Droplet Shape and Wetting Behavior under the Influence of Cyclically

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Biological and Environmental Phenomena at the Interface

Droplet Shape and Wetting Behavior under Influence of Cyclically Changing Humidity. Nikolay Kubochkin, and Natalia A. Ivanova Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00159 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Droplet Shape and Wetting Behavior under Influence of Cyclically Changing Humidity N.S. Kubochkin, N.A. Ivanova* Photonics and Microfluidics Lab, Tyumen State University, Tyumen, Volodarskogo 6, 625003, Russia * Corresponding author: [email protected]

Abstract Relative humidity plays a crucial role in wetting and spreading phenomena by affecting the evaporation rate, evaporation modes and the spreading dynamics via precursor film formation, surface modification and surface tension alteration. We examined the effect of the periodically varied RH between low (20%) and high (85%) levels on the wetting of the droplet of non-hygroscopic (pure surfactants) and hygroscopic (ethylene glycol, glycerol) liquids on a hydrophobic surface. It was revealed that the changing RH induces two modes of the transition between the wetting states of the droplet: with hysteresis and without hysteresis. Droplets of both non-hygroscopic and hygroscopic liquids exhibit the shape hysteresis during the first cycle: (i) droplets of surfactants irreversibly spread saving an initial volume; (ii) ethylene glycol and glycerol droplets irreversibly absorb the moisture increasing the volume and the base diameter. Further, cyclically changing RH results in the droplet breathing effect, i.e., the non-hysteresis transition of the droplet shape between two wetting states corresponding to the minimum and maximum RH levels. In case of the glycerol droplet for three cycles of the RH variation, the volume hysteresis (the droplet volume increases in each cycle) was observed. This is determined by the moisture absorption due to high hygroscopicity of glycerol. We also revealed that for all liquids studied, the droplet spreading at each increase in RH started at reaching the RH threshold level.

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Introduction Wetting phenomena take place in every process in nature where solid meets liquid, from tiny droplets of dew on leaves in cold mornings to tear films protecting eyes. In a number of industrial and manufacturing processes such as coating and painting,1 ink-jet printing,23

cosmetics producing,4-5 agriculture,3,6-8 and many others, wetting and spreading dynamics

plays a crucial role. All these processes generally occur at continuously changing relative humidity (RH), which however is most often considered a constant,7 and in some cases RH is not taken into account at all.9 In the meanwhile, humidity of the surrounding can drastically alter droplet dynamics. An obvious example of how humidity affects a liquid droplet behavior is the evaporation process. Evaporation rate, evaporation regime and, consequently, the droplet lifetime strongly depend on what the surrounding atmosphere is and on the saturation extent of such atmosphere.10 Bourges-Monnier and Shanahan11 showed that a contact angle of water and n-decane droplets on hydrophobic and hydrophilic surfaces preserves when the surrounding atmosphere is saturated with vapour of the same liquid as the droplet. The humidity influence appears to be a crucial factor for the understanding of nanofluids wetting and spreading. Chhasatia et al.11 investigated the RH influence on spreading and evaporation of inkjet-printed colloidal droplets. They found out that the final wetting perimeter of a colloidal droplet and the particle deposition morphology are in dependence of the RH of the surrounding air and that the droplet contact angle corresponding to the droplet pinning is higher for higher evaporation rates (i.e. the low humidity). Similar results were obtained by Bou-Zeid et al.12 who showed that the RH dramatically affects a contact line motion during a blood droplet spreading. Since blood includes colloids (red blood cells, white blood cells and platelets), RH affecting the evaporation rate can serve as an accelerator or a retarder of the contact line pinning. Another way of humidity influence on spreading dynamics is the formation of a precursor liquid film due to the adsorption of moisture on a solid near a droplet contact line.13 In this case, three effects such as the disjoining pressure in the precursor film, Marangoni flows in the droplet-film transition zone, and surface energy misbalance near the droplet contact line, come into play, and they may significantly contribute to the spreading process. Wang et al.14 revealed that a water precursor film, which is formed when increasing the RH, enhances the spreading of ionic liquids over mica surface. They


