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Evaporation-induced foam stabilization inlubricating oilsV. Chandran Sujaa, A. Karb, W. Catesb, S. M. Remmertb, P. D. Savageb, and G. G. Fullera,1

aDepartment of Chemical Engineering, Stanford University, Stanford, CA 94305; and bShell Global Solutions (US), Inc., Houston, TX 77082

Edited by Howard A. Stone, Princeton University, Princeton, NJ, and approved June 21, 2018 (received for review April 4, 2018)

Foaming in liquids is ubiquitous in nature. Whereas the mecha-nism of foaming in aqueous systems has been thoroughly studied,nonaqueous systems have not enjoyed the same level of exami-nation. Here we study the mechanism of foaming in a widely usedclass of nonaqueous liquids: lubricant base oils. Using a newlydeveloped experimental technique, we show that the stability oflubricant foams can be evaluated at the level of single bubbles.The results obtained with this single-bubble technique indicatethat solutocapillary flows are central to lubricant foam stabiliza-tion. These solutocapillary flows are shown to originate fromthe differential evaporation of multicomponent lubricants—anunexpected result given the low volatility of nonaqueous liquids.Further, we show that mixing of some combinations of differentlubricant base oils, a common practice in the industry, exacerbatessolutocapillary flows and hence leads to increased foaming.

lubricant foaming | solutocapillary flows | single-bubble interferometry |spontaneous dimpling

L iquid foam, by definition, is a dispersion of a gas in a liquid.Such foams are generated through the accumulation of gas

bubbles originating from external gas entrainment or through therelease of dissolved gases in the liquid. These liquid foams arecommon and desirable in many applications such as food man-ufacturing processes, personal and health-care products, deter-gency, firefighting, and flotation of minerals (1–3). In contrast,excessive foam in lubricants is undesirable and detrimental asfoaming leads to excessive wear in machine parts, decreased lubri-cation, inadequate heat removal, lubricant oxidation, and overallenergy losses (4). Lubricant foaming is particularly problematicfor critical but hard-to-monitor machinery like wind turbines(5, 6), and hence there is a significant interest in formulatinglubricants where foaming is either avoided or destabilized (7, 8).

The current industrial efforts aimed at improving lubricantformulations for foam performance seek to identify optimalbase oil–additive combinations that satisfy increasingly stringentlubricant foaming guidelines established by international stan-dards and original equipment manufacturers (7). Currently, theidentification of such lubricant base oil–additive combinations isa costly and time-consuming endeavor, primarily due to the lackof experimental techniques that can provide direct mechanisticinsights into lubricant foaming. Existing experimental techniqueslike the ASTM D892 (9), Flender foam test (10), and the foamrise test (11) are bulk foam tests and only provide information onthe stability and density of the aggregate foam. However, single-film experiments using the well-known Scheludko cell (12) canyield mechanistic insights into thin liquid films but are knownto have shortcomings, particularly relevant to probing foamingmechanics (1). These shortcomings include the inability to usefull bubbles and simulate bubble coalescence at flat liquid–airinterfaces. Hence, researchers have traditionally used bulk foamexperiments to interpret foaming mechanics indirectly (4, 13, 14)and guide lubricant formulation (15, 16).

In addition to assisting with the aforementioned problem ofefficient lubricant formulation, this work also addresses the fun-damental question concerning the origin of foaming in lubricantbase oils—the primary nonaqueous phase of lubricants (15).Since lubricant base oils are typically surfactant-free, foaming in

lubricants has mostly been attributed to the effects of viscosity(4). However, viscosity alone cannot account for foam stability;in fact, base oils with identical viscosities can show almost twoorders of magnitude difference in the volume of sustained foamacross the different base-oil categories under identical test con-ditions (17). These five different base-oil categories (or groups)established by the American Petroleum Institute for aiding lubri-cant interchangeability (API 1509, Annexure E) differ in termsof (among other things) the refining method, viscosity index, pro-portion of saturates, and volatility (15). It is thus evident thatthere exist additional foam stabilization mechanisms responsi-ble for the observed differences in foam performance of the fivebase-oil groups.

