Confinement-Induced Alteration of Morphologies of OilâWater

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Confinement induced alteration of morphologies of oil-water emulsion Soumita Maiti, Nitish Singh, and Animangsu Ghatak Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00067 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Confinement induced alteration of morphologies of oil-water emulsion Soumita Maiti1, Nitish Singh1,*, Animangsu Ghatak1,2,* [1] Department of Chemical Engineering, Indian Institute of Technology Kanpur, 208016 [2] Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, 208016 (India) [*] Prof. A. Ghatak, Corresponding-Author, Author-Three, E-mail: [email protected] Nitish Singh, Corresponding-Author, Author-Two, E-mail: [email protected]

Abstract Reversible alteration between different emulsion morphologies like core-shell and Janus is conventionally triggered by altering the interfacial energy between different phases. In contrast, here we show that the morphology of dispersed droplets can change also when the emulsion is sufficiently confined between two parallel plates. In particular we use three immiscible phases: silicone oil, paraffin oil and aqueous solution of surface active agents like Agarose, SDS, AOT, CTAB to generate oil in water emulsion consisting of complex morphologies of the dispersed droplets. At the unconfined state, the core-shell drops appear with paraffin oil at core and silicone oil at the shell. However, the morphology of oil droplets changes to Janus when the emulsion is confined between two parallel plates. We have shown that the meniscus of the continuous phase that forms between the parallel plates, alter the pressure field in the emulsion and the total energy of the system, which trigger such morphological transition.

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Introduction Emulsions are dispersion of one liquid phase in the form of fine droplets into another. Emulsions are extensively used in numerous applications in our daily life [1], e.g. medicines [2], cosmetics [3], paints [4], food items [5], and so on. In these various applications, the properties of the emulsion depend upon several parameters: prominent ones among them are shape, size and morphology of dispersed droplets. The droplet shape can be simply spherical, ellipsoidal or ones having complex morphologies [6-12] like core-shell or Janus type, multiple core-shell (triple, quadruple and higher), core-shell Janus and so on. These droplets are stabilized against coalescence by adding surface active agents like molecular surfactants [13] that decrease the interfacial tension between different phases or by using solid colloidal particles that increase the electric double layer [14,15] or steric repulsion [16-18]. In some cases, alteration of pH [19,20] or temperature [21,22] of the emulsion too gives stability to the emulsion. Beyond emulsion stability, many applications demand also change in emulsion morphologies, e.g. core-shell to Janus and vice-versa, which require small yet definite alteration in interfacial energy between the dispersed and the continuous phases. A surface active agent is conventionally used for altering the morphology of the dispersed phase [23]. In some cases, two different surfactant molecules, which result in two different emulsion morphologies, are used in suitable relative quantity in order to reversibly switch between these morphologies [6]. Yet in others, an additional external trigger is generally employed to drive transition from one emulsion morphology to another: temperature, pH or UV/blue light irradiation [6, 7, 9, 24]. Furthermore, alteration in emulsion drop morphology is affected also by using an anionic liquid motif [19] or by changing the relative conductivity of the two jetting solutions during electrodynamic co-jetting [25]. Here we show that confinement of an emulsion can also be used as a trigger for alteration in morphology of emulsion drops. In essence, we have used silicone, paraffin oil and water as three


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immiscible phases. In order to form stable oil in water emulsion we have used four different surface active agents: Agarose, a bio-polymer and three commonly used surfactants such as Sodium

dodecylsulphate

(SDS),

Dioctyl

sodium

sulfosuccinate

(AOT)

