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Controlled orientation of droplets in ellipsoid-filled polymeric emulsions by particle parameters and flow conditions Chaoying Mao, Yajiang Huang, Junlong Yang, Miqiu Kong, Yuan Wang, Qi Yang, and Guangxian Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02240 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017
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Controlled orientation of droplets in ellipsoid-filled polymeric emulsions by particle parameters and flow conditions Chaoying Mao,† Yajiang Huang,†,* Junlong Yang,† Miqiu Kong,‡ Yuan Wang,§ Qi Yang† and Guangxian Li†,* †
College of polymer science and engineering, State key laboratory of polymer materials
engineering of China, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu 610065, China. E-mail:
[email protected].
[email protected]. ‡
School of Aeronautics and Astronautics, Sichuan University, No.24 South Section 1, Yihuan
Road, Chengdu 610065, China. §
College of Chemical Engineering, Sichuan University, No.24 South Section 1, Yihuan Road,
Chengdu 610065, China.
ABSTRACT: The effect of particle parameters (aspect ratio, AR, and concentration) and flow conditions (gap spacing and shear rate) on droplet orientation deformation behavior in polystyrene (PS) particle-filled binary polymeric emulsions is investigated by using rheo-optical technique and confocal microscopy. Interesting vorticity orientation behavior is achieved by tailoring experimental conditions to gain rigid anisotropic droplets during slow confined shear flow. PS ellipsoids with high AR are found to reside both at the fluid interface in a monolayer side-on state and inside droplets, leading to the formation of rigid anisotropic droplets due to interfacial/bulk jamming effect at appropriate particle concentrations. In unconfined bulk
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samples, droplets with vorticity orientation can also be observed under the wall migration effect and confinement effect arising from nearby droplets. However, too strong wall confinement effect remarkably facilitates the coalescence of vorticity-aligned droplets during slow shear, eventually rendering the formation of long string-like phase aligning along the flow direction. High shear rates generate refined droplets with lower particle coverage and weak rigidity, which restrain the formation of anisotropic droplets and thus suppress droplet vorticity orientation.
KEYWORDS: polymeric emulsions, ellipsoids, vorticity orientation, shear flow The ability of microscopic or nanoscale solid particles to adsorb irreversibly at fluid-fluid interfaces due to large detachment energy offers great opportunity in creating novel low or high viscosity Pickering emulsions with superior stability,1 and therefore has attracted lots of attention in many cross disciplines such as pharmaceutical, food, cosmetics, and material fields. Generally, the addition of interfacially active particles can refine droplet size both under quiescent2 or flow conditions,3 playing a role analogous to that of conventional compatibilizers. The underlying stabilization mechanisms are suggested to be related to the reduction of interfacial tension,4 the formation of dense rigid particle layer encapsulating the droplets5 or particle-induced bridging.6 Because of the increasing importance of Pickering emulsions both in industry and academia, the influence of particle size,7 particle concentration,8 particle wettability,9 component ratio10 or particle-particle interaction11 on the formation of emulsion structure has been investigated extensively. However, the deformation and alignment of particle-stabilized Pickering emulsions during flow have received considerably less attention. Our previous work showed that interfacially active polystyrene (PS) ellipsoids were able to induce
arrested
coalescence
of
polyisobutylene
(PIB)
droplets
dispersed
in
the
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polydimethylsiloxane (PDMS) matrix during slow shear flow.12 Moreover, the anisotropic droplets formed due to arrested coalescence were found to align along the vorticity direction and rotate periodically around their major axes. This interesting phenomenon was interpreted based on the Jeffery orbit theory13 in the framework of fluid mechanics rather than by the elastic effect of droplets as usually observed at relative high shear rates.14-15 Although a plausible interpretation has been proposed for this abnormal vorticity alignment behavior, a comprehensive understanding about how particle parameters and flow conditions affect the orientation behavior of polymeric emulsions is lacking. Particle-induced arrested coalescence usually occurs when particles jam the interface at a sufficiently high surface coverage16 or disperse within droplets leading to strong internal elasticity.17-18 This phenomenon retards the relaxation process of coalescing droplets to minimize total system energy, resulting in the formation of anisotropic droplets. The exact shape of arrested droplets depends on the concentration of particles and the point where coalescence is halted.17 Though arrested coalescence is often studied in spherical particle-filled emulsions, the role of particle aspect ratio and shear conditions in the formation of anisotropic droplets has not yet been fully explored. For multiphase Newtonian fluids in a simple shear flow, the droplet deformation is mainly controlled by the viscosity ratio ( ηr = ηd / ηm , where ηd and ηm are the viscosities of droplet and matrix phase, respectively) and the capillary number ( Ca = ηmγ& RV / σ , where γ& , RV and σ denote the shear rate, the droplet radius and the interfacial tension, respectively).19 The deformation
degree
of
droplets
can
be
expressed
by
a
deformation
parameter
D * = ( l f − lv ) / ( l f + lv ) , where l f and lv are the droplet dimensions in the flow direction and
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vorticity direction, respectively.20-21 If D*>0, droplets orient along the flow direction and if
D*0.2, droplets would enter into a confined region.12 The presence of confinement exerted by the flow geometry will greatly alter the droplet dynamics (including deformation, coalescence, breakup and relaxation) under flow.23-31 For instance, Migler26 has revealed a transition from dispersed droplets to a string-like state in flowing polymer emulsions due to confinement effect. Vananroye et al.30 found that confinement suppressed breakup for low viscosity ratios (ηd/ηm) whereas opposite phenomenon occurred for high viscosity ratios. De Bruyn et al.24 reported that geometrical confinement promoted the coalescence between droplet pairs. Although the critical role of confinement on the droplet dynamics under flow has received consideration, few studies concentrate on the case of particle-filled emulsions. In this paper, we prepare PS ellipsoids with a series of aspect ratios by combining soap-free emulsion polymerization and film-stretching method. Then, special emphasis is devoted to disclosing the flow behavior of PIB/PDMS (10/90) emulsions tailored by interfacially active PS ellipsoids with different particle concentrations and aspect ratios by using in-situ rheo-optical technique and confocal laser scanning microscopy. Finally, by changing the gap spacing and relative rotation rate between shear plates, desired flow conditions (such as confinement degree and shear strength) to achieve vorticity alignment behaviour of droplets are examined. The experimental results are discussed in terms of arrested coalescence, confinement effect and viscoelasticity for a comprehensive understanding of the underpinning role of particle parameters
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and flow conditions on the vorticity orientation behavior of particle-laden emulsions under flow.