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observed that the equilibrium contact angle takes on a minimum value at RH = 70%, at which the surface is fully covered with water molecules as showed by AFM studies. Tavakoli and Kavehpour15 obtained curious results: a water droplet deposited onto a cooled hydrophobic surface, spreads contrary to expectations – a cold-induced surface tension increase should shrink the droplet. A reason for the spreading is a precursor film appearance near the droplet due to condensation of moisture from the atmosphere onto the surface. The researchers also investigated the role of the initial contact angle in the cold-induced spreading by distorting of the droplet shape and find out that depinning reveals itself firstly in the droplet side with a higher contact angle. Bou-Zeid et al.16 studied a humidity influence on wetting behavior of water-glycerol mixtures of various concentrations. The researchers found out that a power exponent increases linearly with the RH and proposed a modified power law where RH is taken into account by introducing an empirical equation for the power exponent. The disjoining pressure acting in the adsorbed precursor film is considered as one of possible mechanisms for enhanced spreading. Tiberg and Cazabat17-18 studied the role of the precursor films in wetting of pure trisiloxane surfactants. According to the researchers, RH affects the spreading rate albeit does not change the precursor film form. The researchers proposed that the enhanced spreading at high humidity values is probably governed by the interaction between atmospheric water and ethylene oxide parts of surfactants investigated. Interesting results on the contact angle relaxation in case of the air bubble attached to the solid in the water surrounding were obtained by Drelich.19 The change of the contact angle, as reported, is occurring due to the water droplets condensation and their coalescence with the bulk water in the close vicinity to the three phase contact line. Humidity of the surrounding air is considered as one of the key factors in the famous superspreading phenomena.3 Zhu et al.20 showed that the superspreading effect cannot be observed in dry atmosphere. A thin precursor water film at the droplet edge causes the water-surfactant solution surface tension gradient (Marangoni forces), which may appear to be the driving forces for droplet spreading. Although there are a few works devoted to the humidity influence on wetting and spreading processes, all of them pay attention to processes occurring at different certain humidity levels and do not consider dynamic cases when RH does not preserve throughout


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the experiment. In our previous study,21 we reported on the influence of continuously increasing RH on pure surfactants spreading over hydrophobic substrates. We showed that the RH increase stimulates a droplet being in the quasi-steady state at some constant RH, to spread over hydrophobic surface. Moreover, a critical value of the RH has to be reached to initiate further contact line motion. We have found that there is the only paper devoted to investigation of the droplets behavior at a changed humidity. Besides, the question now arises of the droplets wetting behavior at a variable humidity in the most general case, which includes both increasing and decreasing humidity cases. The answer, for example, can be of great importance for the effective pesticides applications in agriculture because the weather conditions and, particularly, the changeful humidity affect to a great extent the longevity of the pesticide droplets on foliage and the coverage of the plant leaves.3,6-8 A great potential is also expected for many droplet-based applications11,22 where a reversible change of the RH can be considered as a controlling tool for the droplet shaping. In this paper, we explore the influence of periodically changing RH of air in between 20 and 85% on the droplet wetting behavior and its reshaping on a hydrophobic surface by using non-hygroscopic (Silwet L-77 and Triton X-100 surfactants) and hygroscopic (ethylene glycol, glycerol) liquids. Materials and Methods The substrates used were silicon wafer plates hydrophobized with poly[4,5-difluoro-2,2bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (purchased from Sigma-Aldrich) also known as PTFE AF (Teflon AF). Teflon polymer 0.5 g was dissolved in 200 ml of Fluorinert F75 solvent. The silicon wafer plates (5 cm2) were cleaned in acetone (30 min of ultrasonication), then were rinsed in distilled water and dried. The cleaned silicon wafers were stored in a Petri dish. The solution of Teflon AF was deposited on each plate. The Petri dish was covered to avoid settling dust particles on wafer surfaces covered with Teflon AF. The plates were left overnight to evaporate the solvent. Commercial nonionic organosilicon surfactant Silwet L-77 (Momentive, Germany), nonionic hydrocarbon surfactant Triton X-100, ethylene glycol and glycerol (Sigma Aldrich) were under the investigation. All liquids were used without any further purification.


