Dispersion Stability and Lubrication Performance Correlation of Vegetable Oil-In-Water Emulsions with Nanoparticle-Shielded Oil Droplets

lubricants

Article Dispersion Stability and Lubrication Performance Correlation of Vegetable Oil-In-Water Emulsions with Nanoparticle-Shielded Oil Droplets

Reza Taheri *, Buyung Kosasih, Hongtao Zhu and Anh Kiet Tieu

School of Mechanical, Materials, Mechatronics, and Biomedical Engineering, University of Wollongong, Wollongong 2522, Australia; [email protected] (B.K.); [email protected] (H.Z.); [email protected] (A.K.T.) * Correspondence: [email protected]

Received: 3 May 2018; Accepted: 6 June 2018; Published: 9 June 2018

Abstract: Vegetable oil-in-water (VO/W) emulsions are bio-based metal working lubricants. The emulsions’ lubrication performance depends on the stability of oil droplets. In this paper, the oil droplets’ dispersion stability and lubrication of emulsions containing TiO2/SiO2 nanoparticles (NPs) as dispersant and lubrication agents have been investigated. Enhanced dispersion of NP-shielded oil droplets was found. Increasing the NPs’ mass fraction initially lowers the average size of NP-shielded droplets up to the saturation of the droplets’ surface with NPs at 0.5 wt % mass fraction. NPs also form NP agglomerates in emulsions, more so after the droplets’ surfaces have been saturated with NPs. There is an apparent minimum quantity of NPs (~0.5 wt %) required to ensure sustained dispersions of the droplets which is thought to be related to the oil concentration and the droplets’ total surface-area-to-volume ratio. Below the required quantity of NPs, partially shielded and fully shielded droplets coexist. The partially shielded droplets initially attract other droplets and undergo limited coalescence but retain their long-term stability. A small quantity of NPs improves the antiwear property of the lubricants. However, emulsions with NPs have slightly higher friction than the NP-free emulsion due to the reduced strength of the triboﬁlm. Despite the increased friction, the triboﬁlm formed in presence of NPs can easily be removed from the surface with water, indicating cleaner surfaces after the lubrication (i.e., less oil residue on the surfaces), which, for the sake of cleanliness, is favourable in many applications.

Keywords: nanoparticle-induced stability; clean lubrication; bio-based vegetable oil-in-water emulsion; TiO2/SiO2 composite nanoparticles

1. Introduction Vegetable oil-in-water (VO/W) emulsions are promising renewable and environmentally friendly metal working lubricants [1]. Polar vegetable oil droplets of VO/W emulsions comprise fatty acid triglycerides and have a strong adsorption propensity to metal surfaces, hence, they provide good lubricity in the boundary regime lubrication [2–4]. In order to perform the lubrication role, droplets in emulsions must be in a stable dispersed state. Ultrasonic homogenising has been used to prepare stable droplets in emulsions [5]. However, long-range hydrophobic attraction forces exist between the vegetable oil droplets in VO/W emulsions that cause coalescence; hence, the oil/water phase separation occurs over time [5–7]. Surfactants, surface-active polymers, and solid nanoparticles (NPs) have been used to improve the dispersion stability of such emulsions [8]. NPs are particularly more attractive stabilising agents as they possess both dispersant and lubrication properties that can be combined in composite NPs [9,10].

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In NP-stabilised emulsions (known as Pickering emulsions), the stability of droplets is attributed to the presence of steric layers of NPs (NP shields) on the oil/water interface of droplets, similar to egg–shell structures [11,12]. The formation and stability of NP shields on the droplets’ surface depends on the shape, size, and mass fraction of NPs (wt %), as well as inter-NP interactions and adsorption characteristics of NPs on the droplets’ surface [10–14]. Formation of dense, closely packed NP shields on the droplets’ surface is preferred for better dispersion characteristics. Hence, an adequate quantity of NPs is necessary to ensure the formation of uniform NP shields [13,14]. The required quantity of NPs depends on the NP/oil concentration ratio. A low quantity of NPs results in partial coverage of the droplets with NP shields. By increasing the NPs’ mass fraction, NPs continuously accumulate on the droplets’ surface and increase the stability of droplets up to a saturation point (adsorption plateau of NPs on the droplets’ surface). Above the adsorption plateau of NPs, excess NPs form NP agglomerates in the emulsion that deteriorates the stability [12,15]. On the other hand, inhomogeneous NPs with nonuniform geometries cannot form closely packed NP shields on the droplets’ surface. In such cases, partial coverage of droplets with NPs occurs. If giving rise to the repulsive double-layer electrostatic forces between the droplets, such partial coverage with NPs can provide stability [8]. For instance, with particular NPs having ionisable surface groups on the droplets’ surface, parts of the NPs that reside in water become ionised (OH− formation), charging NPs asymmetrically. The charged NPs on the droplets’ surface attract counter-ions from water, create charge ﬁelds around the droplets and electrostatically improve the coalescence stability [16]. This makes ionisable NPs such as silica good dispersion agents [13] and common elements in composite NPs with combinative dispersant and lubrication properties. In addition to dispersing effects, some NPs such as TiO2 show good lubricity in different environments [17,18]. To beneﬁt from the synergic lubricity effects of TiO2 NPs and dispersant effects of SiO2 NPs, TiO2/SiO2-composite NPs were investigated in soybean VO/W emulsions. Mechanisms with which the dispersion stability of the soybean oil droplets is provided with TiO2/SiO2 NPs are discussed. The stability of emulsions is then correlated with the lubrication properties of the NP-stabilised droplets of the emulsions in the boundary lubrication regime.

