohexane Studied by Surface Forces Measurement and Sum Frequency Generation Vibrational Spectroscopy(1)

We investigated interfacial water, formed by adsorption or phase separation (prewetting transition), on a silica surface in water-cyclohexane binary liquids using a combination of colloidal probe atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. At 33 ± 9 ppm water, the long-range attraction extending to 19.4 ± 2.9 nm appeared, which was caused by the contact of water layers formed on silica surfaces. The attraction range increased with increasing water concentration and reached 97 ± 17 nm at the saturation concentration of water in cyclohexane (C*), indicating that the thickness of the water layer formed on silica was ca. 50 nm. The interfacial energy between the water adsorption layer and bulk solution (γ = 79.3 ± 2.0 mN/m) was estimated from the pull-off force, and was significantly larger than the value for the bulk water/cyclohexane interface (γ = 50.1 mN/m). SFG spectroscopy demonstrated that the interfacial water formed an icelike structure at C*. These results indicated that the interfacial water molecules formed an icelike ordered structure induced by the hydrogen bonding with surface silanol groups, resulting in the free OH groups being more exposed to the bulk solution. On the other hand, the water adsorption layer induced by phase separation at water concentrations above C* was found to be less ordered and its structure at the adsorption layer/bulk interface was almost the same as that of bulk water, although its thickness was almost the same as that formed at C*. To our knowledge, this is the first report of the observation of liquid adsorption layers formed by chemical interaction up to saturation and by the wetting transition above saturation, and their differences in the structure and properties at the molecular level.

Lubrication Properties of Ammonium-Based Ionic Liquids Confined between Silica Surfaces Using Resonance Shear Measurements

To evaluate the friction properties of new lubrication systems, two types of ammonium-based ionic liquids (ILs), N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) and N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]), were investigated by resonance shear measurements (RSM) and reciprocating type tribotests between silica (glass) surfaces. RSM revealed that an IL layer of ca. 2 nm in thickness was maintained between the silica surfaces under an applied load of 0.40 mN ∼ 1.2 mN. The relative intensity of the RMS signal indicated that the friction of the system was lower for [DEME][BF4], 0.12, than that of [DEME][TFSI], 0.18. On the other hand, the friction coefficients μk obtained from the tribotests of [DEME][BF4] were lower than that of [DEME][TFSI] for sliding velocities in the range of 5.0 × 10(-4) m s(-1) to 3.0 × 10(-2) m s(-1) under applied loads of 196-980 mN. The friction coefficients obtained by the tribotest are discussed with reference to the RSM results.

Nanotribological Characterization of Lubricants between Smooth Iron Surfaces

We performed the resonance shear measurement (RSM) for evaluating the nanorheological and tribological properties of model lubricants, hexadecane and poly(α-olefin) (PAO), confined between iron surfaces. The twin-path surface forces apparatus (SFA) was used for determining the distance between the surfaces. The obtained resonance curves for the confined lubricants showed that the viscosity of the confined hexadecane and PAO increased due to liquid structuring when the surface separation (D) decreased to a value less than 24 and 20 nm, respectively. It was also determined that the iron surfaces were lubricated by the hexadecane when normal load (L) was less than 1.1 mN, while the confined hexadecane behaved almost solid-like and showed poor lubricity when L was greater than 1.1 mN. In contrast, PAO between the iron surfaces showed high lubricity even under the high load (L > 2 mN). The surface separation of hexadecane and PAO at a hard wall contact between the iron surfaces was determined to be 4.6 ± 0.5 and 5.0 ± 0.4 nm by applying the fringes of equal chromatic order (FECO) for half-transparent iron films deposited on mica surfaces as substrates. We also characterized hexadecane and PAO confined between mica surfaces for studying the effect of substrates on the confined lubricants.

Alcohol cluster formation on silica surfaces in cyclohexane

Adsorption of ethanol onto silica surfaces from ethanolcyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy and Fourier transform IR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long range attraction was found between the silica surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol%. At 0.1 mol% ethanol, the attraction appeared at a distance of 35 ±3 nm and turned into a repulsive force below 3.5 ± 1.5 nm upon compression. The ATR spectra demonstrated that ethanol adsorbed on the surfaces formed hydrogen-bonded clusters even in a low ethanol concentration range of 0.1–0.5 mol%, where the attractions were especially long-ranged and practically no ethanol cluster formed in the bulk solutions. The spectra also indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol molecules. We account for the observed longrange attraction by the bridging of opposed adsorption layers of ethanol on the silica surfaces.

Force Estimation on the Contact of Poly(l,l-lactide) and Poly(d,d-lactide) Surfaces Regarding Stereocomplex Formation.