In this paper we propose single-bubble experiments using thenewly developed dynamic fluid-film interferometer (DFI) (Fig.1) (1) as a suitable experimental technique to obtain directmechanistic insights into foaming in lubricants. The viability andconsistency of the experimental technique in predicting bulkfoam stability is established by correlating the single-bubble coa-lescence results against foam rise test rankings (ASTM D892, forexample) on five different lubricant base oils, each from a differ-ent base-oil group (SI Appendix, Table S1). Subsequently, usingthe spatiotemporal measurements of single-bubble wall thick-ness (Fig. 1B), solutocapillary Marangoni flows driven by differ-ential multicomponent evaporation are shown to aid lubricantfoam stabilization.

ResultsExperiments. Bulk foam measurements were conducted to obtaina benchmark for the single-bubble stability measurements. These

Significance

Mitigating lubricant foaming is of primary concern to lubricantmanufacturers, as the control of deleterious foams is criticalin high-performance applications. Facilitating the develop-ment of techniques to control foaming, the results from thisresearch has identified that a special type of Marangoni flowdriven by the differential evaporation of lubricant compo-nents plays a central role in promoting foaming in lubricantbase oils. In addition, this research also shows that analyzingthe stability of single bubbles can complement the foam sta-bility data obtained from traditional bulk foam experiments.Hence, this paper provides physical insights into lubricantfoaming and describes a convenient platform that lubricantmanufacturers can use to develop better lubricants that arenot susceptible to foaming.

Author contributions: V.C.S., A.K., W.C., S.M.R., P.D.S., and G.G.F. designed research; V.C.S.performed research; V.C.S., A.K., S.M.R., and G.G.F. analyzed data; and V.C.S., A.K., S.M.R.,and G.G.F. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1 To whom correspondence should be addressed. Email: ggf@stanford.edu.y

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1805645115/-/DCSupplemental.

Published online July 16, 2018.

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Fig. 1. Schematic of the single-bubble experimental setup (DFI) and a typical interferogram obtained from experiments. (A) The experimental setup withlabeled components. (B) The inset shows the initial and final positions of the bubble. Here R is the radius of curvature of the bubble, h(r, θ) is the filmthickness as a function of radial position (r) and angular position (θ), and R0 is the radial extend of the film visible on the interferogram. (C) A typicalinterferogram and its physical film thickness reconstructed using the adjoining reference color map.

bulk foam experiments were conducted by bubbling air at 15±0.45 mL/s into 25 mL of lubricant contained in a funnel for30 s (see Materials and Methods for details). At the end of 30 s,the airflow was cut off and the volume of the sustained foam wasmeasured until the foam collapsed completely. Additional bulkfoam measurements were also performed using the standardASTM D892 test.

The single-bubble experiments were performed using theautomated DFI (Fig. 1A). During an experiment, a single bub-ble (R≈ 0.7 mm) was formed on a capillary submerged in thedesired lubricant. The air–lubricant interface above the bubblewas then lowered (by moving the chamber down) so as to forma thin draining film above the bubble. This signaled the start ofthe experiment. An optical arrangement described in Materialsand Methods, which includes an interferometer, reported the filmthickness in space and time. The internal bubble pressure wasalso measured, and bubble coalescence was identified by dra-matic changes in that pressure. The coalescence time, defined asthe time required for bubble rupture to occur, was determinedaccurately to 0.05 s. All experiments were performed at 20 ◦C.Further, as elaborated in Single-Bubble Results, for some exper-iments the test chamber was covered with a glass cover slip tosuppress lubricant evaporation. The results from these experi-ments are denoted as “closed” or tagged with “(c)” to differen-tiate them from the experiments where the chamber was open,denoted as “open” or tagged with “(o).” Further, the reportedforced convection experiments were performed by pulsing airsinusoidally at a frequency of 13 Hz using a subwoofer (Log-itech) in the open configuration of the single-bubble experimentsdescribed above.