2. Methodology

2.1. Materials and Preparation of Emulsions

TiO2/SiO2-composite NPs (Sigma Aldrich, Castle Hill, Australia) and soybean oil (Masterolfoods, Whitebridge, Australia) were used. TiO2/SiO2 NPs have amorphous structures in which chainlike SiO2 NPs are incorporated into TiO2 NPs, where SiO2 acts as carrier for TiO2 [19,20]. The XRD analysis of TiO2/SiO2 NPs indicated the NPs’ average size to be 35 nm [21]. Soybean oil is composed of 15% saturated, 24% monounsaturated, and 61% polyunsaturated fatty acids. VO/W emulsions were ultrasonically homogenised using 0%, 0.1%, 0.25%, 0.5%, 0.75%, and 1 wt % NPs and 1 vol % soybean oil. Initially, NPs were dispersed in water by 30 min ultrasonication. 1 vol % oil was then added to the NP dispersions and emulsions were emulsiﬁed with 0.16 kJ/mL homogenising energy to ensure the initial production of ﬁne oil droplets with low dispersity [21].

2.2. Characterisation of NPs and VO/W Emulsions Size distributions of oil droplets and NP agglomerates were measured immediately after the preparation (on day 0) and then after 14 and 28 days using a Malvern Zetasizer nano S (Malvern Panalytical, Worcester, UK). The UV–vis spectrophotometry technique was used to study the stability of the droplets after the preparation in the wavelength range of 200–600 nm. Emulsions were diluted in water (1 vol % concentration) and measurements were performed on a Shimadzu UV-1800 spectrophotometer (Shimadzu Corp., Kyoto, Japan). The over-time stability of the emulsions within 96 h after the preparation is discussed in terms of the ratio of the absorbance intensity at different Lubricants 2018, 6, 55 3 of 12 times to the corresponding initial value on day 0. This indicates the relative concentration (RC) of NPs in the emulsions at different times. The interactions between NPs and oil droplets were observed using confocal laser scanning microscopy (CLSM) on a Leica TCS SP8 (Leica Microsystems, Mannheim, Germany). To identify NPs, Rhodamine B ﬂuorescence dye (95%, excitation wavelength 552 nm) (Sigma Aldrich) was used [22]. Dye suspension in water (0.0001 wt % mass fraction) was used to prepare the emulsions for confocal microscopy analysis. Agglomerates of NPs in the aqueous phase were analysed using Jeol JSM6490 SEM/EDS (Jeol Ltd., Tokyo, Japan).

2.3. Lubricity of VO/W Emulsions

The effect of TiO2/SiO2 NPs on the friction reducing property of emulsions was investigated using stainless steel SS316 balls (4 mm diameter) and discs on a UMT2 tribometer (Bruker, Billerica, MA, USA). To ensure boundary regime lubrication, lubricity tests were run at 60 rpm (0.0125 m/s) under a 6N load [23]. Friction tests were designed to assess the film-forming property of emulsions. Initially, water was used as a reference lubricant in stage I for comparison purposes. Water was then replaced with the emulsions in stage II of the tests to assess the film-forming capability of the emulsions by measuring the coefficients of friction (COFs) for 25 min. By further replacing the bulk of emulsions with water, the supply of the oil droplets to the contact zone reduces significantly, hence the adsorption strength of the already adsorbed film determines the lubricity. The strength of this film was assessed in stage III by obtaining COFs after the bulk emulsions were replaced with water. SEM images of the wear tracks on the discs were also obtained using Jeol JCM6000 SEM/EDS (Jeol Ltd.).

