Interfacial Crystallization Within Liquid Marbles

Article Interfacial Crystallization within Liquid Marbles

Edward Bormashenko 1,* , Pritam Kumar Roy 1 , Shraga Shoval 2 and Irina Legchenkova 1

1 Chemical Engineering Department, Engineering Faculty, Ariel University, P.O. Box 3, Ariel 407000, Israel; [email protected] (P.K.R.); [email protected] (I.L.) 2 Industrial Engineering and Management Department, Engineering Faculty, Ariel University, P.O. Box 3, Ariel 40700, Israel; [email protected] * Correspondence: [email protected]

Received: 25 September 2020; Accepted: 12 October 2020; Published: 15 October 2020

Abstract: We report interfacial crystallization in the droplets of saline solutions placed on superhydrophobic surfaces and liquid marbles filled with the saline. Evaporation of saline droplets deposited on superhydrophobic surface resulted in the formation of cup-shaped millimeter-scaled residues. The formation of the cup-like deposit is reasonably explained within the framework of the theory of the coffee-stain effect, namely, the rate of heterogeneous crystallization along the contact line of the droplet is significantly higher than in the droplet bulk. Crystallization within evaporated saline marbles coated with lycopodium particles depends strongly on the evaporation rate. Rapidly evaporated saline marbles yielded dented shells built of a mixture of colloidal particles and NaCl crystals. We relate the formation of these shells to the interfacial crystallization promoted by hydrophobic particles coating the marbles, accompanied with the upward convection flows supplying the saline to the particles, serving as the centers of interfacial crystallization. Convective flows prevail over the diffusion mass transport for the saline marbles heated from below.

Keywords: interfacial crystallization; liquid marble; hydrophobic particle; superhydrophobic surface; coﬀee-stain eﬀect

1. Introduction Interfacial crystallization has attracted much interest of scientists in the field of condensed matter in the past decades [1–6]. Interfacial crystallization enables intelligent control of the morphology of crystals, nucleated in the vicinity of solid/liquid [1–3] or liquid/liquid interfaces [4–6], and some unique crystalline structures have been obtained, including nano-whiskers [4] and hybrid shish–kebab and shish–calabash structures [1,2]. Interfacial crystallization allowed manufacturing of energetic nano-crystals [3] and porous particles [5]. The complicated physico-chemical mechanisms of the interfacial crystallization were addressed in [7–10]. We demonstrate in our communication that evaporated liquid marbles may be used for the interface-driven crystallization. Liquid marbles, introduced in the pioneering investigations of Quéré et al., are non-stick droplets coated with nano- or micron-sized particles, which are usually hydrophobic [11–15]. Liquid marbles are not hermetically coated by the solid particles. The respirability of liquid marbles makes them suitable for the cultivation of microorganisms and cells [16–20]. Liquid marbles may also be used as mini-reactors and bio-reactors [21,22]. Kinetics of the evaporation of liquid marbles was addressed in [23–26]. Use of liquid marbles for non-traditional computing was reported recently [27–30]. Liquid marbles may be actuated by UV and IR light and Marangoni flows [31–33], and demonstrate obvious potential for micro-fluidics applications [34,35]. Actually, liquid marbles opened new horizons for the researchers studying the liquid state of the matter. We report here the use of liquid marbles for controlled interfacial crystallization.

Condens. Matter 2020, 5, 62; doi:10.3390/condmat5040062 www.mdpi.com/journal/condensedmatter Condens. Matter 2018, 3, x FOR PEER REVIEW 2 of 11

2. Results Condens. Matter 2020, 5, 62 2 of 11

We carried2. Results out comparative experiments in which droplets of aqueous NaCl solutions were evaporated underWe conditions carried out comparativeof superhydrophobicity experiments in which when droplets (i) ofnaked aqueous aqueous NaCl solutions NaCl were solution droplets were evaporatedevaporated when under placed conditions on ofsuperhydrophobic superhydrophobicity when surfaces, (i) naked aqueous and NaClwhen solution (ii) dropletsmarbles filled with were evaporated when placed on superhydrophobic surfaces, and when (ii) marbles ﬁlled with aqueous NaClaqueous solution NaCl and solution coated and coated with with lycopodi lycopodiumum powder powder (a strongly (a strongly hydrophobic hydrophobic yellow-tan yellow-tan dust-like powder,dust-like consisting powder, consisting of the ofdry the dryspores spores of of clubmoss plants) plants) were evaporated were ev whenaporated placed onwhen placed on superhydrophobicsuperhydrophobic substrates, substrates, as depicted as depicted in in Figure Figure1. 1.

