Claudia Roman

In this study, molybdenum disulfide (MoS₂)-based water nanofluids were prepared and stabilized using two surfactants with opposite charges: the cationic cetyltrimethylammonium bromide (CTAB) and the anionic sodium lauryl sulfate (SLS). Different MoS₂:surfactant ratios (1:1, 1:2, and 1:3) were examined to identify the optimal formulation ensuring stable dispersion. Stability was evaluated through dynamic light scattering (DLS), zeta potential, and UV–Vis spectroscopy analyses. The results showed that the MoS₂:SLS (1:3) nanofluid achieved the highest stability, characterized by a zeta potential of −38 mV and a mean particle size of approximately 290 nm. Thermophysical properties were then investigated for nanoparticle concentrations of 0.05, 0.1, and 0.2 wt%. The 0.1 wt% nanofluid exhibited the best performance, showing a thermal conductivity enhancement of about 49% and an increased specific heat capacity compared with pure water. This improvement is attributed to uniform nanoparticle dispersion and enhanced phonon transport. Overall, the results demonstrate that the anionic SLS surfactant at a 1:3 ratio effectively enhances the stability as well as the thermal performance of MoS₂–water nanofluids, making them promising candidates for thermal management and energy systems applications.

(1) Background: Developing fully bio-based lubricating greases requires eco-friendly alternatives to conventional harmful components. This study highlights unmodified nanocellulose as an effective structuring agent in vegetable oils, enabling 100% bio-based formulations. (2) Methods: Three bio-based greases were formulated using 1.4 wt.% cellulose nanofibers (CNFs), derived from elm wood pulp through mechanical and chemical pretreatment, as thickening agents in castor oil. Their tribological performance was evaluated under varying temperatures and contact loads and compared to a reference lithium-based grease (LBG) containing 14 wt.% thickener, also formulated with castor oil. (3) Results: Among the CNFs, the unbleached variant (CNF-U) which retained the highest lignin content exhibited the highest coefficient of friction (COF), ranging from 0.09 to 0.14 across test conditions, along with a wear scar diameter of approximately 615 µm at 60 °C. Notable differences in shear stress sensitivity were observed between mechanically and chemically treated nanofibers. The TEMPO-oxidized nanofiber (CNF-TO) grease demonstrated outstanding lubrication stability across contact loads of 10–40 N and temperatures from 25 to 100 °C, maintaining COF values below 0.1—comparable to the reference LBG at 40 N load. Wear scar analysis confirmed that CNF-based greases significantly reduced wear relative to the lithium reference: CNF-B produced the smallest scar diameter (188 µm at 25 °C) while CNF-TO yielded the lowest at 60 °C (457 µm). (4) Conclusions: Nanofiber type and pretreatment significantly impact the tribological performance of CNF-based biogreases. TEMPO-oxidized CNFs provided stable lubrication under varied loads and temperatures, while all CNFs showed strong thermal adaptability, supporting their use in sustainable lubrication.

A method that enabled the formulation of lubricating oleogels using dried cellulose nanofibers (CNFs) as an eco-friendly thickener in castor oil was studied. In their dehydrated state, strong hydrogen bonding between nanofibers and high hydrophilicity are the main obstacles to their dispersion in oil. Hence, clusters of dried CNFs had to be previously detached by their dispersion in water. The resulting hydrogels were then subjected to methanol washes to displace the water from the nanofibers. After centrifugation, the methanol-wetted precipitate was readily dispersed in castor oil, forming an oleogel once the methanol was removed. Optimization was conducted in terms of the following variables: (a) hydrogel processing method; (b) hydrogel pH; (c) methanol/hydrogel ratio; (d) number of washes; and (e) oleogel CNF concentration. Their effect on the oleogel linear viscoelastic behavior was analyzed. In general, they demonstrated a prevailing elastic behavior denoted by a well-developed plateau region. The CNF concentration was found to have a more remarkable impact on the oleogels’ rheological behavior than any other variable studied. Hence, substantial differences were observed between 1 and 2 wt.%. The CNFs exhibited a very remarkable thickening capacity in castor oil, achieving a plateau modulus of ca. 700 Pa with just 2 wt.%. Moreover, the resulting oleogels maintained a uniform texture even after one year of storage. This indicates that the oleogels were both homogeneous and storage stable, effectively overcoming the stability issues associated with direct dispersion of dried CNFs in castor oil.

Sustainable lubricating oleogels were obtained based on elm (Ulmus minor Mill.) pulp cellulose nanofibers in castor oil. Before their nanofibrillation, two samples of the same bleached pulp were subjected to two different pre-treatments, either mechanical PFI (Paper and Fibre Research Institute) refining or chemical TEMPO-mediated oxidation. Their effect on the oleogels’ viscous flow and oscillatory shear behaviors was determined at different temperatures. Very pronounced differences were observed between these oleogels. The oleogel from the chemically-pretreated nanofibers showed evidences of being much more sensitive to both temperature and large shear deformations. Hence, its shear modulus underwent a very remarkable decay of 95.8% when subjected to a non-linear stress value of 100 Pa for 30 min. In turn, under the same conditions, a decay of only 41.4% was observed for the oleogel based on the nanofiber from the mechanically-pretreated nanofibers. Graphical abstract

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Taking advantage of the high thickening capacity of cellulose nanofibers in castor oil, sustainable lubricating oleogels were obtained. Methanol was used as mediator to transfer the nanofibers, originally present as a hydrogel, into the vegetable oil. The effect of the pulp bleaching on the oleogels’ thermo-rheological behavior was analyzed. Viscous flow curves and small/large amplitude dynamic shear tests were carried out on both bleached and unbleached samples of the same elm kraft pulp. Substantial differences were observed, above all when the oleogels were subjected to stress values beyond the onset of their linear viscoelastic regimes. The oleogel prepared with nanofibers from the unbleached pulp was found to endure large shear deformations better than that from the bleached pulp, and was less sensitive to temperature. Hence, the complex shear modulus (|G*|) corresponding to the oleogel based on 1.4 wt.% bleached nanofiber decreased by 55.7% upon a non-linear stress of 200 Pa was applied for 30 minutes. However, a less severe structural breakdown, i.e., a decay in |G*| of 28.9% (half of the previous one), was monitored when the oleogel based on the unbleached nanofiber was submitted to the same testing conditions. It should also be underlined that such an oleogel showed an extraordinary capacity to recovery its structure after the large shear deformation process.

Based on the response surface methodology, a rheological and tribological study carried out on eco-friendly lubricants is described. Such ecolubricants consisted of fibrillated or crystalline nanocellulose in vegetable oil (castor oil, high oleic sunflower oil or their mixtures). Cellulose nanoparticles showed noticeable friction-reducing and anti-wear properties within the boundary and mixed lubrication regimes, which were found to be dependent on nanocellulose concentration, base oil composition and applied normal force. In general, both types of nanocellulose performed equally well. An excellent tribological performance, with large wear scar diameter reductions, was achieved with 3.3 wt.% (or higher) nanocellulose dispersions in castor oil-rich mixtures. The observed behavior was explained on the basis of enhanced viscosity of castor oil-rich suspensions and the preferential action of the most polar components, nanocellulose and ricinoleic acid, in the vicinity of the contact surfaces.