Five different lubricant base oils with comparable viscositiesspanning 37.7 cSt to 50.0 cSt at 20 °C (SI Appendix, Table S1) andtheir mixtures were tested. The tested lubricant base oils belongto five different base-oil groups: two conventional mineral oils(groups I and II), one synthetic group III base oil, one (groupIV) synthetic poly-alpha-olefin oil, and the other (group V) asilicone oil.

Bulk Foam Results. The evolution of the bulk foam volume mea-sured in the group I–IV base oils is shown in Fig. 2 A, i, whilethose measured in silicone oils are shown in Fig. 2 B, i. The

shaded region indicates the SE in the foam volume measure-ments evaluated across three experiments.

As shown in the bulk foam results in Fig. 2 A, i, the group Ibase oils create the most stable foams, followed by groups II, III,and IV. The same relative performance was obtained from theSequence I of the ASTM D892 test (SI Appendix, Table S1). Theresults from the silicone oils (Fig. 2 B, i) showed that the pure50 cSt silicon oils sustain almost no foam (comparable to a groupIV), while the silicone oil mixtures are seen to sustain relativelymore stable foam. Further, the sustained foams are also observedto collapse in discrete steps, where a significant number of bub-bles coalesced simultaneously (SI Appendix, Fig. S1 and MovieS1). This is manifested in Fig. 2 A, i as an almost instantaneouschange in foam volume. It is also worth noting that this foamcollapse behavior is different from the uniform foam collapseobserved in the surfactant-stabilized aqueous foams (1).

Furthermore, as seen from the results, the stability of thefoams and their collapse rates are very different across the testedlubricants, despite the comparable viscosities. This suggests thepresence of a foam-stabilizing mechanism in addition to viscositythat entrains oil and air to different extents across the differentlubricant groups. Isolating the stabilizing mechanism is difficultfrom the bulk foam rise test as we do not obtain any informationon the spatiotemporal evolution of the coalescing bubbles. Tomake quantitative measurements of the wall thickness of the coa-lescing bubbles and identify the foam stabilization mechanism,single-bubble experiments were conducted.

Single-Bubble Results.Group I–IV base oils. The coalescence times of single bubblesmeasured from the DFI are plotted against the fraction oftested bubbles (in a given lubricant) in Fig. 2 A, ii. The distri-bution of the coalescence times of bubbles rupturing naturallyis known to broadly obey the Rayleigh distribution (18, 19).Hence, to rank the bubble stability in the tested lubricants, thecoalescence times can be conveniently fitted to a Rayleigh cumu-lative distribution function (SI Appendix, Supporting InformationText and Fig. S2) constructed through a maximum likelihoodestimate (20). These signature curves, referred to hereafter asthe cumulative coalescence-time curves, capture the distribu-tion of coalescence times observed in a single lubricant—with

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Fig. 2. Comparison of foam stability results from bulk foam experiments with those of single-bubble experiments. (A) The results from the groups I–IVlubricant base oils. (i) The foam volume evolution measured from bulk foam experiments shows group I oils sustain the most foam, followed by groupsII, III, and IV. (ii) The cumulative coalescence-time curves of group I–IV lubricants obtained through fitting the measured coalescence time (shown by openmarkers for open experiments and filled markers for closed experiments) to a Rayleigh cumulative distribution function. Group I lubricants are again seento be more stable, as a majority of bubbles rupture at longer times, followed by group II, III, and IV base oils. Additionally, when the chamber was closed,experiments denoted by (c), the bubbles were seen to rupture at comparatively shorter times. (iii) The evolution of the area-average film thickness of testedbubbles (data shown for bubbles with coalescence times closest to the sample average). Spontaneous oscillations are observed in the film thickness ofbubbles in group I, II, and III base oils when the chamber is open, and the oscillations are suppressed when the chamber is closed. Group IV base oils showedno foaming irrespective of the chamber’s being open or closed. (B) The results from the silicone oil mixtures. (i) The evolution of foam volume from bulkfoam experiments shows that multicomponent silicone oil mixtures foam more than single-component pure silicone oils. (ii) The cumulative coalescence-time curves of different silicone oil mixtures. Silicone oil mixtures are seen to sustain more stable bubbles as compared with pure silicone oils. In addition,when the chamber is closed and evaporation is minimized, denoted by (c), the stability of bubbles in the silicone oil mixtures became comparable to thepure silicone oil. (iii) The evolution of the area-average film thickness for the bubbles with coalescence times closest to the sample average. Spontaneousoscillations are observed in the film thickness of bubbles in all silicone oil mixtures when the chamber is open, and the oscillations are suppressed when thechamber is closed.