and

Cetyl

trimethylammonium bromide (CTAB). The interfacial tension between different phases of these systems is such that in an unconfined state, the dispersed droplets of oil in the continuous aqueous phase, assume core-shell morphology which remains stable over extended period of time and even after agitation for a long time. However, when a drop of the emulsion is confined between two parallel plates, although initially the dispersed droplets of oil continue to remain core-shell, with decrease in gap between the plates, the droplets turn Janus. In particular, the emulsion drop, i.e. the aqueous phase forms a liquid disk with a concave meniscus between the two plates. The excess surface energy of this meniscus alters the energetics of the system so that the core-shell, which remains the energetically favored morphology at the unconfined state, alters to Janus, the more favored one in the confined state. Materials and Methods Materials Agarose (high melt, used for electrophoresis) and Silicone oil (oil bath for up to 250 0C, viscosity 330-370 cSt) were purchased from Loba Chemie Private Limited. Paraffin oil (light) was purchased from Fisher Scientific. The surfactant Sodium dodecyl sulfate (SDS) was procured from Marc; and dioctyl sodium sulfosuccinate (AOT) and Cetyl trimethylammonium bromide (CTAB) were procured from Loba Chemie Private Limited. Sylgard 184 elastomer along with the curing agent was procured from Dow Corning. These chemicals were used as received without purification. Millipore deionized (DI) water was used in all the cases. Methods Preparation of emulsion: Agarose solution (0.1% w/v) (represented as aq-Agarose throughout this paper) was prepared by dissolving required quantity of Agarose powder in DI water heated 3 ACS Paragon Plus Environment

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to elevated temperature ~90o C. Emulsion of different oils: Silicone oil and Paraffin oil were prepared by dispersing the respective liquid in the aq-Agarose solution while stirring it vigorously. In all cases 30ml oil was dispersed in 100ml of the aq-Agarose by stirring at 14,000 rpm for 4 minutes. Paraffin oil, Silicone oil and aq-Agarose were used as three immiscible phases to form the complex emulsion. Silicone and Paraffin oil were used in 1:1 volume ratio. 15 ml of paraffin oil was first added to aq-Agarose to form an emulsion; 15 mL of silicone oil was then added to it with continued stirring which resulted in the complex emulsion. In another set of experiment, required quantity of SDS was dissolved in DI water to prepare 1mM solution of the surfactant, in which silicone and paraffin oil were dispersed in equal volume. Similarly, 0.01mM aqueous solution of CTAB and 0.001 mM aqueous solution of AOT were also prepared for forming the oil (silicone and paraffin oil dispersed in equal volume) in water emulsion. Measurement of surface/interfacial tension: Surface tension of aq-Agarose was measured using pendant drop method [26] in which the contour of a small drop of the liquid, hanging from a vertically placed syringe needle (in air), was extracted using Data Physics OCA 35. The contour was then fitted to Young Laplace equation to obtain the surface tension of the liquid. In order to measure the interfacial tension between two different liquids, e.g. aq-Agarose and Silicone oil or paraffin oil, a hanging drop of aq-Agarose was generated from a syringe needle inside a pool of silicone oil or paraffin oil. Here again the contour of this drop, fitted to Young Laplace equation, resulted in the interfacial tension between the two liquids. In all these cases, the volume of the drops was close to the critical detachment volume as depicted in images presented in Table 1. Density of aq-Agarose, Silicone and Paraffin oil were taken to be 1 gm/cm3 (i.e. density of water), 0.97gm/cm3 and 0.86 gm/cm3 respectively. The surface and interfacial tension values for different liquids and different pairs of liquids are presented in table 1 (details presented in Table S1 of supporting information).


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Drop phase

Aqueous solution of Agarose (0.1%)

Silicone oil



Paraffin oil

Silicone oil

Ambient

Air

Air

Air

Paraffin oil

Silicone oil

Paraffin oil

γ (mN/m) (Pendant drop method)

63.8±0.23

25.5±0.96

24.6±0.70

39.6±0.90

29.7±0.79

0.72±0.083

γ (mN/m) (Owens and Wendt equation)

63.0

25.5

24.3

39.7

29.7

0.78

Table 1. Surface and interfacial tension of different liquids estimated experimentally using pendant drop method (data presented in last but one row) and by using Owens and Wendt equation (last row).