Figure 1. The evaporationFigure 1. The evaporation processes processes of aof naked a naked aqueous aqueous NaCl NaCl solution solution drop (a) anddrop a liquid (a) and marble a liquid marble filled with aqueous NaCl solution (b) placed on a superhydrophobic surface are depicted. filled with aqueous NaCl solution (b) placed on a superhydrophobic surface are depicted. When naked saline droplets are evaporated on superhydrophobic surfaces, very different scenarios of evaporation are possible, namely, the evaporation may take place under the pinned triple line, When nakedand under saline the slidingdroplets triple line,are asevaporated discussed in [36 on]. Thesuperhydrophobic evaporation may be accompanied surfaces, by very different scenarios of evaporationthe Cassie–Wenzel are wettingpossible, transition namely, and may the take evaporation place within themay Cassie take wetting place regime under [36, 37the]. pinned triple line, and underWe the studied sliding evaporation triple of line, the naked as discusse saline dropletsd in under [36]. ambient The evaporation conditions (t = 25 may◦C) and be also accompanied by under the constant temperature of the substrate of t = 70 ◦C. The characteristic time scales of evaporation the Cassie–Wenzelwere τev wetting 24 min fortransition the “slow” andandτev may 4 min take for the place rapid evaporationwithin the processes. Cassie wetting regime [36,37]. We studied evaporationBoth of temperature of the naked regimes suppliedsaline droplets very similar under results, namely, ambient evaporation conditions occurred ( undert = 25 °C) and also the slight variation in the apparent contact angle, i.e., 142◦ < θapp < 149◦, as shown in Figure2, under the constantwhereas thetemperature contact line was of strongly the pinnedsubstrate as demonstrated of t = in70 Movies °C. S1The and characteristic S2. The low contact time scales of evaporation wereangle hysteresis𝜏 ≅ 24 and 𝑚𝑖𝑛 low slidingor f the angles “slow” inherent and for the𝜏 pristine≅ 4 𝑚𝑖𝑛 saline dropletsor f the demonstratedrapid evaporation that the processes. Both of temperatureinitial wetting regime regimes was thesupplied Cassie-like very one. Thesimilar wetting results, regime atnamely, which high evaporation apparent contact occurred under angles are accompanied with the pronounced pinning of the triple line is inherent for the so-called the slight variation“rose petal in e fftheect” [apparent37–40]. However, contact in our angle, experiments, i.e., the 142° situation 𝜃 was much 149° more, complicatedas shown in Figure 2, whereas the contactdue to the line crystallization was strongly of NaCl pinned occurring as in demonstrated a course of evaporation in Movie of the droplets, S1 and and Movie it is S2. The low reasonable to suggest that high pinning of the triple line arises from the high affinity of water to contact angle hysteresisNaCl crystals, and formed low in the sliding vicinity angles of the triple inherent line, as will for be the demonstrated pristine in saline detail below. droplets demonstrated that the initial wetting regime was the Cassie-like one. The wetting regime at which high apparent contact angles are accompanied with the pronounced pinning of the triple line is inherent for the so- called “rose petal effect” [37–40]. However, in our experiments, the situation was much more complicated due to the crystallization of NaCl occurring in a course of evaporation of the droplets, and it is reasonable to suggest that high pinning of the triple line arises from the high affinity of water to NaCl crystals, formed in the vicinity of the triple line, as will be demonstrated in detail below.

