Surfactant-Enhanced Spreading of Sessile Water Drops on

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Surfactant-enhanced Spreading of Sessile Water Drops on Polypropylene Surfaces Xiang Wang, Joachim Venzmer, and Elmar Bonaccurso Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01357 • Publication Date (Web): 22 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Surfactant-enhanced Spreading of Sessile Water Drops on Polypropylene Surfaces Xiang Wang1, Joachim Venzmer2, Elmar Bonaccurso1*

1. Center of Smart Interfaces, Technical University Darmstadt, 64287 Darmstadt, Germany 2. Evonik Nutrition & Care GmbH, Goldschmidtstr. 100, 45127 Essen, Germany

* To whom correspondence should be addressed. Currently at Airbus Group Innovations, 81663 Munich, Germany. Email: [email protected]; Telephone: +49 89 607 27275. Fax: +49 89 607 25408.

Abstract Spreading of water drops resting in equilibrium on polypropylene surfaces was initiated by dispensing surfactant-laden droplets on their apex. Upon contact of the two drops two processes were kicked-off: surfactant from the droplets spread along the water/air interface of the sessile drops and a train of capillary waves propagated along the sessile drops. The contact line of the sessile drops remained initially pinned and started spreading only when surfactant reached it while the capillary waves did not have an apparent effect on initiating drop spreading. However, surfactant influenced the propagation velocity of the capillary waves. Though the spreading dynamics of such nonhomogenously mixed surfactant/water drops on polypropylene surfaces was initially different from that of homogeneously mixed drops, the later spreading dynamics was similar and was dominated by viscosity and surface tension in both cases. These results can help discriminating the path of action of surfactants in bulk and at the water/air interface, which is also relevant for understanding phenomena like superspreading.

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1. Introduction Spreading of a liquid drop on a solid surface, according to classical notions of wetting1–3, takes place when the spreading coefficient = − ( + ) is positive. Here, , , refer to solid, air and liquid phases, respectively, and denotes the static interfacial tension between two phases. A positive value of indicates that an unbalanced capillary force acting on the contact line between liquid drop and solid surface drives the liquid to spread. Surfactants are widely used as aids to facilitate or to fasten spreading by reducing the surface tension of liquids and the interfacial tension between liquid and substrate. Thus, the spreading coefficient can become positive in cases where it was originally negative and the liquid would not have wetted a surface. Among spreading-enhancing agents, trisiloxane surfactants like Silwet® L-77 and Silwet® Gold promote “superspreading” of drops on solid hydrophobic surfaces, which is needed in crop spraying applications, and have been investigated in a number of works4–8. The spreading dynamics of surfactant solutions in general and of superspreading solutions in particular is more complex than that of pure liquids. In fact, pure liquids have a surface tension that is only a function of surrounding pressure and temperature, while the surface tension of complex liquids - such as surfactant solutions - is also dependent on the “age” of the surface. It is thus a dynamic property in contrast to being a static property as for pure liquids. Directly after new liquid surface is generated - e.g. in a drop of aqueous surfactant solution - the surface tension has the same value as that of pure water. The surface tension then gradually decreases until a new equilibrium value is reached. The time required for reaching the static value depends on the diffusion rate and the adsorption rate of the surfactant at the liquid/air interface. Stimulated by the works of Ananthapadmanabhan9 numerous theoretical and experimental investigations have been performed to understand the molecular superspreading mechanism of aqueous trisiloxane solutions on hydrophobic surfaces. Commercially available trisiloxane


