Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self

Invited Feature Article pubs.acs.org/Langmuir

Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to SelfPropulsion Edward Bormashenko* Ariel University, Engineering Faculty, Chemical Engineering Department, P.O.B. 3, 407000 Ariel, Israel ABSTRACT: Liquid marbles are nonstick droplets wrapped by micro- or nanometrically scaled colloidal particles, representing a platform for a variety of chemical, biological, and microfluidics 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. This feature article reviews properties and applications of liquid marbles. Liquid marbles demonstrate potential as microreactors, microcontainers for growing micro-organisms and cells, and microfluidics devices. The Marangoni-flow-driven self-propulsion of marbles supported by liquids is addressed.

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from miniature chemical reactors to flow self-propelled objects driven by Marangoni flows. The actuation of liquid marbles by electric and magnetic fields4 opens a diversity of their microfluidics applications. The presented feature article is devoted to the unusual physical properties of liquid marbles on one hand and their promising applications on the other hand.

INTRODUCTION Liquid marbles, which are droplets coated with colloidal particles, depicted in Figure 1, started their brilliant career from

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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 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 monodisperse 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 electrostatic,17 and electrostatic double-layer interactions.18 Such monodisperse 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 coated by disordered particles separated by water clearings, connecting the liquid filling a marble to the atmosphere.1−4,21,22

Figure 1. Twenty microliter liquid marbles coated with (A) Teflon, (B) lycopodium, and (C) carbon black.

the series of papers published by David Quéré and coauthors, in which droplets were used as templates for absorbing solid, strongly hydrophobic powders (lycopodium and Teflon).1−4 Thus, nonstick (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. 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 have been reported, © 2016 American Chemical Society

Received: September 1, 2016 Revised: November 7, 2016 Published: November 8, 2016 663

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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 and 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) micropowders as a coating for marbles.40 A diversity of liquids were successfully packed within liquid marbles, including water and aqueous sodium dodecyl sulfate solutions,41 glycerol,42 ionic liquids,43 and liquid metals.44

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

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PHYSICS OF LIQUID MARBLES: THEIR EFFECTIVE SURFACE TENSION AND ELASTIC PROPERTIES Liquid marbles demonstrate very unusual properties, common to both liquid and solid interfaces. On one hand, the effective surface tension may be attributed to their surface, as was already suggested by Quéré et al.1,4 On the other hand, they demonstrate elastic properties inherent in liquid surfaces coated with colloidal particles.45−47 Liquid marbles do not coalesce, even when pressed one to another (as shown in Figure 3) or

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

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 nonstick properties to liquid marbles and prevents their coalescence, when contacted.28 The formation of liquid marbles becomes possible because of 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 a liquid/ liquid or liquid/gas boundary). Assuming the particle is small enough (typically less than a few micrometers in diameter so that the effect of gravity is negligible), the energy ΔE required to remove the particle from the interface is given by ΔE = γ12πr 2(1 ± cos θ )2

(1)

in which the sign inside the parentheses 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 γ12 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 = 90°, and as such, it may be on the order of thousands of kBT under ambient conditions.13 However, for 0° < θ < 20° and 433 K < T < 453 K, the energy ΔE required to remove the particle from the fluid−fluid interface falls to ΔE ≤ 10kBT.12,13 Thus, liquid marbles are easily formed from either hydrophobic powders (as shown in Figure 1A,B1−4,28−30) or slightly hydrophilic31,32 powders such as carbon black or graphite (Figure 1C). It was suggested that the Cassie−Baxter wetting occurring at the interface separating aggregates of hydrophilic particles from the liquid promotes the formation of liquid marbles.32 An analysis of the floating of colloidal nonspherical 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

Figure 3. Twenty microliter water marbles coated with 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).

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 γeff 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,52and (6) the pendant marble method.53 The effective surface tension of a liquid surface coated by colloidal particles also may be extracted from the 664

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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 γeff ≅ 45 − 73 mJ/m 2 were 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 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 and 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 springlike force driven by the surface tension, introduced by Lord Rayleigh.57 The analysis of the deformation of liquid marbles (made a in linear approximation) resulted in the following expression for an effective Young modulus of liquid marbles E ̃Young =

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, transverse to the liquid surface, was estimated in ref 59 as E Young ≈ 2πnrγ |cos θY − cot 3 θY|

where θY is the Young angle of the particles and 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 lycopodiumcoated water surface (r ≈ 1 μm; n ∼109 m−2) yields EYoung ≈ 100 Pa, which is at least 2 orders 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 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 a 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