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Figure 2.2. (a) Change in the apparent contact angle in in the the course course of of evaporation evaporation of of a a 5 5 µLµL 25.9% 25.9% w/ww/w aqueous NaCl drop placed on a superhydrophobicsuperhydrophobic surface is shown; the temperature ofof evaporationevaporation

tt = 25 °C.◦C. ((b)) Sketch Sketch demonstrating demonstrating that the formation of cup-shaped residue in a course evaporation of a naked aqueousaqueous NaClNaCl dropdrop placedplaced onon aa superhydrophobicsuperhydrophobic surfacesurface isis presented.presented.

3. Discussion Evaporation of saline droplets placed on a superhydrophobicsuperhydrophobic surface resulted in thethe formationformation of the cup-shaped millimeter-scaled residues, as as shown shown in in Figures Figures 11aa andand3 3aa andand MoviesMovies S1 S1 and S2, and was observed observed for for both both slow slow and and rapid rapid evaporation evaporation regimes. regimes. It seems It seems that that the theformation formation of this of thiscup-like cup-like deposit deposit may may be beexplained explained within within the the fram frameworkework of ofthe the theory theory of of the the coffee-stain coffee-stain eeffectffect developed in [[41,42].41,42]. The authors of [[41,42]41,42] relatedrelated the formation of cocoffee-stainffee-stain deposits to a couple of mainmain physicalphysical reasons,reasons, namely:namely: contact line pinningpinning and intensive evaporationevaporation from the edge of thethe drop.drop. When When the the apparent apparent contact contact angle isangle obtuse, is weobtuse, have onewe morehave geometricone more factor geometric strengthening factor thestrengthening evaporation the rate evaporation in the vicinity rate in ofthe the vicinity triple of line, the whichtriple line, is shown which schematicallyis shown schematically in Figure 4in. ConsiderFigure 4. twoConsider molecules two molecules evaporated evaporated from the sites from labeled the sites “I” labeled and “II” “I” depicted and “II” in Figuredepicted4. Ain molecule Figure 4. evaporatedA molecule fromevaporated the site from labeled the site “I” labeled has a chance “I” has to a be chance absorbed to be byabsorbed a droplet, by whereasa droplet, a whereas molecule a evaporatedmolecule evaporated from site “II”from has site a chance“II” has to a bechance absorbed to be by absorbed the superhydrophobic by the superhydrophobic substrate. This substrate. factor strengthensThis factor strengthens the evaporation the evaporation rate in the rate vicinity in the of vicinity the triple of the line. triple A line. pinned A pinned contact contact line, in line, turn, in inducesturn, induces an outward, an outward, radial radial fluid flowfluid whenflow when there th isere evaporation is evaporation at the at edge the edge of the of drop; the drop; this flow this replenishesflow replenishes the liquid the liquid that is that removed is removed from from the edge the edge and gives and gives rise to rise the to formation the formation of the of co theffee-stain coffee- cup-likestain cup-like deposits deposits shown shown Figures Figure1a ands3 1aa andand discussed 3a and discussed in detail in [detai41,42l in]. [41,42]. Crystallization Crystallization of NaCl atof theNaCl circumference at the circumference of the contact of the area, contact in turn, area, strengthens in turn, thestrengthens effect ofpinning. the effect Thus, of pinning. crystallization Thus, workscrystallization as a positive works feedback, as a positive enhancing feedback, the coenhancingffee-stain the eff ectcoffee-stain and providing effect theand stabilityproviding of thethe apparentstability of contact the apparent angle in acontact course ofangle evaporation, in a course illustrated of evaporation, in Figure 1illustrateda. Our observations in Figure support1a. Our recentobservations experimental support findings recent reportedexperimental in [43 ],findings in which reported the crystallization in [43], in ofwhich CaCl 2thesalt crystallization in a droplet was of CaCl2 salt in a droplet was investigated, and it was revealed that the crystallization rate along the

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Condens. Matter 2018, 3, x FOR PEER REVIEW 4 of 11 Condens.investigated, Matter 2018 and, it3, wasx FOR revealed PEER REVIEW that the crystallization rate along the contact line of the droplet is many 4 of 11 contacttimes higher line of than the droplet in the direction is many oftimes the higher droplet than radius. in the Obviously, direction theof the heterogeneous droplet radius. crystallization Obviously, contact line of the droplet is many times higher than in the direction of the droplet radius. Obviously, theprevails heterogeneous on the homogeneous crystallization bulk prevails nucleation. on the homogeneous bulk nucleation. the heterogeneous crystallization prevails on the homogeneous bulk nucleation.