curves extending to larger times signifying lubricants that sus-tain more stable foams. From these curves, we can see thatgroup I base oils sustain the most stable bubbles, followed bygroups II, III, and IV—which is the same trend obtained fromthe bulk foam experiments. Additionally, the close adherenceof the experimental data to a Rayleigh distribution reinforcesthat all of the tested bubbles coalesced naturally, and thatany influences of thermal drift or contaminating particles wereminimal.

In addition to providing information on bulk foam stability,the single-bubble experiments enable the measurement of thebubble-wall thicknesses that characterize the liquid entrainmentand its drainage leading up to the coalescence of a bubble (Fig.1C). For illustrating the liquid entrainment and drainage, themean film thickness =(2πR2

0)−1

∫∫h(r , θ)drdθ (see Fig. 1B for

notation) is used instead of the film volume, as the former isinsensitive to the size of the bubble. From the entrained mean

film thickness measured from the single-bubble experiments(Fig. 2 A, iii, open), we see a one-to-one correlation with the sus-tained foam volume measured from the bulk foam experiments(Fig. 2 A, i), with the group I base oils entraining the most liq-uid, followed by groups II, III, and IV. In addition, the evolutionof the mean film thickness was seen to be different from thoseexpected in aqueous systems (1). Notably, the tested lubricants(particularly evident in groups I and II) were seen to display thephenomenon of spontaneous dimpling. This effect is revealedin Fig. 2 A, iii, open as rapid temporal fluctuations in the meanfilm thickness, which reflects a dynamic creation and dissipationof dimples in the vicinity of the bubble apex. Movie S2 offers avivid example of these dynamics. Spontaneous dimpling was pre-viously reported in oil–water emulsions as a result of diffusion-driven surfactant redistribution and the resulting Marangoniflows (21). However, since our base oils are surfactant-free, theexistence of the dimpling phenomenon indicates the presence

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of a different physical mechanism leading to spatial variationsin surface tension.

Since spontaneous dimpling is not observed in the group IVbase oil (homogeneous synthetic poly-alpha-olefin), the originof Marangoni flows in group I–III base oils is related to theirmulticomponent nature (Fig. 3). Further, the positive correla-tion of bubble stability ranking with the evaporative mass lossfraction measured using the ASTM D5800 (SI Appendix, TableS1) suggested that these Marangoni flows are driven by the dif-ferential evaporation of the various components in these oils.Since evaporation can easily be controlled in single-bubble mea-surements, we tested this hypothesis by simply covering thechamber with a glass lid that is opaque to near-infrared radi-ation. This effectively minimizes evaporation and convectionduring the DFI experiments. In the absence of evaporation andair convection, the spontaneous dimpling was suppressed. Thisis manifested in Fig. 2 A, iii, closed by the absence of fluctua-tions in the mean film thickness. The stability of the bubbles alsodecreased (Fig. 2 A, ii), clearly suggesting that evaporation andconvection are indeed aiding bubble stabilization. Hence, unlikeprior studies that alluded to the destabilizing nature of evapo-ration (22), clearly evaporation has a stabilizing effect on thebase-oil foams. Furthermore, this evaporation-driven stabiliza-tion is not a result of thermal Marangoni stresses (23), as in thatcase even single-component systems would have foamed. Thus,these observations suggests that Marangoni flows arising fromdifferential evaporation in the multicomponent lubricant baseoils, herein referred to as solutocapillary Marangoni flows, areresponsible for the foam’s stability. To confirm this hypothesisand test the influence of the volatility of mixture components onbubble stability, we formulated controlled silicone oil mixtures.Silicone oil mixtures (group V). Silicone oil mixtures can be usedto construct ideal model systems to systematically study the phe-nomenon of solutocapillary stabilization. These model systemscan easily be formulated to have properties comparable to thebase oils such as bulk viscosity and density. Since 50 cSt sili-cone oil had properties similar to the base oils, it was chosenas the model pure oil. The mixed-oil systems were formulatedby combining 50 cSt with lower-viscosity samples of 20 and 2cSt. As noted in SI Appendix, as the viscosity of the siliconeoils decreases, the surface tension decreases, and the volatilityincreases (SI Appendix, Fig. S3 and Table S1).