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The experiments were performed in a hermetically sealed chamber at 25±2°C. Temperature and RH during the experiments were controlled with RH-meter (CEM DT25) mounted above the top of the droplet and measured with 2% accuracy. A droplet of 36 µl was deposited onto the substrate with a pipette and left in the chamber open until reaching a quasi-equilibrium state when variations of the diameter 𝐷 and the advancing contact angle 𝜃 are negligible. This initial (an equilibration) stage lasted for 300 s to provide the quasi-steady state for all liquids used at initial RHmin = 20%. After the equilibrium, the RH varied periodically. At the beginning of each cycle, the RH was equal to RHmin = 20 ± 2%. Then, convolved tissues saturated with distilled water (14 ml) were placed into the chamber near the substrate. RH started to increase immediately due to the diffusion of water vapour and reached RHmax = 85 ± 2% at the end of the half-period of the cycle (900 s). After that, the chamber was opened, and the tissues were removed. Residual water was gently sopped up with the cloth to make sure that the RH diminished to the initial value RHmin. The whole cycle lasted for 1800 s. Three cycles were applied, Fig. 1. In Fig.1, RH (𝑡) fits well with logarithm within the half-periods, hence, for both increasing and decreasing stages a periodic piecewise function can be the following

{

11.7𝑙𝑛(𝑡) + 8.6, when RH is increasing RH(𝑡) = ―11.3𝑙𝑛(𝑡) + 95.2,when RH is decreasing

As can be seen, the rates of rising and plunging of RH can be considered equal. The side view sequence of droplet profiles was captured using a CCD camera and processed using home-made software to obtain droplet diameter, contact angle and volume as time-dependent functions. Each test was repeated at least five times to assure reproducibility. For further analysis, reduced diameter 𝐷 ∗ (𝑡) = 𝐷(𝑡) 𝐷0, contact angle 𝜃 ∗ (𝑡) = 𝜃(𝑡) 𝜃0, and volume 𝑉 ∗ (𝑡) = 𝑉(𝑡) 𝑉0 were plotted, where 𝐷0, 𝜃0,𝑉0 are diameter, advancing contact angle and volume, respectively, immediately after the droplet deposition.


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Spreading of pure surfactants In case of Silwet L-77 and Triton X-100, during the equilibration stage when the RH is essentially constant, a droplet spreads until reaching a quasi-equilibrium state. After placing watered tissues, RH rises (the first-half period, Fig. 1), 𝐷 ∗ (𝑡), 𝜃 ∗ (𝑡) and 𝑉 ∗ (𝑡) are unchanged until RH reaches the threshold value RH*, and the droplet spreads again,21 Fig. 2(b-d). As is known, water from the surrounding air is adsorbed even on highly hydrophobic surfaces as a thin molecular film.23-25 As a result, a spherical-cup shape of the droplet near the edge is distorted due to a film-droplet transition zone forming with the RH rise. Thus, a number of factors can be in charge of further droplet spreading: the unbalanced surface tensions on the triple line due to water vapour adsorption,21 Marangoni forces due to surface tension gradients and disjoining pressure effects due to adsorbed layer thickening.16,24 Moreover, as is seen in Fig. 2 (a,d), the droplet volume during the first halfperiod increases by ~ 10%, that implies that water is adsorbed not only onto the substrate, near the droplet edge but also onto the droplet surface that makes such effect more complex.14 It is worth noticing that the increase in the droplet base diameter and decrease in the droplet contact angle in saturated with water vapour atmosphere13were observed by Boinovich et. al.13 Fig. 2(a) shows that when the RH decreases (the second half-period, Fig. 1), 𝐷 ∗ (𝑡) and 𝑉 ∗ (𝑡) plunge linearly until attaining quasi-equilibrium values and keep their values until the end of this cycle, whereas receding contact angle 𝜃 ∗ (𝑡) remains unchanged as the RH is decreasing. Thus, the second half-period can be conditionally subdivided into two substages: (I) a contraction of the droplet stipulated by evaporation of water layer adsorbed; (II) a quasi-stationary state when𝐷 ∗ (𝑡), 𝜃 ∗ (𝑡) and 𝑉 ∗ (𝑡) are preserving. It is interesting to note that behavior at the substage (I), Fig. 3(a), coincides with the second mode of simultaneously spreading and evaporation of surfactant solutions26 and can be also compared with the evaporation mode at the constant contact angle described by Picknett and Bexon.27 Fig. 3(b) represents droplet profiles during the first cycle. Notably, at the end of the first cycle, 𝑉 ∗ (𝑡) returns to its initial value, Fig. 2(d), but the droplet, in general, does not take its initial shape, Fig. 3(b), i.e. a hysteresis of droplet base diameter, ∆𝐷 ∗ (𝑡), and contact