The stereocomplex formation of poly(l,l-lactide) (PLLA) and poly(d,d-lactide) (PDLA) was selected in order to investigate the interaction of the two surfaces including hydrogen bonding and van der Waals interaction. Adhesion force measurement using surface force apparatus (SFA) equipped with an optical microscope was conducted on the PLLA and PDLA spin-coated films. The adhesion forces, Fad, phenomenologically followed the linear relation with the applied normal load, L. For the force Fad between PLLA and PDLA films with low molecular weights (PLLA, Mn = 2800; PDLA, : Mn = 2100), the slope of linear fitting of Fad vs L was significantly larger for the heterointerface (PLLA/PLDA) compared with that for the homointerface (PLLA/PLLA and PDLA/PDLA). However, when polymers with higher molecular weights (PLLA, Mn = 8500; PDLA, Mn = 8300) were measured, the slopes of linear fitting lines were almost the same for hetero- and homointerfaces. This indicated that the mobility of the lower molecular weight PLLA/PDLA films promoted the selective interaction of PLLA and PDLA under the applied normal loads. The adhesion between the outermost PLLA layer and PDLA layer prepared by layer-by-layer (LbL) assembly was also measured. It is interesting that the adhesion force was very weak in this case. This weak adhesion could be explained by the much less mobility of the polymer chain due to the stereocomplex formation within the LbL layers. This study demonstrated that the adhesion force due to the selective interaction of PLLA and PDLA between PLLA/PDLA films could be directly measured, and depended on the mobility of the outermost polymer chains, which reflected the different structures of polymer chains in the organized complex films.

In situ polymerization of molecular macroclusters on a silica surface: poly(N-isopropylacrylamide) nanofilms.

We have found that alcohols, carboxylic acids, and amides self-assemble into a unique molecular architecture, a hydrogen-bonded molecular macrocluster, when they are selectively adsorbed onto silica (glass and oxidized silicon) surfaces in nonpolar solvents such as cyclohexane. In our previous study, this phenomenon could be successfully applied to fabricate molecularly flat and defect-free nanofilms of several tens of nanometers thickness. In this study, we prepared a poly(N-isopropylacrylamide) [poly(NIPAAm)] film on the basis of in situ polymerization of a monomer macrocluster layer formed on silica surfaces and investigated how the molecular arrangement of the adsorbed NIPAAm monomers affects the efficiency of the polymerization of them. Poly(NIPAAm) films were prepared by the following two methods: (1) the one-solution method, the in situ photopolymerization of an NIPAAm monomer adsorption layer on silica in one solution (chloroform, cyclohexane, and toluene), and (2) the solution exchange method, adsorption of NIPAAm monomers onto a silica surface from NIPAAm (0.1 mol %) in chloroform, exhange of the solution to 0.005 mol % NIPAAm in cyclohexane, and then polymerization by UV irradiation. By the solution exchange method, molecularly flat, defect-free, and thermoresponsive films were obtained and the thickness could be controlled by the irradiation time, while only several nanometers thickness could be attained by the one-solution method. The structure of NIPAAm adsorption layers formed in each solution condition was characterized by attenuated total reflection Fourier transform infrared spectroscopy. It was revealed that only the solution exchange procedure induced the beta-sheet-like adsorbed structure of NIPAAm in which the double bonds of neighboring NIPAAm monomers were closely located, which should have resulted in effective polymerization.

Preparation of stable silica surfaces for surface forces measurement

A surface forces apparatus (SFA) measures the forces between two surfaces as a function of the surface separation distance. It is regarded as an essential tool for studying the interactions between two surfaces. However, sample surfaces used for the conventional SFA measurements have been mostly limited to thin (ca. 2-3 μm) micas, which are coated with silver layers (ca. 50 nm) on their back, due to the requirement of the distance determination by transmission mode optical interferometry called FECO (fringes of equal chromatic order). The FECO method has the advantage of determining the absolute distance, so it should be important to increase the availability of samples other than mica, which is chemically nonreactive and also requires significant efforts for cleaving. Recently, silica sheets have been occasionally used in place of mica, which increases the possibility of surface modification. However, in this case, the silver layer side of the sheet is glued on a cylindrical quartz disc using epoxy resin, which is not stable in organic solvents and can be easily swollen or dissolved. The preparation of substrates more stable under severe conditions, such as in organic solvents, is necessary for extending application of the measurement. In this study, we report an easy method for preparing stable silica layers of ca. 2 μm in thickness deposited on gold layers (41 nm)/silica discs by sputtering, then annealed to enhance the stability. The obtained silica layers were stable and showed no swelling in organic solvents such as ethanol and toluene.

In situ polymerization of molecular macroclusters on silica: poly(N-isopropylacrylamide) nano-films

Recently, we succeeded to prepare flat and defect-free polymer nanofilms by in-situ photo-polymerization of the adsorbed monomers (e.g. acrylic acid) on silica surfaces through hydrogen-bonding in cyclohexane. Our method has advantages such as simplicity, lowcost, applicability to any shape of substrates. In this study, to further develop this method, we prepared po ly [Ni sopropy lac ry lamide (NIPAAm)] film by using new procedure which we call solution e x c h a n g e m e t h o d ( F i g . 1 ) . NIPAAm monomers are adsorbed on the silicon prism surface in chloroform and polymerized in cyclohexane by UV irradiation. The increase in the film thickness was monitored following the ν NH peak intensity in ATR-FTIR spectra (Fig.2). The film thickness increased drastically after 3 h UV irradiation, while almost no increase in the thickness in case of adsorption and polymerization was performed in one solution (toluene and cyclohexane). Peak positions of Amide I and II of adsorbed NIPAAm indicated that the solution exchange induced the adsorbed structure similar to the N-methyl acetamide crystal, in which the neighboring double bonds of NIPAAm became close, and should promote the polymerization(Fig.3).