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Liquids, Volume 6, Issue 2 (June 2026) – 5 articles

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9 pages, 445 KB

Open AccessArticle

Self-Similar Analysis of Start-Up Fluid Flow over Flat Plate

by Andriy A. Avramenko, Igor V. Shevchuk, Kyryl Fedortsev and Olesya Y. Stepanova

Liquids 2026, 6(2), 18; https://doi.org/10.3390/liquids6020018 - 6 May 2026

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Abstract

Based on the Lie group method (symmetry transformation groups), an analysis of an unsteady (start-up) flow over a flat surface was performed. This approach enabled reducing the number of independent arguments, which significantly simplifies the process of numerical modeling. An unsteady solution was obtained for the velocity profile in the boundary layer. This enabled estimating the dynamics of the velocity profile transformation and its transition to a steady-state mode. It was shown that in the limit of infinite time of the process, the velocity profile tends to the classical steady-state Blasius profile in the boundary layer. The dynamics of the friction coefficient variation over time were elucidated too. Full article

(This article belongs to the Section Physics of Liquids)

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12 pages, 1208 KB

Open AccessArticle

Interfacial Adsorption Behavior of Metal Oxide Nanoparticles at Hydrophobic Ionic Liquid–Water Interfaces

by Chihiro Takeda, Naoki Kanaya, Kotaro Bessho and Shoichi Katsuta

Liquids 2026, 6(2), 17; https://doi.org/10.3390/liquids6020017 - 27 Apr 2026

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Abstract

Metal oxide nanoparticles dispersed in water are difficult to recover because of their small size and colloidal stability. In this study, the interfacial adsorption behavior of Fe₂O₃, CoO, and CuO nanoparticles at hydrophobic ionic liquid (IL)–water interfaces was investigated and compared with that at molecular solvent–water interfaces. When CuO nanoparticle dispersions were shaken with hydrophobic ILs, bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻) salts of 1-butyl-3-methylimidazolium ([BMIm]⁺) and 1-octyl-3-methylimidazolium ([OMIm]⁺), the nanoparticles were removed from the aqueous phase and accumulated at the IL–water interface, while negligible Cu was detected in the bulk IL phase. The removal efficiency decreased with increasing ionic strength below 0.05 mol/dm³ and increased with pH, indicating that electrostatic interactions between charged nanoparticles and the IL–water interface contribute to adsorption. Adsorption isotherms were empirically fitted with the Langmuir equation to estimate the maximum adsorption capacity. For negatively charged Fe₂O₃ and CuO nanoparticles, the maximum adsorption capacities at IL–water interfaces exceeded those at molecular solvent–water interfaces and the theoretical monolayer capacity estimated from nanoparticle size, suggesting multilayer adsorption or aggregation at the interfaces. These results demonstrate the potential of hydrophobic IL–water interfaces for the separation and recovery of metal oxide nanoparticles from aqueous media. Full article

(This article belongs to the Section Physics of Liquids)

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34 pages, 3516 KB

Open AccessReview

Acid Catalytic Effects of Hot Compressed Water and Water–Alcohol Mixtures, and Their Applications as Tunable and Catalyst-Free Solvents

by Shotaro Seki, Yoshito Oshima and Makoto Akizuki

Liquids 2026, 6(2), 16; https://doi.org/10.3390/liquids6020016 - 16 Apr 2026

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Abstract

This paper provides a comprehensive overview of research findings concerning the acid catalytic effect (ACE) of hot compressed water and water–alcohol mixtures, along with the applications of these solvents. The ACE observed during reactions can be categorized into three types: inherent, associated, and interfering. These ACE types originate from the solvent, solutes, and reactor, respectively. Distinguishing and evaluating these ACEs is crucial for elucidating reaction mechanisms and developing reaction models. Water exhibits inherent ACE in both its dissociated and undissociated forms under hot compressed conditions. Hot compressed water–alcohol mixtures possess the capability to tune the characteristics of solvents, including ACE, through their composition. The application of hot compressed water and water–alcohol is prevalent in a variety of fields, including the conversion of biomass and biomass-derived materials, extraction, biodiesel production, organic synthesis reactions, recycling via the decomposition of polymers, and inorganic material synthesis. In these applications, the utilization of water–alcohol mixtures resulted in a higher yield of target products and/or superior properties of products compared to the use of pure solvents, such as water alone or alcohol alone. The observed results can be attributed to the optimization of the roles of water and alcohol in the reaction through mixing them. Full article