Estimation of interfacial tension using Owens and Wendt equation [27]: The interfacial tension between two different liquids 1 and 2 or that between a solid and liquid were estimated also using the relation proposed by Owens and Wendt [27]:

[

g 12 = g 1 + g 2 - 2 g 1d g 2d + g 1p g 2p

]

(1)

Here, g id and g ip represent respectively the dispersive and polar components of surface tension of species i =1 and 2. The surface tension of a material is the summation of the dispersive and polar component of surface tension: g i = g id + g ip . Equation 1 when combined with Young’s equation: g 2 = g 1 cos q + g 12 yields equation 2:

[

g 1 (1 + cos q ) = 2 g 1d g 2d + g 1p g 2p


]

(2)

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Equation 2 can be used for determining the surface tension of a material using the known values d p of g and g of two other materials and the respective static equilibrium contact angles q at the

three phase contact line. We have shown in the supporting information that Equation 2 yields the d = 24.9 mN/m, estimate of surface tension of polystyrene (PS) surface as, g PS = 27.7 mN/m ( g PS d g PSp = 2.83 mN/m) and that of glass cover slip (CS) as g CS = 72.13 mN/m ( g CS = 0.003 mN/m, p g CS = 72.13 mN/m) respectively. Using the surface tension values for PS and CS surfaces, it is

possible to obtain that of aq-Agarose. On PS and CS surface, a 10 µl sessile drop of 0.1% w/w aq-Agarose forms contact angles, q Ag-PS = 84.7 o ± 0.12 o , q Ag-CS = 45.2 o respectively, using which its surface tension is calculated as g Ag = 63.58 mJ/m2. Similarly, surface tension values of Silicone and paraffin oil were estimated as g si = 24.3 mN/m ( g Sid = 23.42 mN/m, g Sip = 0.83 d p = 25.47 mN/m, g Par = 0.003 mN/m) respectively. These mN/m) and g par = 25.5 mN/m ( g Par

values corroborate very closely with that obtained from experiment as in Table 1 (last but one row): g Ag = 63.8 ± 0.2 mN/m, g si = 24.6 ± 0.7 mN/m and g par = 25.5 ± 1 mN/m. Using equation 1, the interfacial tension between aq-Agarose (0.1% w/w solution of Agarose in water) and Silicone oil or Paraffin oil were estimated as, g Si_Ag = 29.7 mN/m and g Par_Ag = 39.7 mN/m respectively and that between silicone and paraffin oil is found to be, g Si_Par = 0.78 mN/m. These interfacial tension values too matched very closely with that obtained from the pendant drop experiment:

g Si_Ag = 29.7 ± 0.8 mN/m and g Par_Ag = 39.6 ± 0.9 mN/m and g Si_Par = 0.72 ± 0.08 mN/m. Optical microscopy of emulsion droplet: A drop (10 µl) of the emulsion was placed on a microscope glass slide and was then covered with a cover-slip to form a sandwiched liquid disk between the plates. The gap between the plates was adjusted by placing spacers of desired height 6 ACS Paragon Plus Environment

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(40 -100 µm) between them at their two ends. The emulsion droplet thus confined was viewed under optical microscope. Since, individual droplets of the dispersed liquid could be identified, the continuous phase of the emulsion was not required to be diluted unlike in previous studies [28,29] for which crowding of dispersed droplets required that the aqueous phase of the emulsion be diluted. Results and Discussion Both Silicone and paraffin oil was found to form stable emulsion when dispersed in aq-Agarose; the emulsion remained stable even after 3 months. Agarose was found to stabilize emulsion with several other oils, e.g. Chlorobenzene, Toluene, and Paraffin oil (supporting information, Figure S1). From consideration of surface energetic, stable emulsion of a pair of liquids 1 and 2 is expected when the interfacial tension between liquids 1 and 2 becomes smaller than the sum of their respective surface tension: g 12 < g 1 + g 2 . The surface and interfacial tension values presented earlier show that for both (Silicone oil, aq-Agarose) and (Paraffin oil, aq-Agarose) this condition gets satisfied.