3. Results and Discussion

3.1. Size Distribution of the Dispersed Droplets Changes of size distributions of the oil droplets over time indicate how stable the droplet dispersions are. Figure1a,b compares the changes of droplet sizes with time in emulsions without and with 0.25 wt % NPs. Without NPs, two overlapped size distributions are observed with average sizes of 170 nm and 550 nm on day 0 (Figure1a). After 14 days, two overlapped distributions enlarge and form distinct small and large groups of droplets with average 200 nm and 2790 nm sizes. After 28 days, aggregation of the droplets with an average size of 2500 nm is observed, which is thought to be due to attractive forces between the hydrophobic oil droplets resulting in the over-time coalescence of the ﬁne droplets [7,24]. On the contrary, in presence of 0.25 wt % NPs, the dispersed oil droplets with a 220-nm average size remain stable for up to 28 days (Figure1b). Similar size distributions are obtained in presence of 0.5 wt % and 0.75 wt % NPs (not shown). We attribute this to the steric barriers rendered by the presence of NP shields on the droplets’ surface [25]. With 0.1 wt % NPs, monodispersed droplets with an average size of 600 nm are obtained immediately after the preparation (Figure1c). Increasing the NPs’ mass fraction from 0.1 wt % to 0.25 wt % lowers the average droplet size to 220 nm. When the NPs mass fraction is increased to 0.5 wt %, adsorption of NPs on the droplets reaches the maximum plateau with a limited average droplets size (~260 nm). This is attributed to the saturation of droplets’ surface with NP shields in the presence of 0.5 wt % NPs. Excess NPs at above 0.5 wt % mass fraction contribute to the formation of NP agglomerates [15]. NP agglomerates are not stable within the emulsions and tend to sediment over time. This reduces the overall stability of the droplets, noted by high dispersity index (≈0.89) of the emulsion with 1 wt % NPs only after 7 days (not shown). LubricantsLubricants 20182018,,6 6,, 55x FOR PEER REVIEW 44 ofof 1212 Lubricants 2018, 6, x FOR PEER REVIEW 4 of 12

FigureFigure 1.1. Over-timeOver-time sizesize distributiondistribution ofof oiloil dropletsdroplets duringduring 2828 daysdays ((aa)) inin emulsionemulsion withoutwithout NPs;NPs; andand b c ((Figureb)) inin emulsionemulsion 1. Over- time withwith size 0.250.25 distribution wtwt %% NPs;NPs; ((ofc)) oil averageaverage droplets sizesize during ofof dropletsdroplets 28 days inin emulsions(emulsionsa) in emulsion withwith without 0.10.1 wtwt %–1% NP–1 s wtwt; and %% NPs immediately after the preparation. NP(b)s in immediately emulsion with after 0.25 the wt preparation. % NPs; (c) average size of droplets in emulsions with 0.1 wt %–1 wt % NPs immediately after the preparation. PolarPolar vegetablevegetable oiloil dropletsdroplets tendtend toto attractattract NPsNPs [[2626].]. However,However, thethe adsorptionadsorptionof of silicasilica (SiO(SiO22)) NPsNPs onon thethePolar oiloil dropletsdroplets vegetable occursoccurs oil droplets atat equilibriumequilibrium tend to attract withwith thethe NP disperseddisperseds [26]. However, freefree NPs,NP thes, indicatingindicating adsorption thethe of formation formationsilica (SiO of2of) NP NPNPs agglomeratesagglomerateson the oil droplets in emulsions occurs at even evenequilibrium with with a a low lowwith amount amount the dispersed of of NP NPss [free14 [14,27 ,NP27]. ].Similars, Similarindicating behaviour behaviour the formation is expected is expected of NPfor forSiOagglomerates SiO2/TiO2/TiO2 NP2 s.inNPs. Therefore,emulsions Therefore, eventhe theformation with formation a low of amount ofNP NP agglomerates agglomeratesof NPs [14,27 is]. is expectedSimilar expected behaviour regardless regardless is expected of of the mass mass for fractionfractionSiO2/TiO ofof2 NP NPs.s. This ThisTherefore, is is evidenced evidenced the formation with with ~5.5 ~5.5 of µm µNPm sizes sizesagglomerates detected detected in in isthe theexpected presence presence regardless of of all all mass mass of fractions fractionsthe mass of ofNPfraction NPss (see (see of Figure NP Figures. This1b1 bas asis an evidenced an example), example), with traced traced ~5.5 to toµ them the sizes NP NPss detectedby by EDS EDS mapping in mapping the presence of of Ti Ti and andof allSi Si elements.mass elements. fractions This This ofis isshownNP showns (see in inFigureFigure Figure 1b2,2 indicatingas, indicating an example), the the tendency tendencytraced to of the of NP NPs Ns Ptos to byfrom from EDS NP NP mapping agglomerates agglomerates of Ti and in in the Si the elements.aqueous aqueous phase This phase is of ofemulsions. emulsions.