FigureFigure 3. ComparisonComparison images images of of evaporation of of a drop vs. a liquid marble. ( (aa,,bb)) represent represent the the side side Figure 3. Comparison images of evaporation of a drop vs. a liquid marble. (a,b) represent the side view and top view of a 10 µLµL aqueous NaCl drop, and ( cc,,d)) represent represent the the side side view view and and top top view view of of view and top view of a 10 µL aqueous NaCl drop, and (c,d) represent the side view and top view of aa 10 10 µLµL liquid marble (liquid (liquid marbles marbles are are made made by by lycopodium lycopodium powder powder and and filled ﬁlled with with 25.9% 25.9% w/ww/w a 10 µL liquid marble (liquid marbles are made by lycopodium powder and filled with 25.9% w/w aqueousaqueous NaCl NaCl solution). The The temperature temperature of of the the superhydrophobic superhydrophobic surface is t = 7070 °C.C. aqueous NaCl solution). The temperature of the superhydrophobic surface is t = 70 °C.◦

Figure 4. Scheme illustrating the formation of the coffee-stain deposits under evaporation of a naked Figure 4. Scheme illustrating the formation of the coffee-stain coﬀee-stain deposits under evaporation of a naked saline droplet evaporated on superhydrophobic surfaces. A molecule evaporated from the state saline dropletdroplet evaporated evaporated on on superhydrophobic superhydrophobic surfaces. surfaces. A molecule A molecule evaporated evaporated from thefrom state the labeled state labeled “I” has a chance to be absorbed by a droplet, whereas a molecule evaporated from site “II” “I”labeled hasa “I” chance has a to chance be absorbed to be byabsorbed a droplet, by whereasa drople at, moleculewhereas evaporateda molecule fromevaporated site “II” from has asite chance “II” has a chance to be absorbed by the superhydrophobic substrate. Red arrows illustrate the internal hasto be a absorbedchance to by be the absorbed superhydrophobic by the superhydrophob substrate. Redic arrows substrate. illustrate Red arrows the internal illustrate ﬂows the resulting internal in flows resulting in the formation of the NaCl residue. flowsthe formation resulting of in the the NaCl formation residue. of the NaCl residue. Now, consider the evaporation of liquid marbles filled with the NaCl solutions. In contrast to Now, consider the evaporation of liquid marbles filled with the NaCl solutions. In contrast to the naked saline droplets, the apparent contact angle of the marbles decreased gradually in a course the naked saline droplets, the apparent contact angle of the marbles decreased gradually in a course of evaporation from ca. 150° to ca. 115°, as shown in Figure 5 and Movie S3. It should be mentioned of evaporation from ca. 150° to ca. 115°, as shown in Figure 5 and Movie S3. It should be mentioned

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Now, consider the evaporation of liquid marbles ﬁlled with the NaCl solutions. In contrast to the naked saline droplets, the apparent contact angle of the marbles decreased gradually in a course of Condens. Matter 2018, 3, x FOR PEER REVIEW 5 of 11 evaporation from ca. 150◦ to ca. 115◦, as shown in Figure5 and Movie S3. It should be mentioned that the apparent contact angle of a liquid marble is not a “true interfacial angle” due to the fact that it is that the apparent contact angle of a liquid marble is not a “true interfacial angle” due to the fact that formed by the micro-rough surface of a marble coated with colloidal particles. it is formed by the micro-rough surface of a marble coated with colloidal particles.

Figure 5. Change in the apparent contact angle in the course of evaporation of a 10 µLµL liquid marble filledﬁlled with 25.9% w/ww/w aqueousaqueous NaClNaCl solutionsolution isis presented.presented. The temperature of the superhydrophobic

surface is t = 2525 °C.◦C.