Fig. 3. Schematic showing the mechanism of solutocapillary Marangoni-mediated bubble stabilization. (A) Evaporation-driven enrichment of theless-volatile component (shown by the dots) in the thin liquid film mak-ing up the wall of the bubble. σ is the surface tension. (B) The developedsurface tension gradients drive flow into the apex of the bubble, leading tothe growth of a dimple. (C) Ambient disturbances destabilize the dimple,causing it to dissipate asymmetrically. At the end of this process the bubblewall returns to the state shown in A, and the process repeats.

The results from the bulk foam experiments (Fig. 2 B, i) andthe single-bubble experiments (Fig. 2 B, ii) clearly show thatfoams in silicone oil mixtures are more stable as compared withthe pure silicone oil. The stability of silicone mixtures is alsoseen to increase with the volatility of the contaminant. In addi-tion to the increased foam stability, we observe that bubblescoalescing in silicone oil mixtures exhibit drainage behavior (Fig.2 B, iii, open and closed) that is similar to that observed ingroup I–III base oils, where spontaneous dimpling occurs. Theseobservations provide additional evidence for the hypothesis thatsolutocapillary flows resulting from differential multicomponentevaporation in group I–III base oils play a key role in stabilizinglubricant foams.

A physical understanding of the solutocapillary foam stabiliza-tion can be attained by considering the coalescence dynamics ofa bubble in a silicone oil mixture (Fig. 3). Initially, as a bub-ble approaches the air–liquid interface for coalescence, the thinliquid film making up the wall of the bubble consists of a homoge-neous mixture of the silicone oils. Almost immediately, the morevolatile silicone oil component starts to preferentially evaporateout of the thin film (Fig. 3A). As a result, the residual mixturebecomes increasingly rich in the less-volatile silicone oil—whichalso has a higher surface tension. Consequently, the surface ten-sion of the liquid mixture in the bubble wall becomes larger thanthat of the ambient liquid. These surface tension gradients driveMarangoni flows to the apex of the bubble, resulting in the spon-taneous growth of a dimple (Fig. 3B). These dimples grow up to acertain volume before ambient disturbances destabilize the dim-ple (24), causing it to dissipate (washout) (Fig. 3C). This processrepeats and is manifested as the observed spontaneous dimpling.Furthermore, as a result of this continuous liquid entrainment,the bubble wall is prevented from thinning down to a thicknessat which molecular forces can rupture the bubble—consequentlystabilizing the bubble.

Characteristics of observed solutocapillary stabilization. As soluto-capillary flows are driven by differences in surface tension causedby differential evaporation, they are different from surfactant-driven Marangoni flows in a number of ways. These differencesare highlighted in Fig. 4 using results from an industrially rele-vant mixture of base oils (10% mixture of group III in group IV).The differences include the following: (i) Bubble stabilizationcan be enhanced by ambient disturbances that vary the evapora-tion rate and/or cause nonuniform changes in the film thickness,and (ii) mixtures of oils are more likely to foam than the purecomponents comprising the mixture. Solutocapillary Marangoniflows with such characteristics have previously been observedand studied extensively in thin liquid films on solid substrates∗(ref. 25 and references therein). However, solutocapillary flowsover liquid–air interfaces remain largely unexplored, apart froma few studies—notably in the context of foaming during liquiddistillation (26, 27) and in aqueous alcohol mixtures (28)—andhave never before been identified as a stabilizing mechanism forfoaming in lubricants.