Experimental Section Preparation of particles. PS microspheres with a diameter of 1.1 µm and a density of ~1.05 g/cm3 were prepared by soap-free emulsion polymerization. Shape-anisotropic ellipsoidal particles were gained by mechanically stretching PS microspheres immersed inside polyvinyl alcohol films above their glass transition temperatures.32 As shown in Figure 1, we obtained four PS particles with various aspect ratios (ARs) ranging from 1 to 6 by controlling the stretching ratios. The dimensions of PS particles (AR1, AR1.5, AR3 and AR6) were listed in Table 1. PS particles with ARs larger than 6.0 are not considered as they tend to bend in the major axis at higher stretching ratios.
Figure 1. SEM images of PS particles with different aspect ratios: (a) 1.0 ± 0.1; (b) 1.5 ± 0.2; (c) 3.0 ± 0.7; (d) 6.0 ± 1.4. Table 1. Dimensions of PS particles as measured from SEM images. Abbreviation
Major axis (µm)
Minor axis (µm)
Aspect ratio
AR1
1.08
1.08
1.0 ± 0.1
AR1.5
1.42
0.94
1.5 ± 0.2
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AR3
2.30
0.78
3.0 ± 0.7
AR6
3.81
0.65
6.0 ± 1.4
Preparation of blends. Pure blend consisting of PIB (Daelim Industrial Co., Ltd., Korea, ρ PIB =0.901 g/cm3) and PDMS (Aldrich, America, ρ PDMS =0.971 g/cm3) with weight ratio of 10/90 and 90/10 were prepared by hand-mixing in a 25 mL beaker for 30 min with a spatula. Both PIB and PDMS are Newtonian fluids with a zero-shear viscosity of about 80 and 118 Pa·s at 30 °C, respectively. The interfacial tension between them is ~1.78 mN/m.33 For particle-filled blends, PS particles with different ARs (1-6) and concentrations, φPS , (0-2.0 wt.% based on the total weight of the blend) were mixed with these two polymers simultaneously. Then all samples were degassed in a vacuum oven at 40 οC for 12 h before further experiments. The obtained morphology of blends is nearly independent of the hand-blending procedure.
Morphological characterization. The shear-induced morphology evolution of emulsions was investigated online at 30 οC by an Olympus BX-51 optical microscope equipped with a Linkam CSS-450 shearing hot stage. Every fresh sample was held in the gap between two parallel quartz disks and sheared by the rotating bottom disk controlled by a precise motor while the top disk kept stationary. All samples were first presheared at 20 s-1 for 300 s to generate a homogenous dispersion and to eradicate the sample history and then sheared at a relatively low shear rate ( γ& =0.05-1 s-1) for ~2000 strain units. The gap spacing (H) was changed from 100 to 1800 µm. The optical images were taken every 60 s. The volume average radius (RV) of ~200 droplets was calculated by using the following equation (for those anisotropic droplets, equivalent radius was calculated), ∞
∞
RV = ∑ni Ri4 / ∑ni Ri3 n =1
(1)
n =1
where ni is the number of droplets with the radius of Ri (measured directly from optical images).
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A confocal laser scanning microscope (CLSM, Laica TCS SP8, Germany) with a 63 × (NA=1.40) oil immersion objective was used to observe the distribution of Coumarin 6-labeled PS particles within emulsions.34 When measuring the three-phase contact angle of individual fluorescent PS microspheres in the emulsions, only those microspheres which were located at the equatorial plane of PIB or PDMS droplets were considered.
