Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self

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Liquid Marbles, Elastic Non-Stick Droplets: from Mini-Reactors to Self-Propulsion Edward Bormashenko Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03231 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 13, 2016

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Liquid Marbles, Elastic Non-Stick Droplets: from Mini-Reactors to SelfPropulsion Edward Bormashenko Ariel University, Engineering Faculty, Chemical Engineering Department, P.O.B. 3, 407000, Ariel, Israel

Edward Bormashenko Ariel University, Engineering Faculty, Chemical Engineering Department. P.O.B. 3 Ariel 407000 Phone: +972-3-906-6134 Fax: +972-3-906-6621 E-mail: [email protected]

1 Environment ACS Paragon Plus

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Abstract Liquid marbles are non-stick droplets wrapped by micro- or nanometrically scaled colloidal particles, representing a platform for a variety of chemical, biological and micro-fluidics applications. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed. The effective surface tension and Young modulus of liquid marbles are discussed. Physical sources of the elasticity of liquid marbles are considered. Liquids and powders used for the fabrication of liquid marbles are surveyed. The paper reviews properties and applications of liquid marbles. Liquid marbles demonstrate a potential as micro-reactors, micro-containers for growing micro-organisms and cells, and micro-fluidics devices. Marangoni-flowdriven self-propulsion of marbles supported by liquids is addressed. Keywords: liquid marbles, non-stick droplets, effective surface tension, effective Young modulus, colloidal particles, self-propulsion. INTRODUCTION Liquid marbles, which are droplets coated with colloidal particles, depicted in Figure 1, started their brilliant career from the series of papers published by David Quéré and co-authors, in which droplets were used as templates for absorbing solid, strongly hydrophobic powders (lycopodium and Teflon).1-4 Thus, non-stick (low adhesion) droplets were introduced, possessing fascinating physical properties and representing an alternative pathway to the famous lotus-effect.5-7 In parallel, it was reported that liquid marbles are also found naturally; aphids convert honeydew droplets into marbles.8 Analogues of liquid marbles might also be relevant in cell biology, where adherent cells sometimes move by rolling.9-10 Restoring historical justice demands recalling that droplets were used as templates for collecting colloidal (latex) particles by Velev et al. in Ref. 11.


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Like many other renowned scientific achievements, liquid marbles started their life as amusing scientific toys; however, it will be remembered that when William Gladstone asked about the practical value of newborn artificial electricity, Michael Faraday replied: “One day sir, you may tax it.” As of today numerous applications of liquid marbles has been reported, from mini-chemical-reactors to flow self-propelled objects, driven by Marangoni flows. Actuation of liquid marbles by electric and magnetic fields4 opens a diversity of their micro-fluidics applications. The presented paper is devoted to the unusual physical properties of liquid marbles on one hand, and their promising applications on the other hand. COLLOIDAL PARTICLES ATTACHED TO DROPLETS GIVE RISE TO LIQUID MARBLES. Colloidal particles attached to liquid surfaces have been subjected to intensive research during the past decades, for both scientific and technological reasons. Colloidal particles deposited on a fluid/vapor and fluid/fluid interfaces give rise to abundant and technologically important products, including foams and emulsions (among them so-called Pickering emulsions).12-14 The interest in colloidal systems was essentially strengthened, when inexpensive manufacturing of mono-dispersed colloidal spheres became possible, leading to a diversity of promising applications.15 It was demonstrated that these spheres form perfect long-range-ordered structures.15 The long-range order evidences the attraction forces acting between colloidal particles. The nature of these forces may be very different, including capillary,16 electrostatic17 and electrostatic double-layer interactions.18 Such mono-disperse particles (latex and poly(methylsilsesquioxane)) were already effectively exploited for manufacturing liquid marbles.19-20 Thus, liquid droplets coated with well-ordered colloidal rafts were reported.20 However, in the vast majority of cases, the marbles are


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coated by disordered particles separated by water clearings, connecting the liquid filling a marble to the atmosphere.1-4,21-22 This respiratory ability of liquid marbles opens pathways for a diversity in their applications, to be discussed below. It is noteworthy that liquid marbles remain stable when supported not only by solids, but also by liquid surfaces, as shown in Figure 2.21-25 This is explained by the fact that the marbles are separated from solid and liquid supports by an air layer,1,21,25-26 in a way that resembles Leidenfrost droplets.27 This air layer provides non-stick properties to liquid marbles and prevents their coalescence, when contacted.28 The formation of liquid marbles becomes possible due to energetic reasoning, considering the energy of colloidal particles attached to the water/vapor or water/oil interface. Consider a single spherical particle located at the interface, separating media numbered 1 and 2 (it may be either liquid/liquid or liquid/gas boundary). Assuming the particle is small enough (typically less than a few microns in diameter, so that the effect of gravity is negligible), the energy ∆E required to remove the particle from the interface is given by: ∆E = γ 12πR 2 (1 ± cosθ ) 2 ,