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surfactant, Silwet® L-77 (Momentive), has been widely investigated4,5,10,11. Experiments involved measuring dynamic contact angles of drops spreading on solid substrates with different hydrophobicity. One commonly used method to analyze drop spreading is recording the spreading radius versus the spreading time and fitting a power law function, ∝ , with being called wetting exponent5,7,12–14. However, different exponents α were reported even for similar aqueous superspreader solutions5,12,15. These discrepancies may have different reasons. For example, most commercial surfactants are not single species but mixtures of different homologues. Moreover, aqueous trisiloxane surfactant solutions slowly hydrolyze over time, depending on pH, concentration, and temperature, which changes their surface tension and their wetting ability16. Also different hydrophobic substrates such as polystyrene, Parafilm, or polypropylene have been used in the experiments, which does not ease the comparability of the results. It was also found that the ability to form bilayer aggregates (e.g. vesicles, lamellae) in solutions is common to superspreading surfactants17–22, which is different to what conventional surfactants do. As reported in23, direct adsorption of bilayer aggregates to the liquid/air and solid/liquid interfaces possibly sustains a surface tension gradient favorable for superspreading. In order to investigate the influence of the surface tension gradient on superspreading, some authors conducted experiments with sessile water drops spreading on polystyrene surfaces by dispensing small droplets of aqueous Silwet® L-77 solutions onto the water drops5,10. The different surface tension between the two coalescing drops generated a surface tension gradient along the sessile drop, and thus triggered its spreading. The study10 was restricted to very small trisiloxane surfactant concentrations (2.5×10-3 wt% and 7.5×10-4 wt%), at which the surface tension gradient equilibrated within 0.5 and 2 s. Further, only partial spreading of the sessile drops occurred, meaning that the trisiloxane concentration was enough to induce spreading but too low to induce superspreading. The limited temporal resolution of the video camera (30 fps) did not allow capturing the spreading dynamics in the initial stage just after the surfactant-laden droplet was dispensed on the sessile water drop.


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In this work, we took advantage of high-speed video imaging (45,000 fps) to examine the spreading dynamics of sessile water drops on polypropylene surfaces driven by a local addition of surfactant-laden droplets. We wanted to investigate experimentally the inception of the contact line movement in order to be able to validate one of the models (positive spreading coefficient vs. surface tension gradient) proposed in the literature. In fact, when a surfactant-laden droplet was gently dispensed on the apex of a sessile water drop, the established surface tension gradient did not cause a direct spreading of the sessile water drop. Initiated by the coalescence of two drops, a train of capillary waves propagated along the curved water/air interface of the sessile drop. At the same time, surfactant spread from regions of lower (apex of the sessile drop) to regions of higher (contact line of the sessile drop) surface tension along the interface. It was found that the surfactant influenced the propagation velocity of the capillary waves, which confirms earlier findings in24. Similarly as the surface tension gradient the capillary waves did neither initiate the spreading of the sessile drop. Spreading started only when surfactant molecules reached the contact line. Analysis of spreading beyond ~10 ms showed that the inhomogeneously mixed surfactant/water drop spread with a similar dynamics as a homogeneously mixed surfactant/water drop25.


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2. Experimental Section 2.1. Surfactants and Surfaces The surfactants used were the anionic surfactant sodium dodecyl sulfate (SDS, Acros Organics, purity ≥98%) and two nonionic trisiloxane surfactants, TSS10/2 and TSS6/3 (synthesized at Evonik Nutrition & Care GmbH, Germany, 100% undiluted). TSS10/2 and TSS6/3 (denoted as M(D′EnPmOH)M, with n=10, 6 and m=2, 3 on average, respectively) are commercially available as TEGOPREN® 5847 and TEGOPREN® 5840. Here, M represents the trimethylsiloxy group (CH3)3SiO1/2–, the term D′ stands for the –O1/2Si(CH3)(R)O1/2–, where R is obtained from a mixture of ethylene oxide and propylene oxide (R=–(CH2)3–O–(CH2–CH2–O–)n–(CH2–CH(CH3)–O–)m). Aqueous surfactant solutions were prepared using ultra-pure water (18.2 , Merck Millipore, Germany). The critical micelle concentration (CMC) is 8.3 × 10 − 3 /! for SDS26 and corresponds to a weight concentration of ~0.25 $% . The concentrations of the aqueous SDS solutions used were 0.5 − 1.5 $%. All TSS10/2 and TSS6/3 surfactant solutions had concentrations from 0.1 $% up to 10 wt%, which were well above CMC (~0.005 $%). Above CMC the nonsuperspreading TSS10/2 and SDS form spherical micelle aggregates in aqueous solutions, while the superspreading trisiloxane TSS6/3 forms bilayer aggregates in aqueous solutions25. The surfactant/water mixtures were hand shaken vigorously in order to disperse the surfactants. They were used within 24 hours after preparation. Thus, the influence of hydrolytic degradation of the surfactants could be neglected. The hydrophobic surfaces used for the spreading experiments were biaxially oriented polypropylene (PP) (FORCO OPPB AT-OPAL, 4P Folie Forchheim, Germany), with a surface energy of ~ 30 &/. Roughness measurements of PP surfaces at various points are performed with an atomic force microscope (AFM, MFP-3D, Asylum Research, Santa Barbara, USA). The average roughness '( is 200 ± 15 * for PP surfaces examined over an area of 50 × 50 +, . Pure