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B

RB A

B

I

RB

I

RB

RAB

RA

(a) (d)

B

I

A

RA

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RA

(b)

A (c)

Volume ratio of silicone oil to paraffin liquid 1:1

3:2

2:1

1:2

1:5

250 µm

tstorage =

(e)

20 minutes

1.5 months

3 weeks

(f)

(g)

100 µm

Figure 1: (a-c) Schematic of different complex emulsion morphologies. (d) Morphology of core-shell emulsion drops in presence of agarose: Shell thickness decreases with increase in volume ratio of silicone and paraffin oils, signifying that paraffin oil forms the core whereas silicone oil remains at the shell. (e-g) An emulsion with core-shell droplets was stored for extended period of time and was examined at different time intervals, which showed that the droplets continued to remain core-shell without any change in morphology.

Generation of complex emulsion: We will now show that using Agarose as the stabilizer it is possible to generate complex emulsion by controlling interfacial tension of three different pairs of liquids. In complex emulsions two different immiscible phases, e.g. two different oils remain dispersed in a third immiscible continuous phase, e.g. water and instead of forming discrete spherical droplets, they assume morphologies like core-shell and Janus as represented schematically by figures 1(a, b) and figure 1(c) respectively. Previously several systems have 8 ACS Paragon Plus Environment

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been shown to form complex emulsions, e.g. hexane and perfluorohexane in an aqueous medium containing a surfactant [6,7]. The condition at which different such morphologies can occur has been deduced by analyzing the interfacial tensions. For example, with I as the continuous medium, A as the core and B as the shell forming liquid, the core-shell morphology in figure 1(a) has been shown to occur[6] when, g AB < g AI - g BI , i.e. when spreading coefficient,

S = g AI - g BI - g AB corresponding to spreading of B on A in a medium I is positive. On the other hand, the Janus morphology in figure 1(c) has been thought to occur [6, 29-32] when

g AI - g BI » g AB . These points are verified by dispersing two immiscible oils: Silicone oil and Paraffin oil in aq-Agarose solution. The interfacial tension between different pairs of liquids: g Si_Ag = 29.7 mN/m, g Par_Ag = 39.7 mN/m and g Si_Par = 0.78 mN/m satisfy the condition that, g Si_Par h , suggesting that their shape may alter from being spherical to a liquid disk, the optical micrograph in figure 2(d) shows that droplets, with diameter significantly smaller than the gap, too turn to Janus implying that confinement of the emulsion drop and not just that of the dispersed oil droplets triggers this transition of morphology. Figure 2(d) shows that the Janus droplets remain randomly oriented suggesting that density difference between silicone and paraffin oil is not an important parameter. At the insets of figure 2(b) and (d) we show also the images of solid poly(dimethylsiloxane) (PDMS) particles that were prepared by this complex emulsion route. Here sylgard 184 elastomer mixed with the curing agent was used in place of silicone oil. For PDMS oligomer-paraffin oil too complex emulsion morphologies like core-shell and Janus were achieved at unconfined and confined state respectively. The emulsion was then heated to ~80o C at which the PDMS portion of the droplet crosslinked leading to solid particles. For core-shell morphology, the particles were hollow, whereas, for Janus morphology the particles were near-hemispheres. It is worth noting that the particles corresponding to core-shell morphology are found to be punctured. Such morphology consisting of punctured shell formed via release of the uncross-linkable liquid at the core has been generated also by other authors [33]. It is worth noting also that none of the particles, however small, was found to be complete sphere (supporting information figure S2).


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Figure 3. An emulsion drop (consisting of Silicone and paraffin oil dispersed in aq-Agarose) was confined between two glass plates, the gap between which was spatially varied from h = 93 µm to 55 µm and back to 93 µm. The 12 ACS Paragon Plus Environment

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optical micrographs (a) to (f) correspond to a specific location where the gap was varied by moving in and out the spacer. Images (a) to (c) correspond to gap being decreased from 93 µm to 55 µm and (d) to (f) correspond to the reverse cycle of the gap being increased from 55µm to 93µm. The sequence of images shows that core-shell morphology alters to the Janus morphology. (g-i) A similar set of experiments carried out using surfactant SDS (1 mM aqueous solution) as the stabilizing agent shows the formation of core-shell emulsion droplets at larger gap, which transforms to Janus morphology when the gap between the plates is diminished. (j, k) Similar phenomenon is observed also when 0.01 mM aqueous solution of CTAB and 0.001 mM aqueous solution of AOT are used as the continuous phase.