(1)

in which the sign inside the bracket is negative for removal into the water phase, and positive for removal into the vapor or oil phase; R is the radius of the particle, θ is the equilibrium contact angle (note that it is different from the Young angle, when the particle is placed at the liquid/liquid interface), and γ 12 is the interfacial tension at the boundary.12-13 It is easily seen from Eq. 1 that the strongest connection of a colloidal particle to an interface takes place for θ = 900 , and as such, it may be on the order of magnitude

of

thousands

of

kBT

at

ambient

conditions.13

However,

for

00 < θ < 200 ;433K < T < 453K , the energy ∆E required to remove the particle from the fluid-fluid interface falls to ∆ E ≤ 10 k B T . 12-13 Thus, liquid marbles are easily


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formed from either hydrophobic powders (as shown in Figures 1A-B1-4,28-30) or slightly hydrophilic31-32 powders such as carbon black or graphite (see Figure 1C). It was suggested that the Cassie-Baxter wetting occurring at the interface separating aggregates of hydrophilic particles from the liquid promotes formation of liquid marbles.32 An analysis of the floating of colloidal non-spherical particles considering the line tension was performed in Ref. 33. It was demonstrated by Krasovitski et al. that a positive line tension may lead to a meaningful energy barrier that may prevent the penetration of particles into a drop, even when it is thermodynamically favorable.33 An analysis of the absorption of colloidal particles to curved interfaces was carried out in Ref. 34. From Eq. 1, it is clear that marbles could be manufactured in two ways: particles could be introduced beforehand into the droplet bulk, or the drop could be rolled on a hydrophobic powder. Actually, both methods have already been reported.32 Scaling laws governing the shape of liquid marbles distorted by gravity were reported in Refs. 1, 4. Liquid marbles may be produced with both synthetic and natural powders, such as lycopodium.1,21,35 The survey of particles used for manufacturing liquid marbles is reported in Ref. 36. Strongly hydrophobic fumed fluorosilica powder37 and latex particles19,30,38-39 were broadly used for preparing liquid marbles. Hydrophobized fumed fluorosilica powders enabled the manufacturing of liquid marbles containing liquids possessing low surface tensions, as will be discussed later. Fernandes et al. reported the use of hydrophobic poly(ionic liquid) micro-powders as a coating for marbles.40 A diversity of liquids was successfully packed within liquid marbles, including water and aqueous sodium dodecyl sulfate solutions,41 glycerol,42 ionic liquids43 and liquid metals.44


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THE PHYSICS OF LIQUID MARBLES: THEIR EFFECTIVE SURFACE TENSION AND ELASTIC PROPERTIES Liquid marbles demonstrate very unusual properties, common for both liquid and solid interfaces. On the one hand, the effective surface tension may be attributed to their surface as was already suggested Quéré et al.1,4 On the other hand, they demonstrate elastic properties inherent for liquid surfaces coated by colloidal particles.45-47 Liquid marbles do not coalesce, even when pressed one (as shown in

Figure 3) to another or made to collide.48 Let us discuss first the effective surface tension of liquid marbles. A variety of independent experimental techniques were applied for the establishment of the effective surface tension γ eff of liquid marbles: 1) the puddle height method,1,4,49 2) analysis of marble shape,4,50-51 3) vibration of marbles,50--51 4) the capillary rise,52 5) the Wilhelmy plate method,52 6) the pendant marble method.53 The effective surface tension of a liquid surface coated by colloidal particles also may be extracted from the analysis of capillary wave propagation along such an interface.47 Planchette et al. demonstrated with these dynamic measurements that the effective surface tension of a particle raft is only slightly dependent on the diameter of the particles.47 However, various groups reported very different values for the effective surface tensions, established by different experimental techniques (the values of the effective surface tension in the range of γ eff ≅ 45 − 73

mJ were m2

reported).49-52 The situation was clarified in Ref. 53, where it was demonstrated that the effective surface tension depends strongly on the marble volume and demonstrates pronounced hysteretic behavior (i.e. it depends on the pathway of inflating or deflating liquid marbles).53