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water exhibits an advancing contact angle of ~ 114° and a receding contact angle of ~ 88° on PP surfaces. The contact angles were measured by the sessile drop technique with a Profile Analysis Tensiometer (PAT1, SINTERFACE Technologies, Berlin, Germany). 2.2. Spreading experiments The spreading of a sessile water drop in equilibrium on a PP surface was initiated by local addition of a surfactant-laden droplet on the drop apex. Firstly, a 10 + pipette with a dispensing accuracy of 0.01 + was used to deposit a water drop with a defined volume of 5 + on the PP surface. Then, a 0.2 − 0.5 + droplet of aqueous surfactant solutions was gently dispensed on the apex of the sessile drop. The controlled release of surfactant-laden droplets was achieved with a syringe connected with a home-made mechanical manipulator. The syringe needle with an outer diameter of 400 + was beforehand hydrophobized with 1,1,1,3,3,3-hexamethyldisilazane (Roth GmbH, Germany) in a desiccator at room temperature for 12 ℎ. This prevented the droplets from wetting the needle and allowed the syringe to generate droplets with controlled size. The surfactantladen droplet was approached to the sessile drop in a quasi-static way (~ 10 +/) to minimize the impact between the two drops. A high-speed video camera (FASTACAM SA-1, Photron Inc., USA) at a rate of 45,000 fps was used to capture the spreading process from the side, which allowed for detecting and tracking the capillary waves generated by the coalescence of the two drops. A cold backlight source with a diffuser was used for illumination. The experimental apparatus rested on a vibration-free optical table. The experiments were conducted at room temperature (22 °0 ) and relative ambient humidity of 35%. To check reproducibility, each experiment was repeated at least four times on fresh PP surfaces. 2.3. Image processing The instantaneous spreading contours of sessile water drops on PP surfaces were extracted by thresholding the captured images with a self-programmed MATLAB® (MathWorks Inc., USA)


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algorithm. With the pixel/length calibration, spreading radius and contact angle of the drops on PP surfaces were calculated.


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3. Results and discussion 3.1. Spreading of surfactant on the curved water/air interface of sessile drops Figure 1 shows the spreading profiles of a sessile water drop on a PP surface after a water or a surfactant-laden droplet is dispensed locally on the apex. In the first column of Fig. 1, when a water droplet was brought into contact with the sessile drop, interfacial waves (red arrow) were initiated at the neck region and propagated downward along the water/air interface to the bottom of the

sessile

drop.

Waves

with

wavelength

smaller

than

a

critical

wavelength,

12 = 234 ⁄(56 − 5 )7, are dominated by surface tension only and the effect of gravity can be neglected27–29. Here, is the surface tension of the drop and 7 is the acceleration due to gravity. 56 and 5 are the mass density of water and air. For the water/air interface, 12 is ~17 . This is much larger than the drop size. Therefore, the interfacial waves along the water/air interface are pure capillary waves. In the surfactant-free system, we found that the capillary waves reached the bottom of the sessile drop after ~2.2 . The contact line of the sessile drop remained pinned, as shown in the superimposed contours sequence (last row in Fig. 1). Only small oscillations of the contact angle of the sessile drop around its equilibrium value occurred. The capillary waves thus did not initiate the movement of the contact line. If the dispensed droplet contained surfactant, the transfer of surfactant to the large sessile drop generated a surface tension gradient between apex and bottom of the sessile drop. It was reported that due to the surface tension gradient the water drop started to spread directly, on the basis of experimental observations with a video camera recording at 30 fps, i.e. with a time resolution of ~33 5,10. Experimental results from our high-speed video imaging showed, however, that the contact line of the sessile drop did move, but only after a lag-time of ~1.6 , which approx. corresponded to the propagating time of capillary waves from drop apex to drop bottom in the system with surfactant. This lag-time was not observed in Refs.5,10 due to the limited time resolution. Our work revealed an initial stage in which the contact line was pinned even when a


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surface tension gradient was established by dispensing surfactant-laden drops containing spherical micelles (SDS, TSS10/2) and with bilayer aggregates (TSS6/3). Despite having a different chemical structure and solubility, the three surfactants initially lead to similar “distortions” in the spreading drop contours (see last three columns in Fig. 1).