Reversibility of the phenomenon: In order to examine if the transition from core-shell to Janus was reversible, experiments were done using spacers of two different heights between the plates, so that the gap h could be varied along the length as shown in figure 3. Such an experiment allowed also the gap between plates at any given lateral location to be altered by simply sliding in and out one of the spacers, while keeping the other one fixed. Thus, the confinement of the emulsion drop could be dynamically varied. For figure 3(a-f), the gap at a typical location of the emulsion drop was first decreased from 93 µm to 50 µm by sliding out the right spacer and was then increased back to its original value by sliding it in. Optical micrographs in figure 3(a-b) show that the morphology continued to remain core-shell till h = 65 µm was reached but turned to Janus for h = 55 µm. The morphology however continued to remain Janus during the reverse cycle even when the gap was increased from 55 µm to 93 µm. Several sets of experiments showed this sequence of events leading finally to apparently irreversible transition to the Janus morphology. Hypothesizing that this irreversibility was possibly because of energy barrier associated with the viscous dissipation in the shell forming liquid, following sequence of steps was carried out: first the emulsion (with core-shell dispersed droplets) was confined between two parallel plates which resulted in the transition to Janus phase as in figure 3(a-c).The plates were then separated out and using a sharp razor blade the liquid layer adhered to these plates was scraped and collected in a vial. This process was repeated several times so that ~8 cc of liquid could be collected, which was then subjected to agitation at 1000 rpm for ~45 minutes. The 13 ACS Paragon Plus Environment

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emulsion was then examined by confining a drop of it, between two glass plates kept separated by h = 138 μm , which indeed showed reconfiguration of the Janus morphology to core-shell (Figure 4a,b). This observation showed that, core-shell was the equilibrium morphology of the dispersed droplets in the emulsion which reversibly altered to Janus when the emulsion was sufficiently confined. Generality of the phenomenon: In order to examine the generality of the phenomena, experiments were done also using three different surfactants as the emulsion stabilizer: SDS, AOT and CTAB. 1 mM of aq-SDS solution was used as the continuous medium in which silicone and paraffin oil was dispensed as before. The resultant emulsion was examined by confining a drop (10 µl) of it between two parallel plates as before. The sequence of images in figure 3(g-i), show that the dispersed droplets of oil were smaller in size than that attained for aqAgarose as the stabilizer. Initially, these droplets assumed core-shell morphology when the gap between the plates was h = 93 µm, but transformed into Janus when the gap was reduced to

h = 55 µm. The Janus droplets continued to remain so even when the confinement was relaxed, i.e. the gap was increased to h = 93 µm signifying that here too viscous energy barrier prevented the Janus droplets from transforming back to core-shell. Similar set of observations were made also when 0.01 mM aqueous solution of CTAB (Figure 3j) and 0.001 mM aqueous solution of AOT (Figure 3k) were used respectively as the emulsion stabilizer. These experiments confirm that the concentric arrangement of core and the shell liquid within the dispersed droplets turns unstable when the emulsion is confined, and this phenomenon occurs irrespective of the surfactant used.


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(a)

(b)

(c)

25 μm

25 μm

25 μm

Figure 4: (a) A small quantity of the emulsion was first confined between two parallel plates separated by h = 46 μm leading to the formation of Janus droplets. The emulsion drop was scrapped by using a sharp razor blade and was collected in a vial. (b) Optical micrograph of a small sample of this liquid shows that the dispersed droplets continue to be Janus. (c) On agitation of this liquid, the Janus morphology of the dispersed droplets transforms to core-shell morphology. For both (b) and (c) the emulsion was examined by confining it between two parallel plates separated by h = 138 μm .

I H

pI

h

patm

patm

pI = patm

B

I

RA

A

RB pB1

pA

pB2

RB

B RA

I

pI

A

pA

pB1 pB2

pI

q

L(q )

(b)

(a)

Figure 5. Schematic of core -shell morphology of dispersed droplets in emulsion.