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Now let us discuss the elastic properties of liquid marbles. Remarkably, liquid marbles demonstrate pronounced elastic properties and can sustain a reversible deformation of up to 30%, as demonstrated in Refs. 54-55. Actually, there exist two very different sources of elasticity of liquid marbles, the first of which is common for droplets and liquid marbles. When droplets and marbles are deformed from their initial spherical shape, they increase their surface.56 This gives rise to the restoring “spring-like” force driven by the surface tension, introduced by Lord Rayleigh.57 The analysis of the deformation of liquid marbles (made in linear approximation) resulted in the following expression for an effective Young modulus of liquid marbles: 4γ eff ~ , EYoung = R0

(2)

where R0 is the radius of the non-deformed marble. Considering the non-linear terms 4γ eff ~ of the strain ε yielded: EYoung = (1 − ε + ε 2 ) .58 R0 There exists one more mechanism of elasticity of liquid marbles, arising from the elastic properties of colloidal rafts coating a marble. Vella et al. treated the collective behavior of a close-packed monolayer of non-Brownian colloidal particles placed at a fluid-liquid interface.45 In this simplest case, however, the close-packed monolayers may be characterized using an effective Young modulus and Poisson ratio.45 These authors proposed an expression for the effective Young modulus EYoung of the “interfacial particle raft” in the form45:

EYoung ≅

1 −ν γ , 1+φ d

(3)

where γ is the surface tension of a liquid, d is the diameter of the solid particles, is the Poisson ratio of solid particles, and is the solid fraction of the interface. They


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concluded that the elastic properties of such an interface are not dependent on the details of capillary interaction between particles.45 The quantitative estimations and the experimentally established values for the close-packed rafts built from monodisperse particles are supplied in Ref. 45. The bending modulus of air-water interfaces covered by a monolayer of bi-disperse particles was experimentally established in Ref. 46. However, the estimation of elastic properties of colloidal layers coating liquid marbles is a challenging task, due to the fact that marbles are usually coated by polydisperse particles or their agglomerates, separated by arbitrarily shaped water clearings.21 An attempt to calculate the effective Young modulus was undertaken in Ref. 59. Now let n be the surface density of identical colloidal particles with a radius of r, attached to the liquid/vapor interface (the surface is supposed to be flat; the elasticity in this case is caused by the change in the liquid area under deformation).59 The effective Young modulus EYoung of such a layer exposed to the stress, transversal to the liquid surface, was estimated in Ref. 59, as:

≈ 2 |cos − cot |

,

(4)

Where θY is the Young angle of the particles, γ is the surface tension at the liquid/vapor interface.59 However, the experimental establishment of the Young angle of colloidal particles remains an extremely challenging task.14 The realistic numerical estimation of EYoung from Eq. 4 for the lycopodium coated water surface (r~1 μm;

θY = 120o , γ = 70

mJ , ~10 m-2 ) yields EYoung~100 Pa, which is at least two orders m2

of magnitude lower than that following from Eq. 3 in the case of close-packed microparticles.45 The effective mechanical scheme of a liquid surface coated with solid


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colloidal particles, considering different sources of elasticity and the viscosity of the liquid is supplied in Ref. 59. A pure macroscopic phenomenological approach was applied in Ref. 60 for the analysis of the elastic collisions of liquid marbles. The collision between two marbles has been considered with the help of a simple model of linear oscillator, moved by a spring with the stiffness, governed by the elasticity and thickness of the elastic shell.60 The elastic energy W of the colloidal raft coating a marble was estimated in Ref 60 as:

2

π ς  2 W = G  S ≅ Ghsrel , R 2  

(5)

where G and R are the elastic modulus the radius of the shell (the value of R may be considered equal to the radius of the marble, for the rough calculation of G the estimation G ≅ EYoung was assumed in Ref. 60, where EYoung was calculated with Eq. 4); S is the contact area, h is the thickness of the shell, which is on the order of magnitude of the characteristic dimension of a colloidal

particle, ς is the radial

displacement of the points of the deformed shell and srel is the relative displacement between the marbles in the course of collision.60 Considering the “elastic” energy given by Eq. 5 is essential for understanding collisions of liquid marbles and scaling laws governing their shape.60 The physics of rolling liquid marbles was addressed in Refs. 3, 4, 42. The mechanism of friction decelerating the friction of liquid marbles depends crucially on the liquid filling a marble. The friction of glycerol marbles is governed by viscous dissipation, whereas the rolling of water marbles is dictated by adhesion forces acting within the contact area.42