Figure 2. SEM image and EDS mapping of Ti and Si elements in NP agglomerates in emulsions with FigureNPFigures (scale 2.2. SEMSEM bars imageimage 5 µm). andand EDSEDS mappingmapping ofof TiTi andand SiSi elementselements inin NPNP agglomeratesagglomerates inin emulsionsemulsions withwith NPsNPs (scale(scale barsbars 55µ µm).m). 3.2. UV–Vis Spectrophotometry Analysis 3.2. UV–Vis Spectrophotometry Analysis Dispersion stability is inferred with UV–vis spectrophotometry. Figure 3a shows the absorbance spectraDispersion of the emulsions stability withis inferred different with mass UV –fractionsvis spectrophotometry. of NPs immediately Figure after 3a showsthe preparation. the absorbance Peak spectra of the emulsions with different mass fractions of NPs immediately after the preparation. Peak

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3.2. UV–Vis Spectrophotometry Analysis

LubricantsDispersion 2018, 6, x stabilityFOR PEERis REVIEW inferred with UV–vis spectrophotometry. Figure3a shows the absorbance5 of 12 spectra of the emulsions with different mass fractions of NPs immediately after the preparation. Peak Δ푎푑푠표푟푝푡𝑖표푛 wavelengths are determined where∆adsorption = 0 and are denoted by dashed lines. Wide wavelengths are determined where ∆wavelengthΔ푤푎푣푒푙푒푛𝑔푡= 0ℎ and are denoted by dashed lines. Wide absorbance peaksabsorbance of emulsions peaks of indicate emulsions the multidisperse indicate the multidisperse behaviour of droplets behaviour and of NPs droplets in the and emulsions NPs in [28 the]. Withoutemulsions NPs, [28 a]. peakWithout at a smallNPs, a wavelength peak at a small (200.5 wavelength nm) is obtained (200.5 linked nm) is to obtained the NP-free linked droplets. to the With NP- 0.25free wtdroplets. % NPs, With a greater 0.25 wt peak % wavelengthNPs, a greater compared peak wavelength to the NP-free compared emulsion to the is observed NP-free atemulsion 243.5 nm. is Thisobserved shift isat attributed 243.5 nm. to This a higher shift is local attribute densityd to in a the higher emulsion local with density NPs in compared the emulsion to the with NP-free NPs emulsioncompared [ 29to] the by NP the- virtuefree emulsion of adsorption [29] by of the the virtue NPs onof adsorption the droplets’ of surface. the NPs Increasing on the droplets the NPs’’ surface. mass fractionIncreasing to 0.5the wtNP %s’ increasesmass fraction the adsorptionto 0.5 wt % of increases NPs on the adsorption droplets, which of NP increasess on the droplets, the absorbance which intensityincreases while the absorbance the peak wavelength intensity while increases the slightly peak wavelength to 251 nm. NPs’ increases adsorption slightly on to droplets 251 nm. reaches NPs’ theadsorption maximum on droplets plateau atreaches 0.5 wt the % maximum while further plateau increasing at 0.5 wt the % NPs’while mass further fraction increasing results the in NP thes’ formationmass fraction of NP results agglomerates in the formation only. This of is NP inferred agglomerates by a signiﬁcant only. This increase is inferred in the absorbance by a significant peak wavelengthincrease in the to absorbance 306 nm with peak 1 wt wavelength % NPs [28 ,to30 ].30 Moreover,6 nm with 1 ﬂattening wt % NP ins [28 the,30 absorbance]. Moreover, intensity flattening is observedin the absorbance with 1 wt intensity % NPs below is observed 300 nm. with This is1 duewt % to NP thes full below adsorption 300 nm. of This UV in is thedue wavelength to the full rangeadsorption of 200–300 of UV nm in the by thewavelength NP agglomerates. range of 200–300 nm by the NP agglomerates.

Figure 3. (a) UV–vis absorbance spectra of emulsions with 0 wt %–1 wt % NPs (peak wavelengths are Figure 3. (a) UV–vis absorbance spectra of emulsions with 0 wt %–1 wt % NPs (peak wavelengths are shown by dashed lines); and (b) relative concentration (RC) within 96 h after the preparation. shown by dashed lines); and (b) relative concentration (RC) within 96 h after the preparation.

Relative concentration (RC) is used to assessassess thethe over-timeover-time dispersiondispersion stabilitystability of thethe dropletsdroplets (Figure(Figure3 3b).b). RCRC isis defineddeﬁned as the ratio of absorbance absorbance intensity to the the corresponding corresponding initial initial value value obtained afterafter thethe preparationpreparation [ 31[31].] For. For instance, instance, 100% 100%RC RCindicates indicates stable stable initial initial dispersions. dispersions. Figure Figure3b shows3b shows relatively relatively high high stability stability (RC (>RC 95%) > 95%) of the of emulsionsthe emulsions with with 0.25 wt0.25 % wt and % 0.5 and wt 0.5 % wt NPs % for NP ups for to 96up h,to indicating96 h, indicat theing formation the formation of stable of stable droplets. droplets. On the On other the hand,other hand, the NP-free the NP emulsion-free emulsion shows shows a 10% reductiona 10% reduction in RC within in RC within 48 h. Interestingly, 48 h. Interestingly, with excess with NPs excess (1 wtNP %),s (1RC wtremains %), RC remains at above at 95% above for 4895% h, thenfor 48 decreases h, then decreases to 87% and to 87% 84% and after 84% 72 after and 9672 h,and respectively. 96 h, respectively This indicates. This indicates the degradation the degradation of the of the emulsion after 72 h, which is related to the propensity of excess NPs to form sedimenting µm- sized NP agglomerates which consequently lower the dispersion stability of the droplets.