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Analysis of the effect of confinement: Transition from core-shell to Janus occurs via a sequence of two consecutive events: first the liquid core migrates from the concentric location within the shell to one side of it; followed by that, the thin film of the shell liquid ruptures leading to the Janus drops. The migration of the core liquid can be rationalized by estimating the Laplace pressure across various interfaces as depicted in schematic diagrams in figure 5. Here, the pressure across the interface between liquid A and B and that between B and I can be written as, p A - p B1 =

2g AB 2g and p B2 - p I = BI respectively, in which p B1 and p B2 are pressure in the RA RB

shell liquid B at the vicinity of interfaces with core A and the continuous phase I respectively. For an unconfined emulsion, in the continuous phase, p I = p atm , and in shell liquid pressure is at equilibrium everywhere, so that p B1 = p B2 = p atm +

2g BI . For a confined emulsion, however, the RB

pressure field changes because of the appearance of the meniscus between the plates confining the emulsion. For gap h between plates, the pressure p I now alters to p I = p atm result, the pressure p B2 is now expressed as, p B2 = p atm -

2g I . As a h

2g I 2g BI + . Since, pressure p B1 is h RB

expected to remain unaltered, a radial pressure gradient develops within the shell liquid, which in the limit of linear variation can be written as:

p - p B2 ¶p B 2g I = - B1 =. Here, L(q ) defines L(q ) hL(q ) ¶L

the liquid shell thickness at any angular location as shown in figure 5. For concentric morphology of core and shell, L remains q symmetric: L = RB - RA .But as the core displaces off-center, the shell thickness L and pressure p B , both vary with q . As a result, a pressure gradient develops in q direction which can be deduced as,

¶p B ¶p B ¶L(q ) 2g I ¶L(q ) = × =. ¶q ¶L ¶q hL(q ) ¶q


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Since, the variation in shell thickness is such that

¶p B ¶L(q ) < 0, > 0 , this relation shows that ¶q ¶q

implying that p B decreases with increase in q . In other word, the pressure field is maximum at the location of minimum shell thickness, L0 = L(q = 0) , but decreases away from it, which is expected to drive the liquid out of the location, q = 0 . This gradient in pressure gets stiffer with decrease the shell liquid thickness L(q ) implying an unstable situation in which, any perturbation from the concentric morphology is expected to grow, with the minimum shell thickness L0 getting smaller and the core migrating towards the inner periphery of the shell. Thus, the concentric arrangement of the core and shell liquid is unstable with respect to the decrease in pressure field in the continuous liquid phase I and this effect is expected to get stronger for larger interfacial tension g I and smaller gap h between the plates. Nevertheless, the above analysis suggests also that the core is expected to migrate back to the concentric location when gap h is sufficiently increased. This particular effect was verified by carrying out the following experiment: in which paraffin oil (A), silicone oil (B) and aqueous solution of 1 mM SDS (I) were used as the three phases. The optical micrographs in Figure 6 show that at

h = 93 µm, the dispersed droplets of oils were core-shell with the core (silicone oil) and shell (paraffin oil) liquid being concentric (figure 6a). However, as the gap was decreased to

h = 65 µm, the core migrated to one side within shell; nevertheless, the morphology continued to remain core-shell as the thin liquid film of the shell did not undergo rupture (figure 6b) and there was no transition to Janus. Figure 6(c) shows that as the gap was then increased back to

h = 93 µm, the core once again migrated back to the concentric location with the shell.


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(a)

h = 93 µm

(b)

silicone oil

paraffin oil Aq-SDS (1 mM)

Page 18 of 26

65 µm

(c)

thin film of shell

93 µm

50 µm

Figure 6. (a) Optical micrograph shows a core-shell drop of silicone and paraffin oil dispersed in an aqueous solution of SDS. The emulsion was confined between two parallel plates separated by a gap of h = 93 µm. (b) As h was decreased to 65 µm, the core migrated to one side of the shell. (c) The core liquid migrated back to the concentric location with the shell, as h was increased to 93 µm.