(This article belongs to the Collection Feature Papers in Solutions and Liquid Mixtures Research)

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17 pages, 3640 KB

Open AccessArticle

Volumetric Properties of 9 Binary Liquid Mixtures Ethyl Propanoate + Naphthenes (From Cyclohexane to Decylcyclohexane): Experimental Study from 288.15 K to 328.15 K

by Vincent Caqueret, Khaled Abou Alfa and Stéphane Vitu

Liquids 2026, 6(2), 15; https://doi.org/10.3390/liquids6020015 - 26 Mar 2026

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Abstract

In this work, the volumetric properties of nine binary systems composed of ethyl propanoate and n-alkylcyclohexanes (from cyclohexane to decylcyclohexane) were investigated. Densities were measured at atmospheric pressure (101 kPa) over the entire composition range and at temperatures from 288.15 K to 328.15 K. A total of 525 density data points were obtained. Excess molar volumes were derived from the experimental densities and correlated using a Redlich–Kister equation, while mixture densities were modeled with the Jouyban–Acree model. All systems exhibit positive excess molar volumes over the studied temperature and composition ranges, indicating volume expansion upon mixing due to dominant repulsive interactions. The magnitude of the excess molar volume increases with increasing alkyl chain length of the branched naphthenic compound: for an equimolar mixture, V^E is about 0.65 cm³·mol⁻¹ for the methylcylohexane + ethyl propanoate mixture and reaches 0.83 cm³·mol⁻¹ for the heptylcylohexane + ethyl propanoate binary system, although a plateau tendency is observed for longer alkyl chains. Excess molar volumes increase linearly with temperature, with a more pronounced temperature effect for shorter-chain alkylcyclohexanes. The Jouyban–Acree model provides an excellent correlation of the density data, yielding average relative deviations between 0.02% and 0.04%, and allows reliable predictions within the investigated temperature range. Full article

(This article belongs to the Collection Feature Papers in Solutions and Liquid Mixtures Research)

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18 pages, 2189 KB

Open AccessArticle

Optical Evaluation of Microviscosity in 4-Cyano-4′-n-Octyloxybiphenyl Liquid Crystals Using a Viscosity-Responsive Aggregation-Induced Emission Luminogen

by Chaiwattana Sattawat, Takuya Tanaka, Yuki Sawatari, Yuuto Iida, Yoshimichi Shimomura, Ryohei Ishige and Gen-ichi Konishi

Liquids 2026, 6(2), 14; https://doi.org/10.3390/liquids6020014 - 24 Mar 2026

Viewed by 668

Abstract

We report an optical method to estimate local microviscosity in thermotropic liquid crystals using viscosity-responsive aggregation-induced emission luminogens. Pendant-type luminogens were designed by covalently attaching 4-cyano-4′-n-octyloxybiphenyl mesogens (n = 8, 10) to a bis(N,N-dialkylamino)anthracene emissive core. When introduced at 1.0 wt% into 8OCB and 10OCB, thermal and optical analyses showed that the intrinsic liquid crystal properties were essentially unchanged, indicating good structural compatibility. Temperature-dependent fluorescence and polarization measurements revealed that emission changes are governed mainly by microviscosity rather than macroscopic phase disruption. Effective microviscosity was evaluated from absolute fluorescence quantum yields using the Förster–Hoffmann relation. On this basis, the microviscosity in the nematic phase is 21 mPa·s for 8OCB upon cooling, which correlates with the enhancement in fluorescence. In the smectic phase, although the director distribution parameter remains nearly constant, the effective microviscosity is ca. 21 mPa·s for 10OCB and ca. 54 mPa·s for 8OCB, and the fluorescence varies smoothly with temperature, reflecting changes in local segmental mobility within the layered structure. These values are broadly consistent with reported viscosity ranges/trends for cyanobiphenyl-type liquid crystals. Full article

(This article belongs to the Section Chemical Physics of Liquids)

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