4γeff R0

⎛ ς ⎞2 π W = G⎜ ⎟ S ≅ Ghsrel 2 ⎝R⎠ 2

(2)

0

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 1−ν γ 1+ϕd

(5)

where G and R are the elastic modulus the radius of the shell (the value of R may be considered to be equal to the radius of the marble, but 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 the magnitude of the characteristic dimension of a colloidal particle, ζ 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 to understanding collisions of liquid marbles and scaling laws governing their shape.60 The physics of rolling liquid marbles was addressed in refs 3, 4, and 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

where R0 is the radius of the nondeformed marble. Considering the nonlinear terms of the strain ε yielded 4γ E ̃Young = Reff (1 − ε + ε 2).58

E Young ≅

(4)

(3)

where γ is the surface tension of a liquid, d is the diameter of the solid particles, v is the Poisson ratio of solid particles, and ϕ is the solid fraction of the interface. They 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 bidisperse particles was experimentally established in ref 46. However, the estimation of elastic properties of colloidal layers coating liquid marbles is a challenging task because of the fact that marbles are usually coated with 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;

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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 h) when transferred onto the surface of liquid water, provided that the solution pH of the subphase was above 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 The 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.71−75 665

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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 microreactors, enabling blood typing,81 copolymerization reactions,82 and even ultratrace 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 microphysical devices, in particular, as microcentrifuges and miniature viscometers.88 The connection of the liquid filling a marble to the atmosphere also gave rise to the self-propulsion of liquid marbles containing aqueous ethanol solutions placed on liquid supports.89 The successful synthesis of hydrophobized fumed fluorosilica powders enabled the manufacturing of liquid marbles containing aqueous ethanol solutions, characterized by low surface tension.37,89 When marbles containing aqueous ethanol solutions are supported by water, they are selfpropelled by Marangoni solutocapillary flow, developed by the evaporation of ethanol from a marble followed by its condensation on the water surface. 89,90 Consider the spontaneous increase in the 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.

Ferrofluid 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 an electric field was performed in refs 72 and 73. Liquid marbles may be not only actuated but also manufactured under the 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.

Figure 4. Janus water marble (40 μL) built from carbon black and Teflon hemispheres.

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APPLICATIONS OF LIQUID MARBLES: FROM MINIREACTORS TO SELF-PROPULSION As has already been mentioned, liquid marbles are respiring systems: the bulk of a marble is connected to the atmosphere via water clearings, as shown in Figure 5. Liquid marbles, because of their small volumes (typically on the order of microliters), 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

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

This increase will give rise to the Marangoni flow, resulting in the force F⃗ (shown with the 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 self-propulsion.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 ≅ Figure 5. Scheme of a microreactor based on respiring liquid marbles. 666

|∇γ | a ηw

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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 (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 Marangoni-driven-flow driven motions, usually ranging 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 of mass of a marbles scales with its volume as vcm ≅ V1/2 (details in ref 90). 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 light-controlled displacement of liquid marbles makes it possible to not only transport the 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 laser-driven 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 photoreversible 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 The self-propulsion of liquid marbles opens novel pathways for their microfluidic applications.

miniaturized chemical and biological processes, including the growth and preservation of micro-organisms and cells. The Marangoni thermocapillary and solutocapillary flow-driven propulsion of liquid marbles opens novel pathways for their microfluidic applications, enabling controlled transport and release of the microliter-scaled quantities of liquids contained in the liquid marble.89,92−94

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AUTHOR INFORMATION

Corresponding Author

*Phone: +972-3-906-6134. Fax: +972-3-906-6621. E-mail: [email protected]. ORCID

Edward Bormashenko: 0000-0003-1356-2486 Notes

The author declares no competing financial interest. Biography

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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 the 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 past decade, he has devoted his research to interfaces with prescribed wettability, including nonwetted surfaces and nonstick droplets.

CHALLENGES IN THE FUNDAMENTALS AND APPLICATIONS OF LIQUID MARBLES Only a few works reporting liquid marbles coated with monodisperse 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 ≈ 0.01−0.1 μL) also remains a challenging experimental task. The opposite problem of the use of liquid puddlelike macroreactors where the surface area-to-volume ratio is maximized rather than minimized may also be of interest.97 Opening and closing of the liquid marble particle achieved via acoustic levitation enables the possibility to manipulate liquid marbles coated with nonferromagnetic particles and opens new pathways for the use of liquid marbles as microreactors.98

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

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REFERENCES

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CLOSING REMARKS Liquid marbles, as highly mobile nonstick droplets, present an alternative to superhydrophobicity. They may be actuated by chemical and physical stimuli, including electric and magnetic fields and 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 667

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