Figure 7. (a-f) Sequence of images capture the transformation of core-shell to Janus droplet. The transformation occurs via drainage of the shell liquid forming a thin film that separates the core from the continuous phase outside; the thin film eventually ruptures leading to the Janus droplet. (g, h) The images show conversion of a core-shell droplet of diameter d = 35 µm to Janus. (i, j) Similarly, the images represent a droplet of d = 40 µm. (k-n) Sequence of images depicts transition of a core-shell emulsion drop confined between the parallel plates, with h = 48 µm to Janus when SDS is used as the emulsion stabilizer instead of Agarose. (o) The time for rupture of the thin liquid shell,

t tran , as obtained from images (a-j), is plotted against the ratio of volume of shell ( VSilicone ) and

core liquid ( VParaffin ) which shows that

ttran ~ (VSilicone VParaffin )

-2

.


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Rupturing of thin film of shell liquid: In order to examine the final phase of transition of the morphology of the core-shell droplet, a high frame rate camera was used. The optical micrographs in Figure 7 capture this sequence of events in which the core sphere first slides to one side (figure 7a-c), so that the thickness of the liquid shell increases at one side, at the expense at the other. The liquid at the shell continues to drain out from the thinner portion resulting in further thinning of the shell, although the drop morphology continues to remain coreshell. At a critical state, the thin film of the liquid shell ruptures (figure 7c). Followed by it, the thin layer of the shell liquid dewets off the core, finally resulting in the Janus morphology. Despite this morphological transition, the over-all size and shape of the complex droplet remains unaltered. It is worth pointing out that this mode of appearance of the Janus morphology is different from other known mechanisms, for example, fusion of two individual droplets of the two immiscible oils: Silicone and vegetable oil in aqueous solution of a surfactant resulting in a single Janus drop [34, 35]. In such cases, Janus is the energetically most favorable state; therefore, the core-shell morphology does not occur at all, instead, Janus morphology occurs directly following collision of dispersed droplets. Furthermore, since such occurrence of Janus state depends essentially upon the probability of collision of droplets of two different oils, this process continues over long period of time, e.g. ~72 hours in contrast to that presented in figure 7 in which transition of core-shell to Janus occurs over fraction of a second. Here t = 0 is taken as the state at which the core almost comes into contact with the shell. The analysis of time of occurrence of different events then shows that whereas both the thinning of the liquid shell and then dewetting followed by rupture happens over relatively longer time, the rupturing of the thin film occurs rather catastrophically very much similar to rupturing of ultra-thin liquid film driven by disjoining pressure [36]. Preliminary observations suggest also that time, ttran for rupture of


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the shell liquid film varies with the relative quantity of the two oils present in the complex

(

droplet, ttran ~ VSilicone VParaffin

)

-2

.

Analysis of energy of the system: In order to examine if transition from core-shell to Janus is energetically favorable, the total energy of the system was estimated; it consisted of that of all interfaces including that of the meniscus of the emulsion droplet sandwiched between the parallel plates. For the core-shell as shown in figure 1(a), this energy can be written as

P i = 4pRA2 g AB + 4pRB2 g BI + pHlg I 2 . Here H is the initial gap between the parallel plates and l is the length of the meniscus or the perimeter of the sandwiched liquid disk. We have assumed that the continuous phase of the emulsion completely wets the surface of the plates so that the contact angle is zero. In the context of a single oil droplet, l can be taken as ~ 2 RB . As the gap between the plates is reduced to h , the morphology alters to Janus, for which the energy is given as, P f = 2pRB2 g BI + 2pRB2 g AI + pRB2 g AB + phlg I 2 . The core-shell can spontaneously alter to Janus, when P f £ P i . We can consider a specific case in which the constituent oils forming the complex drop are in equal volume, so that, corresponding to a core-shell, the relation between 13 RA and RB is derived as: RA = RB 2 . Substituting it into above expressions result in the 2 condition for transition from core-shell to Janus: g AI - g BI - 0.76g AB < (H - h )lg I 4 RB . Noting

that in our experiments, A, B and I represent Silicone oil, Paraffin oil and aq-Agarose respectively and putting the values of the corresponding interfacial energies, e.g.

g Si_Ag = 39.7 mJ/m2, g Par_Ag = 29.7 mJ/m2 and g Si_Par = 0.78 mJ/m2, the left hand side of the above expression is calculated as 9.4 mJ/m2. Similarly, putting representative numbers, e.g.