3.3. CLSM Analysis of Emulsions

To explain the above observation, we hypothesize that TiO2/SiO2 NPs form anticoalescing shell- like NP layers on the droplets’ surface [8,11,21]. To examine this, confocal laser scanning microscopy

Lubricants 2018, 6, 55 6 of 12 emulsion after 72 h, which is related to the propensity of excess NPs to form sedimenting µm-sized NP agglomerates which consequently lower the dispersion stability of the droplets.

3.3. CLSM Analysis of Emulsions

To explain the above observation, we hypothesize that TiO2/SiO2 NPs form anticoalescing shell-likeLubricants NP 2018 layers, 6, x FOR on PEER the REVIEW droplets’ surface [8,11,21]. To examine this, confocal laser scanning6 of 12 microscopy (CLSM) was used. Soybean oil and TiO2/SiO2 NPs are nonfluorescent. In order to (CLSM) was used. Soybean oil and TiO2/SiO2 NPs are nonfluorescent. In order to visualise the NPs, visualise the NPs, Rhodamine B fluorescence dye was used (SiO2 NPs attract Rhodamine B dye in waterRhodamine due to electrostatic B fluorescence interactions dye was [used22]). (SiO Figure2 NP4 sshows attract theRhodamine bright-field B dye and in CLSM water imagesdue to of theelectrostatic NP-free emulsion interactions immediately [22]). Figure after 4 the shows preparation the bright (day-field 0) and and CLSM after 7 images days. On of theday NP 0, NP-free-free emulsion immediately after the preparation (day 0) and after 7 days. On day 0, NP-free droplets with droplets with varying sizes were detected in the dye solution (Figure4a,b). This confirms the initial varying sizes were detected in the dye solution (Figure 4a,b). This confirms the initial stability of NP- stability of NP-free droplets of ultrasonically prepared emulsions. Larger droplets are observed after free droplets of ultrasonically prepared emulsions. Larger droplets are observed after 7 days (Figure 7 days (Figure4c,d), indicating the aggregation of fine droplets due to the strong attraction forces 4c,d), indicating the aggregation of fine droplets due to the strong attraction forces between the bare betweenhydrophobic the bare oil hydrophobicdroplets. This oilagrees droplets. with previous This agrees results with indicating previous that results the average indicating size of thatNP- the averagefree droplets size of NP-freeincreases droplets over time increases in absence over of coalescence time in absence barriers. ofcoalescence barriers.

Figure 4. Corresponding bright-field (BF) and confocal laser scanning microscopy (CLSM) images of Figure 4. Corresponding bright-ﬁeld (BF) and confocal laser scanning microscopy (CLSM) images of NP-free droplets in 0.0001 wt % Rhodamine B dye solution, (a) and (b) after the preparation (day 0); NP-free droplets in 0.0001 wt % Rhodamine B dye solution, (a,b) after the preparation (day 0); (c,d) after (c) and (d) after 7 days (scale bars 5 µm). 7 days (scale bars 5 µm). Figure 5 shows the CLSM images of emulsions with 0.25 wt % and 0.5 wt % NPs on day 0. In the presence of NPs, formation of anticoalescing NP shields on the droplets’ surface is expected. This is confirmed by tracing the Rhodamine B dye attached to the NPs to the oil/water interface of the droplets. However, these shielding layers appear different in 0.25 wt % and 0.5 wt % NPs. When the surface of the droplets is saturated with NPs at 0.5 wt % mass fraction (Figure 5b), the NP-shielded