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angle, ∆𝜃 ∗ (𝑡), occurs, Fig. 2(b,c). The latter can be explained by the spreading process, which is caused by the RH increasing as is shown in our previous work.21 Considering the second cycle of the RH increasing/decreasing, one can see that 𝐷 ∗ (𝑡) does not change when the RH starts to increase, while 𝜃 ∗ (𝑡) and 𝑉 ∗ (𝑡) start to increase instantaneously, Fig. 2(a). The latter fact allows us to conclude that water adsorbed onto the droplet apex forms a crescent-like layer and, hence, makes contact angle larger. After 𝜃 ∗ (𝑡) reaches maximum value, a triple contact line starts to move again. As can be seen in Fig. 2(b), at this moment the RH is around threshold value RH* again. Probably, due to a small 𝐷 ∗ (𝑡) increase ~ 5%, such spreading can be explained by the crescent-like layer formed which is slipping along the droplet surface. This assumption requires further investigations. The droplet contraction during the RH decreasing (the half-period of the second cycle) is similar to that of the first cycle. The droplet behavior during the third cycle repeats its behavior during the second cycle. It is worth noting that in the second and third cycles the hysteresis of ∆𝐷 ∗ (𝑡) and ∆𝜃 ∗ (𝑡) is unsubstantial, hence, the droplet spreads during the first increase in RH, and further variation of RH leads to a non-hysteresis switching between two wetting states. We denote this process as a breathing droplet effect hereafter. Note again that aforementioned behavior, Fig. 2, is intrinsic for water-soluble pure Silwet L-77 as well as pure Triton X-100. Spreading of pure hygroscopic liquids Ethylene glycol and glycerol are hygroscopic liquids; hence, droplets of such liquids tend to increase their volumes involuntary due to absorption of water from surrounding atmosphere. Thus, wetting behavior of those liquids is expected to be different from that of pure surfactants: two competitive processes – evaporation and absorption of water – will affect the droplet behavior. Glycerol. The wetting behavior of pure glycerol droplet on PTFE at the cyclically changing RH is shown in Fig. 4. During the equilibration stage, when RH = 20% (around 5 minutes), the droplet does not attain the equilibrium state: an insignificant increase in 𝑉 ∗ (𝑡), 𝜃 ∗ (𝑡) and 𝐷(𝑡) is still observed, which is related to the high hygroscopicity and viscosity of glycerol. When RH increases, the glycerol droplet starts to intensively absorb moisture and as a consequence, to expand its wetting perimeter. This fact might be proved


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with the increase in 𝐷 ∗ (𝑡) and 𝑉 ∗ (𝑡) and the decrease in 𝜃 ∗ (𝑡), Fig. 4(a). As is seen in Fig. 4(b-d), the triple contact line movement, similar to surfactants, starts only when the RH* reaches (RH ∗ ≈ 75%). However, in contrast with pure surfactants, the values of 𝜃 ∗ (𝑡), 𝑉 ∗ (𝑡) and 𝐷 ∗ (𝑡) for glycerol decrease during the second half-period. Despite the decrease in 𝑉 ∗ (𝑡) due to the evaporation, a droplet does not reach the initial shape and the volume, which it was deposited with. Spreading behavior in next two cycles of the RH increasing/decreasing is the same as the first one: during every cycle one can observe volume increase due to surrounding water accumulation and contact angle diminishing, Fig. 4. Interestingly, that values of ∆𝑉 ∗ , ∆𝐷 ∗ и 𝑅𝐻 ∗ are decreasing for every cycle (∆𝑉1∗ < ∆𝑉2∗ < ∆𝑉3∗ and ∆𝐷1∗ < ∆𝐷2∗ < ∆𝐷3∗ ), Fig. 4(b,d), which allows us to conclude that pure glycerol accumulating moisture from the atmosphere until the water-saturated state is reached. It is known that being in the ambient, glycerol is able to absorb up to 50% of water in comparison with its initial volume.28 During three whole cycles, glycerol droplet diameter and volume are up by ~20% and ~50%, respectively. Apparently, an increase in a number of cycles will lead to reaching the watersaturated state. Fig. 4(a-c) reveals that when RH is between RHmin and RH ∗ , a slight increase in 𝜃 ∗ (𝑡) takes place due to water adsorption onto the droplet surface. Fig. 3(c) shows the schematic representation of droplet profiles throughout this time span (increasing of humidity from RHmin up to RH ∗ ). It is clearly seen that water adsorbs predominantly on the droplet apex forming the crescent-shaped layer (as in case of Silwet L-77) and, hence, leading to the 𝜃 ∗ (𝑡) increase. Ethylene glycol. Fig. 5 shows the influence of the cyclically changing RH on wetting behaviour of an ethylene glycol droplet. As all liquids investigated, ethylene glycol has no immediate response on the RH changing, i.e. the critical RH ∗ requires for the contact line motion. Alike glycerol, the ethylene glycol droplet reshaping is governed solely by the balance of moisture evaporation/sorption processes. However, in contrast with glycerol, ∆𝜃 ∗ , ∆𝐷 ∗ and ∆𝑉 ∗ take place only with the first humidity rise, Fig. 5(a-d). After that, only interplay between evaporation and absorption mechanisms occurs – the droplet responses to the cyclically changing RH by the breathing droplet effect. We assume it is due to reaching the equilibrium water-saturated state within the first half-period, as ethylene