We establishestablish that that the the eventual eventual shape shape of theof solidthe so residue,lid residue, built ofbuilt the mixtureof the mixture of colloidal of colloidal particles particlesand NaCl and crystals, NaCl depends crystals, strongly depends on thestrongly temperature on the oftemperature evaporation of and evaporation the concentration and the of concentrationthe solution. When of the the solution. marble When was slowly the marble evaporated was slowly under evaporated ambient conditions, under ambient (the characteristic conditions,

(thetime characteristic of the process time was ofτ evthe process35 min )wasthe process𝜏 ≅ 35 yielded min) the a singleprocess relatively yielded a large single (ca. relatively 1 mm) NaCllarge (ca.crystal 1 mm) that NaCl formed crystal at the that bottom formed of theat the evaporated bottom of saline the evaporated marble. saline marble. The situation was rather different different for rapidly evaporatedevaporated liquid marbles. When evaporation was conducted under high temperatures (t = 7070 °C),◦C), the the characteristic characteristic time time of of the the process process was was 𝜏τev ≅5 5 min and twotwo di differentfferent scenarios scenarios of crystallization of crystallization were observed.were observed. When the When NaCl solutionsthe NaCl were solutions unsaturated were (14.9%unsaturatedw/w and (14.9% 8.1% w/ww/w), and the crystallization8.1% w/w), the took crystallization place mainly took at the place base mainly of the marble, at the yieldingbase of the marble,flat residues yielding shown the inflat Figure residues6. The shown most in interesting Figure 6. The and most intriguing interesting result and was intriguing obtained underresult was the obtainedrapid (t = under70 ◦C ,theτev rapid 5 min ( 𝑡) evaporation = 70°C, 𝜏 ≅5 of min saturated) evaporation NaCl solutions. of saturated In this NaCl case, solutions. small crystals In this of case,NaCl small (the characteristic crystals of NaCl size (the of crystals characteristic was ca. size 10–50 of µcrystalsm) grew was uniformly ca. 10–50 over µm) the grew entire uniformly marble shell.over theIn other entire words, marble interface shell. In crystallization other words, interface stimulated crystallization by the particles stimulated coating aby marble the particles took place coating [1–6 a]. Themarble NaCl took crystals place merged[1–6]. The with NaCl the crystals colloidal merged particles, with thus the forming colloidal the particles, uniform thus hemisphere-like forming the uniformdented shells hemisphere-like depicted in Figuresdented7 shellsb and 8depictedd. The SEM in Fi imagesgures 7b of theand shells, 8d. The representing SEM images the of mixture the shells, of representingNaCl crystals the and mixture colloidal of particles,NaCl crystals are depicted and colloidal in the particles, inset (c) ofare Figure depicted7. The in the residues inset (c) depicted of Figure in 7.Figures The residues7b and8 ddepicted are markedly in Figures stronger 7b and than 8d the are residues markedly emerging stronger at than room the temperature. residues emerging It should at roombe emphasized temperature. that It in should this case, be emphasized the interfacial that crystallization in this case, promotedthe interfacial by the crystallization hydrophobic promoted particles bycoating the hydrophobic the marble was particles observed. coating The the mass marble transport was promotingobserved. The the interfacialmass transport crystallization promoting under the interfacialslow and rapid crystallization evaporation under is depicted slow and schematically rapid evaporation in Figure is depicted8a,c. schematically in Figure 8a,c.

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FigureFigureFigure 6. Photos 6. PhotosPhotos of ofthe of thethe residues residues residues emerging emerging emerging fromfrom from thethe evaporation evaporation of of of10 10 10µLµ LµL liquid liquid liquid marble marble marble coated coated coated by by by Figure 6. Photos of the residues emerging from the evaporation of 10 µL liquid marble coated by lycopodiumlycopodium powderpowder andand ﬁlled filled with with 14.9% 14.9%w /ww/w(a) and(a) and 8.1% 8.1%w/w w/w(b) aqueous (b) aqueous NaCl solutionNaCl solution are shown. are lycopodiumlycopodium powder powder and and fi lledfilled with with 14.9%14.9% w/w ((aa)) andand 8.1% 8.1% w/w w/w (b )( baqueous) aqueous NaCl NaCl solution solution are are shown.The temperature The temperature of the superhydrophobic of the superhydrophobic surface issurfacet = 70 isC. t=70°C. shown.shown. The The temperature temperature of of the the supe superhydrophobicrhydrophobic surfacesurface is◦ is t=70°C. t=70°C.