Figure 1. Snapshots of drop contours during the spreading of sessile water drops on PP surfaces triggered by local dispensing of water or surfactant-laden droplets on top. The last row shows a superposition of all contours from 0 to 15.62 . Drop spreading models on solid surfaces can be grouped into two types, each having a different physics behind. The first type grounds on thermodynamics: a positive spreading coefficient leads a liquid to spread on a solid surface until the surface tension balance is restored. The second type requires the generation of a surface tension gradient that results from a non-uniform surfactant


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concentration at the liquid/air interface5. The surface tension at the drop contact line must be greater than that at the drop apex. We showed above that none of these two conditions is sufficient for initiating drop spreading. Figure 2 shows three possible surfactant coverage scenarios at the water/air interface of a sessile drop (height ℎ~1.5 ) before its contact line starts moving. When a dispensed surfactantladen droplet comes into contact with the sessile drop, a region of low surface tension on the drop apex is created and the transfer of surfactant to the sessile drop occurs (Fig. 2a). The coalescence of surfactant-laden droplet and sessile drop generates a capillary wave propagating along the water/air interface of the sessile drop. The surface tension at the contact line of the sessile drop is ~ 72 &/ and the surface tension of a trisiloxane-laden droplet dispensed at the apex of the sessile drop is ~ 22 &/25. Consequently, the resulting surface tension gradient ; between apex and bottom of the ~ 1.5 high drop is ~ 33 &/,. Upon deposition ( = 0) the contact line was still pinned, implying that the magnitude of ; was not sufficient to initiate drop spreading. The spreading coefficient of the drop on the PP surface ( = 30 − (72 + ), with unknown value of ) is negative. Figure 2b shows that during surfactant transfer to the sessile drop, at a critical time 2 of the order of 1 the water/air interface is covered by surfactant. The surfactant distribution is not uniform: the surface density is high close to the drop apex and low close to the drop bottom. The surface tension gradient over a small critical height 2 of the order of ~ 100 + at 2 will greatly increase to ~ 500 &/, , which can be strong enough to drive the drop to spread. The spreading coefficient at the contact line, however, is still negative in this scenario. Figure 2c shows the case where the water/air interface is fully covered by surfactant at ~1.6 . If the surfactant is uniformly distributed, the surface tension gradient along the water/air interface will become zero. The presence of surfactant at the contact line decreases the liquid surface tension, thus making the spreading coefficient positive. The unbalanced capillary force at the contact line will drive the drop to spread.


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The bulk diffusion coefficient of SDS is ~ 8 × 10−10 2 ⁄30 and of trisiloxane surfactants ~ 1 × 10 , ⁄ 31,32. Surface and bulk diffusion coefficients are assumed to be of the same order

of magnitude33. Therefore, the experimental lag-time of ~1.6 is too short for surfactants to reach the contact line by surface or bulk diffusion. This implies that another surfactant transfer mechanism is active at the water/air interface.

Figure 2. Schematic representation of surfactant coverage at the water/air interface of the sessile drop. (a) Upon contact surfactant transfer to the sessile drop occurs immediately, establishing a surface tension gradient ?. (b) The water/air interface is gradually covered by surfactant and at a critical time 2 ? is established over a critical height 2 . (c) The water/air interface is fully covered by surfactant, ? becomes zero and becomes positive. Cartoon is not to scale. Despite many studies of surfactant spreading on extended flat water layers or pools15,24,34,35, investigation of surfactant spreading along curved water surfaces like drops is limited. The surface tension gradient is assumed to be the driving force for the spreading of insoluble34 and soluble surfactants15 on water pools, as can also be shown for water or ethanol nanodrops spreading on water by MD simulations36. The distance @ traveled by the front of the spreading surfactant at time can be described by34:


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B =⁄, (EF as a function of spreading time in25 and present work. (a) TSS10/2-laden drops, (b) TSS6/3-laden drops. Only every 25th data point is plotted for clarity.