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glycol is able to absorb up to 20% of surround moisture. Note also that the contraction stages in each cycle correspond to the mixed evaporation regime26 and are similar to the last stage of the evaporation of pure liquids. We observed that a tiny droplet-consisted liquid film propagates from the droplet edge when RH is increasing, Fig. 6(a). The question of such film formation is obscure. Probably, it could be explained the following way: water, which is contained in ethylene glycol initially, begins to evaporate and adsorb onto the substrate near the droplet edge. This process leads to the formation of nucleation centers, which enhance the adsorption of water. When RH is increasing, water vapour “sits” onto the nucleation centers and forms tiny droplets (the average diameter is 20μm) which, in turn, result in the water film formation. Note that such film was observed only near ethylene glycol droplets at increasing RH on both hydrophobic (PTFE AF) and hydrophilic (a clean glass) substrates. To show the film dynamics, an ethylene glycol droplet was placed on the PTFE substrate in a Petri dish, then water-saturated tissues were added/removed to increase/decrease RH. Images of the film evolution when RH varying were taken with the optical microscope (AxioZoom v16, Carl Zeiss, Germany). Fig. 6(b) shows that the water film formed at high RH level by the tiny droplets assembling recedes due to their evaporation when RH decreases. Fig. 6(c) shows the top view of the droplet and the water film in the saturated atmosphere. The average width of the film between the droplet edge to the dry surface is 530 µm. Conclusions We examined the effect of the cyclically changing RH between two levels of 20 and 85% on wetting behaviour of droplets of pure surfactants and hygroscopic liquids on a hydrophobic surface.

It was found out that the cyclically changing RH induces the

transition between the wetting states, i.e. the cyclically changing the droplet shape. Two modes of the wetting transition with hysteresis and without hysteresis were identified. In case of pure surfactants and ethylene glycol, the transition between two wetting states occurs with hysteresis in the first cycle. Droplets do not take on the initial shapes when RH returns to the initial low value: (i) for droplets of pure surfactants an irreversible spreading with saving an initial volume takes place; (ii) for ethylene glycol droplets the moisture absorption with increase in the droplet volume and the base diameter occurs. Further,


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cyclically changing RH results in the droplet breathing effect, i.e., the non-hysteresis transition of the droplet shape between two wetting states corresponding to the minimum and maximum RH levels. In case of a glycerol droplet for three cycles of RH variation, the volume hysteresis (droplet volume increases in each cycle) was observed that is determined by the moisture absorption due to the high hygroscopicity of glycerol. Note that for all liquids studied, at each increase in the RH the change in the droplet shape (the wetting transition) started when the RH reached a threshold level, which was reported in our previous study.21 We anticipate that the effect of the reversible change in the droplet shape through the RH variation (the droplet breathing effect) has a great potential for optofluidics and digital microfluidics circuits as a method for a droplet shape control. Acknowledgements The research was supported by the Russian Foundation for Basic Research (Grant No. 1831-00231) and the Ministry of Education and Science of the Russian Federation (Grant No. 3.4744.2017/6.7).