Figure 7. (a) Photo of the 10 µL liquid marbles filled with 25.9% w/w NaCl solution and coated by FigureFigure 7. 7. (a()a )Photo Photo of of the the 10 10 µLµ liquidL liquid marbles marbles filled ﬁlled with with 25.9% 25.9% w/ww /NaClw NaCl solution solution and and coated coated by lycopodium. (b) and (c) are the photo and scanning electron microscope (SEM) images of the shell Figurelycopodium.by lycopodium.7. (a) Photo (b) (andofb) the and (c) 10 (arec) µL are the theliquid photo photo marblesand and scanning scanning filled electron wi electronth 25.9% microscope microscope w/w NaCl(SEM) (SEM) solution images images of ofand the the coatedshell shell by emerging from evaporation of the liquid marbles (t = 70 °C), respectively. lycopodium.emergingemerging from( fromb) and evaporation evaporation (c) are the of of the thephoto liquid liquid and marbles scanning ( tt = 70 70electron °C),◦C), respectively. respectively. microscope (SEM) images of the shell emerging from evaporation of the liquid marbles (t = 70 °C), respectively.

Figure 8. Water flows within marbles containing 10 µL of 25.9% w/w saline and evaporated at the FigureFigure 8. 8. WaterWater flows ﬂows within within marbles marbles containing containing 10 10 µLµL of of 25.9% 25.9% w/ww/w salinesaline and and evaporated evaporated at at the the temperatures t = 25 °C and t = 70 °C are shown in insets (a) and (c). The shapes of the eventual NaCl temperaturestemperatures tt == 2525 °CC and and tt == 7070 °CC are are shown shown in in insets insets ( (aa)) and and ( (cc).). The The shapes shapes of of the the eventual eventual NaCl NaCl deposits arising from◦ evaporation◦ of the marbles are shown in insets (b) and (d). The red arrow in depositsdeposits arising fromfrom evaporationevaporation of of the the marbles marbles are are shown shown in insetsin insets (b) ( andb) and (d). ( Thed). The red arrowred arrow in inset in inset (c) indicates the velocity of convective flows u⃗. Figureinset(c) indicates8. ( cWater) indicatesthe flows velocity the within velocity of convective marbles of convective containing ﬂows flows→u. u⃗10. µL of 25.9% w/w saline and evaporated at the temperaturesThe upward t = flow,25 °C supplyingand t = 70 NaCl°C are to shown the hydropho in insetsbic (a )particles and (c). coatingThe shapes the marble,of the eventual is reasonably NaCl The upward flow, supplying NaCl to the hydrophobic particles coating the marble, is reasonably attributeddeposits arising to the thermalfrom evaporation convection, of inevitable the marbles when are a shown marble in is insetsplaced ( bon) and a hot (d plate.). The Consider red arrow that in attributed to the thermal convection, inevitable when a marble is placed on a hot plate. Consider that theinset change (c) indicates in temperature the velocity of the of convectivesaline in the flows cour seu⃗. of evaporation was registered with the thermal the change in temperature of the saline in the course of evaporation was registered with the thermal camera as ca. Δ𝑡 ≅ 25°. In order to attain a qualitative understanding of the process occurring within camera as ca. Δ𝑡 ≅ 25°. In order to attain a qualitative understanding of the process occurring within The upward flow, supplying NaCl to the hydrophobic particles coating the marble, is reasonably attributed to the thermal convection, inevitable when a marble is placed on a hot plate. Consider that the change in temperature of the saline in the course of evaporation was registered with the thermal camera as ca. Δ𝑡 ≅ 25°. In order to attain a qualitative understanding of the process occurring within