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Figure5 shows the CLSM images of emulsions with 0.25 wt % and 0.5 wt % NPs on day 0. In the presence of NPs, formation of anticoalescing NP shields on the droplets’ surface is expected. This is conﬁrmed by tracing the Rhodamine B dye attached to the NPs to the oil/water interface of the droplets. However, these shielding layers appear different in 0.25 wt % and 0.5 wt % NPs. When the surface of the droplets is saturated with NPs at 0.5 wt % mass fraction (Figure5b), the NP-shielded droplets are dispersed uniformly in the emulsion. On the contrary, with 0.25 wt % NPs, large droplets appear toLubricants have 2018 the, 6 tendency, x FOR PEER toREVIEW attract other droplets but still retain their stability (Figure75 ofa). 12 This is related to the intensity of NP shields on the droplets’ surface that determines the extent of stability [32]. Individualdroplets are oil dispersed droplets uniformly in emulsions in the emulsion. with 0.25 On wt the % contrary, and 0.5 with wt %0.25 NPs wt % were NPs, further large droplets investigated appear to have the tendency to attract other droplets but still retain their stability (Figure 5a). This is with CLSM imaging immediately after the preparation (Figure6, background noise is reduced). related to the intensity of NP shields on the droplets’ surface that determines the extent of stability With 0.5[32 wt]. % NPs, uniform coverage of the droplets with NPs is observed (Figure6b), which is enough to stabilise theIndividual droplets oil in droplets the emulsion. in emulsions However, with 0.25 0.25 wt % wt and % mass0.5 wt fraction% NPs were of NPs further is belowinvestigated the required amountwith of NPs CLSM for imaging full coverage immediately of all after the the droplets preparation with (Figure NPs (see 6, background Figure1c). noise Therefore, is reduced). with With 0.25 wt % NPs, the0.5 surface wt % NP ofs, someuniform droplets coverage remains of the droplets partially with covered NPs is observed with NPs (Figure (Figure 6b), which6a) due is enough to the to shortage of the NPstabilise supply. the Thisdroplets promotes in the emulsion. initial partial However coverage, 0.25 wt % of mass some fraction droplets of NP withs is below NPsbelow the required the required amount of NPs for full coverage of all the droplets with NPs (see Figure 1c). Therefore, with 0.25 wt mass fraction.% NPs, the Therefore, surface of with some 0.25 droplets wt % remains NPs, two partially size-dependent covered with pathways NPs (Figure for 6a) the due stabilisation to the of dropletsshortage are proposed. of the NP With supply. initially This promotes fully covered initial droplets,partial coverage uniform of some coverage droplets of with the droplets NPs below with NPs results inthe good required dispersion mass fraction. stability Therefore, owing with tothe 0.25 effectiveness wt % NPs, two of size NP-depend shieldsent on pathways the droplets’ for the surface. On the contrary,stabilisation initially of droplets partially are proposed. covered With droplets initially with fully 0.25covered wt droplets, % NPs areuniform hypothesised coverage of to the undergo limiteddroplets coalescence with NP ands results form in larger good dropletsdispersion (thus stability reduce owing the to the overall effectiveness droplets/water of NP shields interfacial on the area) droplets’ surface. On the contrary, initially partially covered droplets with 0.25 wt % NPs are until an equilibrium is reached where the total droplets/water interfacial area can be stabilised with hypothesised to undergo limited coalescence and form larger droplets (thus reduce the overall availabledroplets/water NPs [33]. At interfacial this state, area) as the until partially an equilibrium covered droplets is reached approach where the other total droplets, droplets/water short-ranged repulsioninterfacial caused area by can the be partially stabilised absorbedwith available NP NP shieldss [33]. At on this the state, droplets as the partially increases covered and droplets improves the stabilityapproach at close other proximity. droplets This, short is- dueranged to therepulsion strong caused and irreversibleby the partially adsorption absorbed NP of NPsshields on on the the droplets’ surface,droplets especially increases on the and large improves droplets the [stability12,14]. atTherefore, close proximity. the mechanism This is due by to whichthe strong the stabilityand of the dropletsirreversible is provided adsorption is related of NPs to on the the overall droplets droplets/water’ surface, especially interfacial on the large area droplets(which is[12 droplet-size,14]. Therefore, the mechanism by which the stability of the droplets is provided is related to the overall and preparation-energydroplets/water interfacial dependent area (which [1]) is and droplet the-size availability and preparation of an-energy adequate dependent quantity [1]) and of NPs the in the aqueousavailability phase. of an adequate quantity of NPs in the aqueous phase.

Figure 5. Cont.

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Figure 5.FigureConfocal 5. Confocal laser scanninglaser scanning microscopy microscopy (CLSM) (CLSM images) images of emulsions after after thethe preparation preparation with (a with) 0.25 wt %; and (b) 0.5 wt % NPs (scale bars 10 µm). (a) 0.25 wt %; and (b) 0.5 wt % NPs (scale bars 10 µm). Figure 5. Confocal laser scanning microscopy (CLSM) images of emulsions after the preparation with (a) 0.25 wt %; and (b) 0.5 wt % NPs (scale bars 10 µm).

(b)

Figure 6. Confocal laser scanning microscopy (CLSM) images of emulsions after the preparation with (a) 0.25 wt %; and (b) 0.5 wt % NPs (scale bars 3 µm).