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References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

(14) (15) (16) (17) (18)

Adams J. W. in Sharma M.K., editor, Surface phenomena and additives in water-based coatings and printing technology, New York, Plenum Press, 1991. Bonn D.; Eggers J.; Indekeu J.; Meunier J.; Rolley E. Wetting and spreading. Rev. Mod. Phys. 2009, 81, 739-805. Venzmer J. Superspreading—20 years of physicochemical research. Curr. Opin. Colloid Interface Sci, 2011, 16 (4), 335-343. Lodge R. A.; Bhushan B.; J Appl Polym Sci. Wetting properties of human hair by means of dynamic contact angle measurement. 2006, 102(6), 5255-5265. Trybala A.; Bureiko A.; Kovalchuk N.; Arjmandi-Tash O.; Liu Z.; Starov V. Wetting properties of cosmetic polymeric solutions on hair tresses. COLCOM, 2015, 9, 12. Ramsey R. J. L.; Stephenson G. R.; Hall J. C. A review of the effects of humidity, humectants, and surfactant composition on the absorption and efficacy of highly watersoluble herbicides. Pesticide Biochem. Physiol. 2005, 82, 162-175. Tang X.; Dong J.; Li X. A comparison of spreading behaviours of Silwet L – 77 on dry and wet lotus leaves. J Colloid Interface Sci. 2008; 325, 223-227. Yoon S. M.; Ko T.-J.; Oh K. H.; Nahm S.; Moon M.-W. Water wetting observation on a superhydrophobic hairy plant leaf using environmental scanning electron microscopy. Appl. Microsc. 2016, 46(4), 201-205. Zhou, Z.; Cao, C.; Cao, L.; Zheng, Li; Xu, J.; Li, F.; Huang, Q. Effect of surfactant concentration on the evaporation of droplets on cotton (Gossypium hirsutum L.) leaves. Colloids Surf B Biointerfaces. 2018, 167, 206-212. Bourge`s-Monnier C.; Shanahan M. E. R. Influence of evaporation on contact angle. Langmuir. 1995, 11, 2820-2829. Chhasatia, V. H.; Joshi, A. S.; Sun, Y. Effect of relative humidity on contact angle and particle deposition morphology of an evaporating colloidal drop. Appl. Phys. Lett. 2010, 97, No. 231909. Bou-zeid W.; Brutin D. Effect of relative humidity on the spreading dynamics of sessile drops of blood. Colloids Surf A Phys Chem Aspects. 2014, 456, 273-285. Boinovich L.; Emelyanenko A. Characterizing the physicochemical processes at the interface through evolution of the axisymmetric droplet shape parameters. In: Advances in Contact Angle, Wettability and Adhesion. Vol. 3, Mittal KL (editor)., Beverly, MA: Scrivener Publishing LLC; 2018. p. 99-129. Wang, Z.; Shi, F.; Zhao, C. Humidity-accelerated spreading of ionic liquids on a mica surface. RSC Adv. 2017, 7, 42718-42724. Tavakoli, F.; Kavehpour, P. Cold-induced spreading of water drops on hydrophobic surfaces. Langmuir. 2015, 31, 2120-2126. Bou-zeid W.; Brutin D. Beyond Tanner's law: role of contact line evaporation on the spreading of viscous droplet. Interf. Phen. Heat Transfer. 2015, 3(3), 221-229. Tiberg F.; Cazabat A. M.; Spreading of thin films of ordered non-ionic surfactants. Origin of the stepped shape of the spreading precursor. Langmuir. 1994, 10, 2301-2306. Tiberg F.; Cazabat A. M.; Self-assembly and spreading of non-ionic trisiloxane surfactants. Europhys. Lett. 1994, 25(3), 205-210.