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The upward ﬂow, supplying NaCl to the hydrophobic particles coating the marble, is reasonably attributed to the thermal convection, inevitable when a marble is placed on a hot plate. Consider that the change in temperature of the saline in the course of evaporation was registered with the thermal camera as ca. ∆t 25◦. In order to attain a qualitative understanding of the process occurring within the marbles under their evaporation, let us understand the hierarchy of time scales involved in the process. The characteristic time scale of evaporation is τev 5 min. The characteristic time scale of the diﬀusion mass transport within the marbles/saline droplets is supplied by Equation (1):

(2R)2 τ (1) di f f D where R is the radius of a marble, and D is the diffusion coefficient of NaCl in water; substitution of 2 R 1.0 10 3 m and D 1.5 10 9 m (see [44]) yields the estimation τ 2.7 103s 45.0 min. × − × − s di f f × The characteristic time scale of thermal equilibration in the marbles heated from below τeq may be estimated as: (2R)2 τ (2) therm α 2 where α 0.14 10 6 m is the thermal diffusivity of saline (see [45]); calculation with Equation (2) × − s yields the estimation τtherm 0.5 min. Thus, the hierarchy of time scales supplied by Equation (3) takes place: τ τev τ (3) di f f therm We recognize from Equation (3) that the diffusion mass transport is a slow process on the time scales of evaporation and thermal equilibration, occurring in the saline marbles subject to heating. Therefore, it is reasonable to attribute the dramatic influence exerted by temperature on the morphology of the eventual residue to the upward convective flows (shown schematically in Figure8c), which supply saline to the hydrophobic particles, coating the marble and promoting the interfacial crystallization, and yielding entire shells shown in Figure7b. Contrastingly, slow evaporation of the liquid marbles yielded flat deposits, as shown in Figure8b, containing the large (~1 mm) NaCl crystal formed at the base of the marble. This conclusion is supported by the direct calculation of the Peclet number supplied by Equation (4): 2Ru Pe = (4) D where u is the characteristic velocity of the convective flows (see Figure8c). Direct visualization of the convective flows taking place within the heated marbles under t = 70 ◦C supplied the value of the mm typical velocity of convective flows estimated as u 0.3 s (see Movie S2). Substitution of the aforementioned values of the physical parameters yields the following estimation: Pe 4 102 1. This means that within the regime of rapid evaporation of the saline marbles, the bulk × convective flows prevail over the diffusion mass transport, thus justifying the suggested mechanism of shells’ formation. Our observations are in accordance with those reported in [46], in which crystallization during evaporation of saturated Na2SO4 and NaCl droplets was studied. The authors of [46] reported that on a hydrophobic surface, contrary to what happens for the hydrophilic one, the NaCl crystallization appears not at the liquid–air interface but rather at the solid–liquid interface. In our experiments, this was the case for both naked NaCl droplets evaporated on superhydrophobic surfaces and also for NaCl liquid marbles. Let us quote [46]: ‘Sodium chloride crystals are found to form preferentially in contact with a nonpolar area (air or hydrophobic solid). This is an important observation, as hydrophobic treatments have been used frequently in the past to prevent crystallization and damage on buildings. Our observations suggest that this may be counterproductive due to the tendency of NaCl to crystallize on nonpolar surfaces’. Our results support this conclusion; indeed, the sodium chloride crystals prefer Condens. Matter 2020, 5, 62 8 of 11 to grow on hydrophobic surfaces in contact with NaCl solutions, filling naked droplets placed on superhydrophobic surfaces and saline marbles coated with hydrophobic particles. The dented shape of the shell emerging from the evaporation of saline marbles, as shown in Figure8c,d, is noteworthy. This shape resembles the shape of Leidenfrost droplets deposited on hot plates, as discussed in [47]. It is reasonable to attribute the formation of the dent to a gas pocket, formed by the evaporation of water when a marble is heated from below.