H = 100 µm, h = 50 µm, RB = 40 µm, l ~ 2 RB ~ 100 µm and g Ag = 63.58 mJ/m2, the right hand side is estimated as 32 mJ/m2. Since the estimated value of the right hand side significantly 20 ACS Paragon Plus Environment

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exceeds that of the left hand side, the above inequality holds and transition from core-shell to Janus morphology is indeed expected to occur. In essence, the excess energy required for the Janus over and above that of the core-shell is supplied by the decrease in air-water interfacial energy at the meniscus of the liquid disk. It is worth noting also that only a fraction of that energy is used for the transition of the morphology, rest gets dissipated. Summary Although the morphology of dispersed droplets in an emulsion is routinely characterized by optical microscopy, in which an emulsion drop is confined between two parallel plates, the effect of confinement is largely neglected. Here for the first time we have shown that the excess surface energy of the meniscus formed at the periphery of such confined drop of the emulsion can be an important parameter. Particularly, it can alter the total energy of the system thereby triggering the change in morphology of a complex emulsion drop. We have used Silicone oil, Paraffin oil as the two dispersed phases and aqueous solution of several surfactants as the continuous phase to show that morphology of complex emulsion droplets alters from core-shell to Janus when confined. We have examined also the dynamics of this phenomenon which reveals that such morphological transition occurs via rupturing of thin film of the shell liquid engulfing the core. It is then natural to expect that viscous energy dissipation associated with such rupture of a thin film and subsequent flow of the viscous shell liquid acts as an energy barrier for such transition, both in forward and the reverse direction. More work however is necessary to conclude about the velocity and pressure field within the dispersed droplets and in the surrounding continuous phase that lead to the core-shell to Janus transition and dynamics of this process. Supporting Information: Agarose as a stabilizer; estimation of surface tension using Owens and Wendt equation, estimation of surface and interfacial tension of different liquids by pendant drop method; PDMS particles of different shapes as proof of formation of different morphology21 ACS Paragon Plus Environment

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liquid drops; estimation of surface tension of aq-SDS (1 mM) and aq-CTAB (0.01 mM) solutions (PDF). Acknowledgement: AG acknowledges the financial support from Department of Science and Technology, Government India, in the form of grant SB/S3/CE/036/2013. SM acknowledges help from Shubham Tiwari in carrying out pendant drop experiments. Reference 1. Aserin, A. Multiple Emulsion: Technology and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2007. 2. Ge, X. H.; Huang, J. P.; Xu, J. H.; Luo, G. S. Controlled stimulation-burst targeted release by smart decentered core-shell microcapsules in gravity and magnetic field. Lab chip 2014, 14, 4451–4454. 3. Patravale, V.; Mandawgade, S. Novel cosmetic delivery systems: an application update. Int. J. Cosmet. Sci. 2008, 30, 19–33. 4. Zhang, Q.; Wang, W. J.; Lu, Y. Y.; Li, B. G.; Zhu, S. Reversibly coagulatable and redispersible polystyrene latex prepared by emulsion polymerization of styrene containing switchable amidine. Macromolecules 2011, 44, 6539–6545. 5. Augustin, M. A.; Hemar, Y. Nano-and micro-structured assemblies for encapsulation of food ingredients. Chem. Soc. Rev.2009, 38, 902–912. 6. Zarzar, L. D.; Sresht, V.; Sletten, E. M.; Kalow, J. A.; Blankschtein, D.; Swager, T. M. Dynamically

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Graphical Abstract


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Confinement-Induced Alteration of Morphologies of OilâWater

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