Finally, we note that solutocapillary stabilization is more pro-nounced in the top layer of bubbles in a foam. This is becausethe bubbles at the top experience unconfined evaporation andconvection, as they are exposed to the ambient air. However, thebubbles below the top layer are somewhat protected from evap-oration, as evaporation within the foam is expected to saturate attsat ∼ M

RTVA∼ 10−2s (for a 1 mm bubble within a foam contain-

ing 1% 2-cSt silicone oil), where M is molar mass of the volatilespecies, R the universal gas constant, T the temperature, andVA

is the volume-to-surface area ratio of a bubble (SI Appendix).

∗Rodriguez-Hakim M, Fuller GG. Ninety-First ACS Colloid & Surface Science Symposium,July 9–12, 2017, New York.

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Fig. 4. Features of solutocapillary flows observed in the lubricant base oils. (A) The distribution of coalescence times shows that (i) under sinusoidallyforced convection (FC) (13 Hz), the stability of the bubbles increased and (ii) bubbles in mixtures of lubricant base oils are more stable than in either of theircomponent base oils. (B) Mean thickness profiles of 10% mixture of group III in group IV showing the characteristic oscillations of solutocapillary Marangoniflows, and its absence in pure group III and group IV. Experimentally obtained interferograms for the corresponding cases are shown above.

Hence, the bubbles inside the foam are primarily stabilized bythe viscous resistance to the thinning of the bubble walls. Asa consequence, coalescence is relatively more intensive insidethe bulk of the foam than from the top (SI Appendix, Fig. S4).Furthermore, the rupture of bubbles from the top layers resultsin a cascade of bubble collapses in layers below, resultingin the stepwise foam collapse discussed above and shown inFig. 2 A, i.

DiscussionIn conclusion, we have shown that the relative stability of foamsin nonaqueous liquids—specifically that of lubricant base oils—can be determined by analyzing the coalescence dynamics ofsingle bubbles. This suggests single-bubble experiments are anattractive alternative to probe foaming characteristics of liquids,as they can provide additional mechanistic insights into foamingprocesses.

Using single-bubble experiments, four different (groups I–IV) lubricant base oils (each belonging to a different base-oilgroup) and silicone oil mixtures with comparable viscosities weretested to study the foam stabilization mechanism in these non-aqueous systems. The tests revealed that the dominant physicalmechanism stabilizing the otherwise thermodynamically unsta-ble bubbles were solutocapillary Marangoni flows. As thesesolutocapillary Marangoni flows originate through differentialevaporation in multicomponent liquids having components withdifferent volatilities and equilibrium surface tensions, the resultssuggest that mixing of some types of liquids can contribute toincreased foaming.

These results are particularly significant for the lubricantindustry, where foam control in high-performance applications iscritical. In many lubricated environments (apart from completelyconfined situations such as in hermetically sealed machinery)lubricant evaporation can be expected, as is the case with com-monly used gear-box arrangements having air breathers andlubricant recirculation systems connected to open air sumps. Insuch cases we can expect solutocapillary-mediated foaming. Fur-thermore, as solutocapillary stabilization is driven by differentialevaporation, ambient conditions that enhance evaporation suchas high temperature, ambient vibration, and air convection thatare prevalent in lubricated environments could also exacerbatesolutocapillary-mediated foaming. In such applications, lubricantproperties including base oil selection and blending must be

carefully considered to minimize solutocapillary-mediated foamstabilization.

Chemically, the lubricants discussed mainly comprise aliphaticalkenes/alkanes (groups I–IV) and polydimethyl siloxanes (sili-cone oils), which make up a wide class of nonaqueous systemscommonly encountered in our daily life. Consequently, evap-oration-driven solutocapillary-mediated foaming could apply toa broad class of nonaqueous systems and complement the estab-lished mechanisms of foam stability in nonaqueous systems (26,29). In fact, evaporation-driven solutocapillary-mediated foam-ing might be the common source of foaming in nonaqueoussystems with very low surface tension such as silicone oils thathave a low propensity to adsorb surface active species at theinterface. Further, the experimental technique and results pre-sented in the study will be useful to further characterize foamingin systems such as aqueous alcohol mixtures (28), frying oils(29), and liquid mixtures subjected to distillation (27), whereevaporation is known to be important.