(b) 3.4. Friction and Wear FigureIt 6was. Confocal observed laser that scanning both 0.25 microscopy wt % and 0.5 (CLSM wt %) massimages fractions of emulsions of NPs aftercan provide the preparation stability for Figure 6. Confocal laser scanning microscopy (CLSM) images of emulsions after the preparation with withthe emulsions (a) 0.25 wt in % which; and ( bthe) 0.5 full wt coverage % NPs (scale of droplets bars 3 occurs µm). with the latter. However, the adsorption µ (a) 0.25of TiO wt2/SiO %; and2 NPs (b on) 0.5 the wt droplets % NPs’ surface (scale bars is in 3equilibrim). um with the NP agglomerates in the aqueous 3.4. Frictionphase, theand presence Wear of which is detrimental to lubricity. To examine this, friction tests were designed 3.4. Frictionwith andemulsions Wear having 0 wt %, 0.25 wt %, and 0.5 wt % NPs (on day 0) and the results are shown in It was observed that both 0.25 wt % and 0.5 wt % mass fractions of NPs can provide stability for theIt emulsions was observed in which that the both full 0.25 coverage wt % and of droplets 0.5 wt % occurs mass with fractions the latter. of NPs However, can provide the adsorption stability for theof emulsions TiO2/SiO2 inNP whichs on the the droplets full coverage’ surface of is droplets in equilibri occursum withwith the latter.NP agglomerates However, the in the adsorption aqueous of TiOphase2/SiO, the2 NPs presence on the of droplets’which is detrimental surface is in to equilibrium lubricity. Towith examine the NPthis, agglomerates friction tests were in the designed aqueous phase,with theemulsions presence having of which 0 wt is% detrimental, 0.25 wt %, and to lubricity. 0.5 wt % ToNPs examine (on day this, 0) and friction the results tests wereare shown designed in with emulsions having 0 wt %, 0.25 wt %, and 0.5 wt % NPs (on day 0) and the results are shown Lubricants 2018, 6, 55 9 of 12

Lubricants 2018, 6, x FOR PEER REVIEW 9 of 12 in Figure7. Water lubrication results in COF ≈ 0.48 (stage I). By introducing emulsions in stage II, Figure 7. Water lubrication results in COF ≈ 0.48 (stage I). By introducing emulsions in stage II, COF COF reduces significantly for all emulsions. The least friction coefficient is obtained without NPs reduces significantly for all emulsions. The least friction coefficient is obtained without NPs (COF ≈ (COF ≈ 0.11) due to the formation of a tribofilm on the surface owing to the adsorption of fatty acid 0.11) due to the formation of a tribofilm on the surface owing to the adsorption of fatty acid films. films. COF increases to 0.13 with 0.25 wt % NPs, postulating the detrimental effect of NPs on the COF increases to 0.13 with 0.25 wt % NPs, postulating the detrimental effect of NPs on the film- film-forming capability of the oil droplets [21]. With 0.5 wt % NPs COF further increases to 0.15 due forming capability of the oil droplets [21]. With 0.5 wt % NPs COF further increases to 0.15 due to the to the abrasion of NPs between the surfaces. After replacing the emulsions with water (stage III), abrasion of NPs between the surfaces. After replacing the emulsions with water (stage III), the film the film formed by the NP-free emulsion remains intact on the surface, as evidenced by the slight formed by the NP-free emulsion remains intact on the surface, as evidenced by the slight increase in increase in COF to 0.13. On the other hand, replacing the emulsions having 0.25 wt % and 0.5 wt % COF to 0.13. On the other hand, replacing the emulsions having 0.25 wt % and 0.5 wt % NPs with NPs with water increases COF significantly. Therefore, with NPs, the strength of the film formed on water increases COF significantly. Therefore, with NPs, the strength of the film formed on the surface the surface reduces, as evidenced by the increased COFs by replacing the bulk emulsions with water. reduces, as evidenced by the increased COFs by replacing the bulk emulsions with water. This This suggests cleaner friction surfaces with less oil residue after the lubrication by rinsing the surfaces suggests cleaner friction surfaces with less oil residue after the lubrication by rinsing the surfaces with water. with water.

FigureFigure 7 7.. COFCOF tracestraces during during water water–emulsion–water–emulsion–water lubrication lubrication processes: processes: “ “stagestage I”: I”: water water lubrication lubrication;; ““stagestage II II”:”: emulsions emulsions with with 0 0 wt wt % %,, 0.25 0.25 wt wt % %,, and and 0.5 0.5 wt wt % % NP NPss lubrication lubrication;; and and “ “stagestage III III”:”: water water lubrication.lubrication. Different Different stages stages are are sep separatedarated by by dash dasheded lines. lines.