For validating the conclusions on the effect of interfacial surfactants/aggregates on spreading, we injected a surfactant-laden droplet at the bottom of a sessile water drop resting on a pierced PP surface. The needle was carefully prepared before injecting the surfactant via the hole in the substrate close to the drop base and care was taken that no surfactant contacted the drop interface during injection. The injected surfactant would establish a surface tension gradient of similar strength as in Fig. 2 along the solid/water - but not the water/air interface - this time. However, surfactant spreading along the solid/water interface is only diffusive and much slower than along the air/water interface. In fact, we did observe spreading of the sessile drop only up to ~1 W* after injection ( vs. curves not shown here), even if due to the slowness of the surfactant diffusion process we could not resolve the three spreading stages that are so clear in Fig. 4. The lag-time corresponds roughly to the time needed for a trisiloxane molecule with diffusion coefficient ~10 , ⁄ 31,32 to diffuse from the center of the drop base to the interface or to the contact line. This simple control experiment


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confirmed that the presence of surfactant in the bulk or at the solid/water interface did not initiate directly the spreading of the sessile drop, while the surfactant dispensed at the water/air interface acted more rapidly due to the faster surface spreading. This is consistent with the findings in Ref. 23 where the authors show that the diffusion of trisiloxanes from the bulk is not fast enough to maintain the surface tension gradients necessary for superspreading.


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4. Conclusions In this paper we have studied the surfactant-enhanced spreading of sessile water drops on hydrophobic polypropylene surfaces by high-speed video imaging. This was achieved by bringing surfactant-laden droplets in contact with the apex of the sessile water drop at the water/air interface or dispensing them into the liquid close to the drop base. In the first case, the surfactant droplet coalesced with the sessile water drop and the coalescence caused the generation of capillary waves that propagated along the water/air interface. These capillary waves did not trigger the spreading of the sessile drop, as we verified, because their propagation time does not match with the inception of contact line motion. We have found that the group velocity of the capillary waves was influenced by the nature of the surfactant: surfactants generating a stronger surface tension gradient made the capillary waves propagate faster. We have observed that the initially established surface tension gradient between the apex and the bottom of the sessile water drop did not cause the sessile drop to spread directly. From our data we must conclude that only when surfactant moved close to or at the contact line after a well-defined time the drop started spreading. This critical time depends on the surface spreading time of surfactant at the water/air interface and depends on viscosity, density and surface tension of the surfactant and the water drop. This finding was validated by injecting surfactant directly into the drop. We observed that the drop spread only after the time needed by surfactant molecules to diffuse from the bulk to the drop water/air interface or to the contact line. The two times – surface spreading of surfactant and bulk diffusion – differed by nearly 4 orders of magnitude in our experiments. Sessile drop spreading upon dispensing surfactant on its apex could be separated into three (four) stages characterized by different dynamics. The zeroth stage is when the contact line is still pinned and surfactant is spreading at the water/air interface towards the contact line. The first stage lasts for approx. 4 and is characterized by a slow dynamics common to all surfactants and similar for all


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concentrations investigated. The second stage lasts for approx. 8 and is characterized by a faster dynamics that is common to all surfactants and similar for all concentrations that were investigated. Despite being faster, the spreading is still not as fast as inertial spreading would be. At the beginning of the third stage at ~12 the spreading dynamics starts diverging and can be clearly correlated with one of the three types of surfactants used: conventional organic, non-superspreading trisiloxane, or superspreading trisiloxane. We compared the spreading dynamics of the sessile drops to the spreading dynamics of surfactant drops investigated in a previous work, with the only difference being that surfactant was homogeneously dispersed in those drops. All other experimental parameters were similar. We found that the spreading dynamics in the third stage, i.e. after ~12, were similar for both types of drops. From this comparison we could draw two important conclusions: first, during the very initial stages of spreading, superspreaders do not offer advantages compared with other surfactants; it takes a time of ~12 until superspreaders can develop their particular property. Second, once superspreading starts the past history of the droplet or the way the surfactant was dispended is not relevant any more. This is because the spreading of surfactant at the water/air interface is slower than the propagation of capillary waves, but faster than the movement of the contact line of the drop. Thus, as long as the surfactant concentration is high enough, there will always be enough surfactant molecules at the contact line that keep the spreading coefficient positive and the contact line moving.

Acknowledgements We acknowledge beneficial discussions with Cameron Tropea, Longquan Chen, Marcus Lopes, and Stephen Garoff. This research was supported by the German Research Foundation (DFG) within the Cluster of Excellence 259 “Smart Interfaces—Understanding and Designing Fluid Boundaries”.


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Surfactant-Enhanced Spreading of Sessile Water Drops on

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