Materials and MethodsBulk Foam Experiments. Bulk foam stability measurements were conductedin accordance with the industry standard ASTM D892 test. Foam rise mea-surements (reported in Fig. 2 A, i and B, i) were conducted using a bulk foamapparatus developed in-house, the details of which are reported elsewhere(1). All experiments were video-recorded at 30 frames per s. The volume offoam was measured every 10 frames, about 0.3 s after the bubbling wasstopped (the time taken for the last bubbles to finish rising through bulkliquid).

Single-Bubble Experiments. Single-bubble coalescence experiments wereconducted using the DFI; the specific details regarding its construction arementioned elsewhere (ref. 1 and references therein). At the start of everysingle-bubble experiment reported in this paper, 5 to 6 mL of the lubricantbase oil is filled in the DFI chamber. A bubble of 1.2± 0.15 µL is madeat the tip of a standard 16-gauge capillary (o.d.: 1.651± 0.013 mm, i.d.:1.194± 0.038 mm). The bubble size is chosen to be close as possible to thebubble size having the largest bubble number density in a freshly formedfoam (30), and at the same time large enough to avoid instabilities associ-ated with manipulating small bubbles on capillaries (31). After the bubbleis created on the capillary, the chamber is moved down (with the bubbleremaining stationary) at a constant velocity of 0.15 mm/s till the bubble isone radius away from the oil–air interface. This is the initial state of thesystem before all of the experiments (Fig. 1B).

At this point, the experiment starts with the pressure transducer mea-suring the pressure inside the bubble at 20 Hz. Subsequently, the bubble is

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raised a distance of 1.5 times its radius from its initial position and heldat that final position. (This final position is comparable to the equilib-rium position attained by a free bubble through the balance of buoyancyand capillary forces.) Simultaneously, the top camera records the evo-lution of the film of liquid between the bubble and the lubricant–airinterface. As the film drains and its thickness becomes comparable to thewavelength of light, interference patterns are seen by the top camera(Fig. 1C). Finally, the experiment ends as the film ruptures and the bub-ble coalesces at some critical film thickness. The film thickness is obtainedby mapping the colors in the recorded interference patterns to physi-cal thickness using the classical light intensity–film thickness relations (12)assuming homogeneous and nondispersive films. A Python 2.7-based soft-ware was developed in-house (1) to aid thickness mapping and visualizingthe thickness profiles.

Oil Samples. The four base oils (groups I–IV) were obtained from ShellGlobal Solutions (US), Inc. As shown in SI Appendix, Table S1, these baseoils have comparable densities (determined by ASTM D-4052), viscosities

(determined by ASTM D-445), and surface tension (determined by pen-dant drop method). The mass loss due to evaporation (determined byASTM D-5800) was different between the oils. Further, spectroscopic analy-ses were conducted on all of the base oils to ensure the absence of surfaceactive polydimethylsiloxane and fluoroalkyl groups. The 50-cSt silicone oil(Shin Etsu) was selected both as the group V lubricant base and the com-positionally pure model system to contrast the foaming behavior of theheterogeneous (groups I–III) lubricant oils. Finally, to study the effect of solu-tocapillary flows, controlled blends of 50-cSt silicone oil with 2 cSt and 20 cSt(Shin Etsu) were made. The reported mixtures (Fig. 2 B, ii) include 0.5% and5% by-volume mixture of 20 cSt in 50 cSt, and 0.5% by-volume mixture of2 cSt in 50 cSt.

ACKNOWLEDGMENTS. We thank Dr. John Frostad for his help and guid-ance with the initial stages of this study, Mariana Rodriguez-Hakim for thevaluable insights she provided us through her research on solutocapillaryflows in thin liquid films over solid substrates, and Prem Sai for creating theschematic illustrations in the manuscript. This work was supported by ShellGrant No. PT60980.

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