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RESPONSE TO STIMULI AND ACTUATION WITH EXTERNAL FIELDS OF LIQUID MARBLES Liquid marbles are highly sensitive to external stimuli, in particular to the pH of the supporting liquid.19,22,30,61 In Ref. 19, liquid marbles coated with pH-responsive sterically stabilized polystyrene (PS) latex exhibited long-term stability (over 1 hour) when transferred onto the surface of liquid water, provided that the solution pH of the subphase was above pH 8. In contrast, the use of acidic solutions led to immediate disintegration of the reported liquid marbles.19 Photoresponsive (UV- and IRirradiation responsive) liquid marbles were introduced.62,63

Electrochemically

induced actuation of liquid metal marbles was reported.64 Various groups have demonstrated that liquid marbles could be actuated with magnetic24,40,65-70

and electric fields.71-75 Ferro-fluid liquid marbles may be

accelerated up to 25 cm/s by a magnetic field.65 Lin et al. demonstrated that the magnet may reversibly open a liquid marble also allowing the liquid to be taken from the marble.67 Liquid marbles, coated with magnetic lanthanide-doped upconversion nanoparticles that can convert near-infrared light into visible light, were reported in Ref. 70. Quantitative analysis of the deformation of liquid marbles by electric field was performed in Refs. 72-73. Liquid marbles may be not only actuated but also manufactured under assistance of applied electric fields.76 Marbles built from hemispheres possessing various electric properties (so-called Janus-marbles, shown in

Figure 4) may be easily actuated by electric fields.14,77A simple method of continuous processing of Janus marbles was reported in Ref. 78.

APPLICATIONS OF LIQUID MARBLES: FROM MINI-REACTORS TO SELF-PROPULSION


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As has already been mentioned, liquid marbles are respiring systems: the bulk of a marble is connected to atmosphere via water clearings, as shown in Figure 5. Liquid marbles, due to their small volumes (typically on the order of µl), provide optimal conditions for miniaturized chemical processes. Such processes have many advantages related to the reduced use of chemical reagents and solvents, precisely controlled reaction conditions, and greatly shortened reaction time. This makes liquid marbles extremely suitable for miniaturized gas sensing, depicted schematically in

Figure 5.79-80 Marbles were also effectively used as micro-reactors, enabling blood typing81, copolymerization reactions,82 and even ultra-trace molecular detection, reported with plasmonic liquid marbles.83 Liquid marbles were successfully exploited as a platform for Janus particle synthesis.84 The respirability of liquid marbles makes them extremely suitable for the cultivation of microorganisms and cells.85-87 The small volumes of liquid marbles also enable their applications as micro-physical devices, in particular as micro-centrifuges and miniature viscometers.88 The connection of the liquid filling a marble to the atmosphere also gave rise to self-propulsion of liquid marbles containing aqueous ethanol solutions placed on liquid supports.89 Successful synthesis of hydrophobized fumed fluorosilica powders enabled the manufacturing of liquid marbles containing aqueous ethanol solutions, characterized by low surface tensions.37, 89 When marbles containing aqueous ethanol solutions are supported by water, they are self-propelled by Marangoni solutocapillary flow, developed by the evaporation of ethanol from a marble followed by it condensation at the water surface.89-90 Consider the spontaneous increase in evaporation of alcohol from the marble in the direction of –x (recall that alcohol evaporates from a marble much faster than water), as depicted in Figure 6. This

r increase will give rise to the Marangoni flow, resulting in the force F (shown with the


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green arrow in Figure 6), driving the marble in the direction of –x. In parallel it develops a fascinating instability, transporting marbles.89 The displacement of marbles in turn enhances the evaporation, withdrawing vapor from the layer separating the marble from the supporting liquid.89 The addition of alcohol to the water supporting the marbles suppresses the selfpropulsion.89 The propulsion of liquid marbles is mainly stopped by water drag.89-90 The stationary velocity of the center of mass vcm of the marbles increases with an increase in the concentration of alcohol within the marble, and attains a speed of 15 cm/s; it may be roughly estimated as:

vcm ≅

∇γ

ηw

a,

(6)

where ∇γ is the modulus of the gradient of the surface tension driving a marble due to the condensation of the alcohol, a is the radius of the contact area of a marble (see

Figure 6) and ηw is the water viscosity.90 The observed velocity of self-propulsion is relatively large when compared to the characteristic velocities of the Marangonidriven-flows driven motions, ranging usually from µm/s to mm/s.91-92 The scaling laws relating the dynamic parameters of the motion to the physical properties of the system, including the viscosity and the density of the supporting liquid, the coefficient of diffusion of the ethanol vapor and geometrical parameters of a marble, were suggested in Ref. 90. The velocity of the center mass of a marbles scales with its 1

volume as vcm ≅ V 2 (see Ref 90 for the details). The thermal Marangoni flow, induced by NIR laser irradiation allowed the light-driven propulsion of marbles with a maximal velocity of 2.7 cm/s.93 The lightcontrolled displacement of liquid marbles makes it possible to not only transport the