SEMSEM images images of of wear wear tracks tracks on on the the respective respective discs discs of of friction friction tests tests are are shown shown in in Figure Figure 88.. In spite ofof the the low low friction friction coefficient coefﬁcient without without NP NPs,s, wide wide furrows, furrows, fractures, fractures, and and signs signs of of plastic plastic deformation deformation areare observed observed on thethe surfacesurface lubricatedlubricated without without the the NPs NP (Figures (Figure8a,b), 8a,b indicating), indicating a dominant a dominant two-body two- bodyabrasive abrasive wear wear due todue high to asperityhigh asperity contacts contacts [34]. Wear[34]. Wear reduces reduces in presence in presence of 0.25 of wt 0.25 % NPs,wt % where NPs, wheremild scufﬁng mild scuffing grooves grooves alongside alongside the sliding the direction sliding aredirection observed are (Figure observ8edc,d). (Figure This is 8c,d attributed). This to is attributedthe entrapped to the NPs entrapped within NP thes contact within zonethe contact and in zone between and in the between surfaces. the These surfaces. entrapped These entrapped NPs work NPass rigid work third as bodies rigid third between bodies the surfaces,between mendthe surfaces, the surface, mend and the change surface, the and wear change mechanism the wear from mechanismabrasive two-body from abrasive to mild two three-body.-body to mi Thisld three promotes-body. a lowerThis promotes wear rate a inlower presence wear ofrate 0.25 in wtpresence % NPs ofcompared 0.25 wt % to NP thes lattercompared [35,36 to]. the latter [35,36].

Lubricants 2018, 6, 55 10 of 12 Lubricants 2018, 6, x FOR PEER REVIEW 10 of 12

Figure 8. SEM images of the wear tracks on discs lubricated with (a) and (b) emulsion without NPs; Figure 8. SEM images of the wear tracks on discs lubricated with (a,b) emulsion without NPs; (c) and (d) emulsion with 0.25 wt % NPs. (c,d) emulsion with 0.25 wt % NPs.

4. Conclusions 4. Conclusions The dispersion stability and lubricity of soybean oil droplets in VO/W emulsions with varying The dispersion stability and lubricity of soybean oil droplets in VO/W emulsions with varying mass fractions of TiO2/SiO2 NPs were investigated. The dispersion stability of the droplets is massattributed fractions to the of TiO formation2/SiO2 NPsof partial/full were investigated. anticoalescing The dispersion NP shields stability on the of oil/water the droplets interface is attributed of the todroplets. the formation With 1 of vol partial/full % soybean anticoalescing oil, 0.5 wt % NP s shieldsresult in on full the coverage oil/water of interface the droplets of the with droplets. NPs Withproviding 1 vol %a shielding soybean oil,layer 0.5 around wt % NPsthe droplets result in against full coverage coalescence. of the 0.25 droplets wt % withNPs can NPs also providing providea shieldingstability for layer the around droplets the despite droplets the againstpartial coalescence.coverage of the 0.25 droplets wt % NPs at 0.25 can wt also % providemass fraction. stability This for theindicates droplets two despite size-depende the partialnt stabilisation coverage of path the dropletsways for at droplets 0.25 wt in % the mass presence fraction. of Thisa low indicates quantity two of size-dependentNPs. Fully covered stabilisation droplets pathways have better for and droplets more in uniform the presence NP shields of a low on quantity the surface of NPs.and thusFully coveredpreserve droplets their fine have size. better Partially and morecovered uniform droplets NP coalesce shields oninitially the surface until an and equilibrium thus preserve is reached their ﬁne at size.which Partially available covered NPs can droplets stabilise coalesce the total initially droplets/water until an interfacial equilibrium area is due reached to the atstrong which adsorption available NPsof NP cans on stabilise the droplets the total’ surface droplets/water and the close interfacial-range repulsion area due forces to the between strong adsorption them. NP agglomerates of NPs on the droplets’degrade surfaceover time and and the reduce close-range the overall repulsion stability forces of droplets between especially them. NP in agglomerateshigh mass fractions degrade of NP overs time(1 wt and %). reduceDespite the the overall small increase stability in of COF droplets with NP especiallys, the presence in high of mass 0.25 wt fractions % NPs ofimprovesNPs (1 wtwear %). Despiteprotection the due small to increase the changed in COF wearwith mechan NPs, theism presencefrom two of-body 0.25 to wt mild % NPs three improves-body. The wear presence protection of dueNPs to also the changedpromotes wear less oilmechanism residue on from the two-body surface after to mild the three-body.lubrication process, The presence which of, from NPs alsothe promotescleanliness less point oil residue of view on, is thefavourable surface afterin many the lubricationapplications. process, which, from the cleanliness point of view, is favourable in many applications.

AuthorAuthor Contributions: Contributions: Conceptualization,Conceptualization, R.T.R.T. and B.K.;B.K.; Data curation, R R.T.;.T.; Funding Funding acquisition, acquisition, B B.K.,.K., H H.Z..Z. andand A.K.T.; A.K.T.; Investigation, Investigation, R.T.;R.T.; Methodology,Methodology, R.T.R.T. andand B.K.;B.K.; Resources, B B.K.,.K., H H.Z..Z. and and A A.K.T.;.K.T.; Supervision, Supervision, B B.K.;.K.; Writing—original draft, R.T.; Writing—review & editing, R.T., B.K., H.Z. and A.K.T. Writing—original draft, R.T.; Writing—review & editing, R.T., B.K., H.Z. and A.K.T. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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