4. Conclusions We conclude that the saline droplets placed on superhydrophobic surfaces and liquid marbles filled with saline demonstrate interfacial crystallization over the course of water evaporation. Evaporation of naked saline droplets resulted in the formation of cup-shaped millimeter-scaled residues. The formation of the cup-like deposit is reasonably explained within the framework of the theory of the coffee-stain effect [41,42]. Evaporated saline marbles heated from below gave rise to dented shells built of a mixture of colloidal particles and NaCl crystals. Qualitative analysis of the processes taking place under the evaporation of saline marbles supports the assumption that the diffusion mass transport is a slow process on the time scales of evaporation and thermal equilibration of heated marbles. Formation of the entire shells built of the lycopodium and NaCl crystals under rapid evaporation of the heated saline marbles takes place under the high values of the Peclet numbers [48]. We relate the formation of these shells to the interfacial crystallization promoted by the hydrophobic particles coating the marbles, accompanied by the upward convection flows supplying the saline to the hydrophobic particles.

5. Materials and Methods The following materials were used in order to prepare the liquid marbles: deionized water (DI) from Millipore SAS (France) (specific resistivity ρˆ = 18.2 MΩ cm at 25 C, surface tension · ◦ γ = 72.9 mN/m; viscosity η = 8.9 10 4 Pa s) used in the experiments; sodium chloride (NaCl) supplied × − · by Melach Haaretz Ltd., Israel; and lycopodium powder (average diameter 50 µm) supplied by Fluka. Liquid marbles containing aqueous solution of NaCl were coated by lycopodium powder. Then, 5–20 µL drops containing three diﬀerent concentrations of aqueous NaCl solution (25.9% w/w; 14.9% w/w and 8.1% w/w) were used to prepare the liquid marbles. Saline droplets were deposited using a precise micro-syringe on the superhydrophobic surface. The superhydrophobic surface was covered with a layer of aforementioned powders. Rolling of droplets yielded the formation of marbles enwrapped with the speciﬁed hydrophobic powders and containing the solution. Evaporation of liquid marbles and naked saline droplets was performed on a superhydrophobic surface. The evaporation process, carried out on the superhydrophobic surface, was studied at ambient conditions (t = 25 ◦C) and 70 ◦C, where 70 ◦C was the temperature of the hot plate. The relative humidity was RH = 44%. A Nikon 1 v3 camera, digital microscope BW1008-500X and filed emission scanning electron microscope (MAIA3 TESCAN) were used to capture images and movies of the marbles during evaporation. A Rame-Hart 500 goniometer was used to capture the side view of the liquid marbles during evaporation. Temperature distribution was studied using a Therm-App Pro TAS19AV-M25A-HZ LWIR 7.5-14 µm thermal camera. The resolution, accuracy,sensitivity and the frame rate of the camera were 640 480 pixels × (>300,000 pixels), NEDT (noise equivalent differential temperature) < 0.03 ◦C and 25 frames/s, respectively.

Supplementary Materials: The following are available online at http://www.mdpi.com/2410-3896/5/4/62/s1, Movie S1 represents evaporation of a 5 µL 25.9% w/w aqueous NaCl drop placed on the superhydrophobic surface (side view) (speed 64X). The temperature of the superhydrophobic surface is t = 25 ◦C; Movie S2 represents evaporation of a 5 µL 25.9% w/w aqueous NaCl drop placed on a superhydrophobic surface (top view) (speed 4X). The temperature of the superhydrophobic surface is t = 70 ◦C; Movie S3 represents evaporation of a 10 µL liquid marble made by lycopodium powder and ﬁlled with 25.9% w/w aqueous NaCl solution (side view). The temperature of the superhydrophobic surface is t = 70 ◦C. Condens. Matter 2020, 5, 62 9 of 11

Author Contributions: S.S. and E.B. proposed the concept. P.K.R.and I.L. designed and performed the experiments. E.B. wrote the paper. The results were analyzed by S.S., E.B., I.L. and P.K.R. Authorship must be limited to those who have contributed substantially to the work reported. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors are thankful to Yelena Bormashenko for her kind help in preparing this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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