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(19) Drelich, J. Instability of the Three-Phase Contact Region and Its Effect on Contact Angle Relaxation. J. Adhes. Sci. Technol. 1999, 13(12), 1437-1455. (20) Zhu S.; Miller W. G.; Scriven L. E.; Davis H. T. Superspreading of water – silicone surfactant on hydrophobic surfaces, Colloids Surf A: Phys Chem Aspects. 1994, 90, 63-78. (21) Ivanova N. A.; Kubochkin N. S.; Starov V. M. Wetting of hydrophobic substrates by pure surfactants at continuously increasing humidity. Colloids Surf A Physicochem Eng Asp. 2016, 519, 71-77. (22) Dong L.; Agarwal A.K.; Beebe D.J.; Jiang H. Adaptive liquid microlenses activated by stimuli-responsive hydrogels. Nature, 2006, 443, 551-554. (23) Liebe H. J.; Wolfe V. L. Test of wall coatings for controlled moist air experiments, Rev. Sci. Instrum. 1984, 44, 1702-1705. (24) Starov V. M. Surface forces action in a vicinity of three phase contact line and other current problems in kinetics of wetting and spreading. Adv. Colloid Interface Sci. 2010, 161, 139-152. (25) Boinovich L.; Emelyanenko A. Wetting and surface forces. Adv. Colloid Interface Sci. 2011, 165 (2), 60-69. (26) Semenov S.; Trybala A.; Rubio R. G.; Kovalchuk N.; Starov V.; Velarde M. G. Simultaneous spreading and evaporation: recent developments. Adv. Colloid Interface. 2014, 206, 382-398. (27) Picknett R. G.; Bexon R. J. The Evaporation of Sessile or Pendant Drops in Still Air. Colloid Interface Sci. 1977, 61(2), 336-350. (28) Physical Properties of Glycerine and Its Solutions, Glycerine Producers' Association, New York, 1963.


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List of Figures Figure 1. Relative humidity pattern applied: cycles (each cycle lasts for 1800 s) are divided by dotted lines. At the beginning of each cycle RH = 20%, the maximum RH reaches 85%. Figure 2. Wetting behavior of Silwet L-77 droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph shows the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □; (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye. Figure 3. Schematic representation of wetting dynamics of Silwet L-77 and glycerol droplets on PTFE. (a) Silwet L-77 droplet profiles during the second-half period of the 1st cycle (contraction regime, RH decreases). (b) Silwet L-77 droplet profiles during the 1st cycle of RH varying (300 s – RH=20%, 1200s – RH=85% and 2100 s – RH=20%). (c) A crescent-like layer formation on the glycerol droplet throughout RH increasing. Figure 4. Wetting behavior of a glycerol droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph shows the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □. (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye. Figure 5. Wetting behavior of an ethylene glycol droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph show the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □. (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye. Figure 6. Water film near the contact line of the ethylene glycol droplet: (a) a side view of a water film formation at continuously rising RH, (b) a top view of the water film receding at continuously diminishing RH (a part of the droplet edge is shown), (c) a top view of the ethylene glycol droplet in saturated atmosphere in Petri dish. The film area and the droplet area are contoured by the white and the black dotted lines, respectively.


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Figure 1. Relative humidity pattern applied: cycles (each cycle lasts for 1800 s) are divided by dotted lines. At the beginning of each cycle RH = 20%, the maximum RH reaches 85%.


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Langmuir

Figure 2. Wetting behavior of Silwet L-77 droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph shows the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □; (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye.


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Figure 3. Schematic representation of wetting dynamics of Silwet L-77 and glycerol droplets on PTFE. (a) Silwet L-77 droplet profiles during the second-half period of the 1st cycle (contraction regime, RH decreases). (b) Silwet L-77 droplet profiles during the 1st cycle of RH varying (300 s – RH=20%, 1200s – RH=85% and 2100 s – RH=20%). (c) A crescent-like layer formation on the glycerol droplet throughout RH increasing.


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Langmuir

Figure 4. Wetting behavior of a glycerol droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph shows the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □. (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye.


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Figure 5. Wetting behavior of an ethylene glycol droplet on PTFE. (a) The top graph shows RH pattern applied, the vertical lines are to distinguish the humidity cycles; the bottom graph show the time evolution of the reduced diameter - ○, the reduced contact angle - △, and the reduced volume - □. (b-d) Representation of reduced diameter, contact angle and volume, respectively, throughout the humidity cycles. The cycles are numbered 1, 2, 3. Arrows are guide to eye.


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Langmuir

Figure 6. Water film observation near the contact line of the ethylene glycol droplet: (a) a side view of a water film formation at continuously rising RH, (b) a top view of the water film receding at continuously diminishing RH (a part of the droplet edge is shown), (c) a top view of the ethylene glycol droplet in saturated atmosphere in Petri dish. The film area and the droplet area are contoured by the white and the black dotted lines, respectively.


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Droplet Shape and Wetting Behavior under the Influence of Cyclically

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