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materials encapsulated within the liquid marble but also to release them at a specific place and time, as controlled by external stimuli and to exploit liquid marbles as laserdriven towing engines to push or pull objects.93 One more light-inspired Marangoni-flow-supported motion of liquid marbles was observed, when marbles were deposited on a water solution containing photosensitive surfactants.94 Irradiation of the solution generated photo-reversible Marangoni flows, that transported the liquid marbles toward UV light and away from blue light when the thickness of the liquid substrate was large enough.94 Below a critical thickness, the liquid marbles moved in the opposite direction to that of the surface flow, at a speed increasing with decreasing liquid layer thickness.94 Selfpropulsion of liquid marbles opens novel pathways for their micro-fluidic applications.

CHALLENGES IN THE FUNDAMENTALS AND APPLICATIONS OF LIQUID MARBLES Only a few works reporting liquid marbles coated with mono-disperse particles have been reported.19-20 The extension of this investigation, including the study of elastic properties of marbles coated by long-ordered, close-packed rafts of particles, may shed light on mechanical properties of the layers of colloidal particles attached to liquid surfaces.45,59,60 The investigations devoted to preparing optically transparent marbles are scarce,95 whereas transparent liquid marbles may give rise to a variety of optical applications.96 The manufacturing of very small-volume liquid marbles (V~0.01-0.1µl) also remains a challenging experimental task. The opposite problem of the use of liquid puddle-like macro-reactors where surface-area-to-volume ratio is maximized rather than minimized may be also of interest.97 Opening and closing of the liquid marble particle achieved via acoustic levitation, enables a


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possibility to manipulate liquid marbles coated with non-ferromagnetic particles, and opens new pathways for use of liquid marbles as micro-reactors.98

CLOSING REMARKS Liquid marbles, as highly mobile non-stick droplets, present an alternative to superhydrophobicity. They may be actuated by chemical and physical stimuli, including electric and magnetic fields, UV and IR-light. They possess unique physical properties, being characterized by both the effective surface tension and Young modulus. The small volumes and high mobility of liquid marbles provide optimal conditions for miniaturized chemical and biological processes, including growing and preservation of micro-organisms and cells. The Marangoni thermo- and solutocapillary flows driven propulsion of liquid marbles opens novel pathways for their micro-fluidic applications, enabling controlled transport and release of the microliterscaled quantities of liquids contained in the liquid marble.89, 92-94

ACKNOWLEDGMENTS The author is thankful to Mrs. Ye. Bormashenko, Professor G. Whyman, Dr. M. Frenkel, Y. Shapira and V. Multanen, for their kind help in preparing this manuscript.

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A

B

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C

Figure 1. 20 µl liquid marbles coated with: A. Teflon B. Lycopodium C. Carbon Black.


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Figure 2. Teflon-coated 10 µl water marble floating on the 18% wt. NaCl aqueous solution.


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Figure 3. 20 µl water marbles coated by Teflon (the white marble) and lycopodium (the red marble) do not coalesce when pressed one to another (the upper image) and restore their shape when released (the lower image).


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Figure 4. 40 µl Janus water marble built from carbon black and Teflon hemispheres.


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solid particles

liquid + reagents

gas

water clearings

liquid marble

Figure 5. Scheme of a micro-reactor based on respiring liquid marbles.


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evaporation evaporation

r F γ1

γ2

Marangoni flow

2a x

Figure 6. Scheme illustrating the origin of the instability driving liquid marbles containing aqueous alcohol solution deposited on a water surface. The blue arrow shows the spontaneous increase of the alcohol evaporation from a marble. The red arrow indicates the direction of the Marangoni flow, increasing in turn the evaporation of alcohol from the area beneath a marble (γ1 >γ2 for two points shown on the water surface).


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TOC IMAGE


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AUTHOR INFORMATION Edward Bormashenko is a professor of chemical engineering and materials science and is the head of the Laboratory of Interface Science at Ariel University in Israel. His research interests include wetting phenomena, surface science, superhydrophobicity, wetting transitions, processes of self-assembly, polymer science, soft matter physics and interaction of plasma with organic materials. He is the author of more than 180 publications in these fields, including the book Wetting of Real Surfaces. In the last decade, he has devoted his research to interfaces with prescribed wettability, including non-wetted surfaces and nonstick droplets.


Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self

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