Results and discussion Particle localization in emulsions. The equilibrium localization of solid particles in polymeric emulsions is mainly controlled by thermodynamic and kinetic effects.35 Based on Young’s equation and the surface tension of materials,36-38 the wetting parameter of PS particles,
ω = ( γ PS−PIB − γ PS−PDMS ) / γ PIB−PDMS , is calculated to be -0.48 corresponding to a theoretical threecontact angle ( θ1 ) of ~119° in the PIB/PDMS (10/90) emulsion (measured through PDMS phase ) and a θ 2 of ~61° in the inverse 90/10 emulsion (measured through PIB phase). By using confocal microscopy (Figure 2a and a'), the three-phase contact angle of a single PS microsphere labeled by a fluorescent dye (Coumarin 6) is determined directly. We enhance the laser intensity to distinguish two polymer phase as a little fluorescent dye migrates from PS microspheres into the PIB phase. We find that θ1 =123°±5° and θ 2 =57±6°, which are consistent with predicted values. Similar confocal microscopy experiment is not performed with ellipsoids as they produce saddle-shaped contact lines at the interface,39 making it difficult to determine their contact angle. However, according to our previous study,12 the change in the shape of a particle has a little influence on its wettability. Therefore, both methods indicate that PS ellipsoids should accumulate at the fluid interface and have a better affinity with PIB phase.
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Figure 2. Localization of PS particles in PIB/PDMS=10/90 (a-d) and 90/10 (a'-d') emulsions. (a, a') The
interfacial position of a single PS microsphere stained with a fluorescent dye, Coumarin 6. (b, b') Fluorescent images and (c, c') overlapped images of fluorescent field and bright field for the distribution of labeled PS ellipsoids (0.5 wt.%, AR=6). The insets A-D are corresponding CLSM images near droplet surface. (d) and (d') are the schematic illustration for the distribution of ellipsoids.
A close inspection of droplet surface (Figure 2A-D) reveals that PS ellipsoids (AR6) in PIB/PDMS emulsions prefer a side-on state (most of them assemble into a side-to-side configuration and a few of them form tip-to-tip configuration) with their major axes parallel to the interface to minimize the total surface energy.39 Some of them form local order structure while others are random. In PIB/PDMS (10/90) emulsion (Figure 2b-c), a monolayer of ellipsoids adsorbs at the droplet surface (the thickness of interface particle layer is ∼0.8 µm which is comparable to ellipsoid minor axis of ~0.7 µm) while most of ellipsoids reside inside droplets, forming particle network. In the inverse 90/10 emulsion (Figure 2b'-c'), PS ellipsoids prominently accumulate on the surface of spherical PDMS droplets, forming multilayer structure (interfacial particle layer thickness thickness is ~2.2 µm) and broken fluorescent contour lines in the CLSM image. The corresponding schematic illustration for the distribution of ellipsoids in
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two different PIB/PDMS emulsions can be seen in Figure 2d-d'.
Aspect ratio. For elongated particles (i.e., larger ARs), the increased degree of freedom could bring a large change in particle-particle interactions in polymeric emulsions.19 ARdependent interactions are ubiquitous for the self-assembly of colloidal particles located at the fluid interfaces,40 which will undoubtedly affect the size and alignment of droplets under flow. Figure 3 illustrates the detail influence of AR of 0.5 wt.% PS particles on the morphology of PIB/PDMS (10/90) blends after shearing at 0.1 s-1 for 20,000 s at a gap spacing (H) of 300 µm. The volume average droplet radii RV (measured on the optical images in Figure 3A-D) of these particle-filled blends are all larger than that of pure blend (Figure 3a). The promoted flowinduced coalescence can be interpreted by the bridging-dewetting mechanism.41 This mechanism requires that interfacially active particles are preferentially wetted by the dispersed phase, which is true in our case as evidenced in Figure 2. As the AR increases from 1.0 to 3.0, the RV declines nonmonotonically from 50 to 36 µm. AR3 particles demonstrate a higher stabilization efficiency than the AR1 particles. This may be ascribed to the fact that due to the shape-induced attractive capillary interactions, higher AR would possess higher interface adsorption energy and thus are less likely to desorb from fluid interface.4243 In this sense, long particles should bring stronger space steric hindrance against droplet coalescence.
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(b) 0.4 Flow alignment
0.2
D*
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0.0
-0.2
Vorticity alignment
-0.4 0
1
2
3
4
5
6
AR of ellipsoids Figure 3. (a) Droplet volume average radius (RV) and (b) deformation parameter (D*) as a function of particle aspect ratio in PIB/PDMS (10/90) emulsions after shearing at 0.1 s-1 for 20,000 s (particle concentration ( φPS )=0.5wt.%, Gap spacing (H)=300 µm). AR=0 means pure system. The insets A-D are corresponding optical images and all scale bars are 100 µm.
However, as the AR increases further to 6.0, the RV does not decrease correspondingly but increases to 56 µm. Here, the effect of another particle parameter, namely the sharpness of corners, may come to play an important role besides the AR. With increasing stretching ratio, the shape of ellipsoids changes from prolate for AR3 to acicular for AR6 particles with sharpest corners (Figure 1). Due to the preferential wettability of particles to droplets, the contact lines between particles and droplet phase are unstable and thus recede across the interface until the films are ruptured. The appearance of sharp corner is suggested to be able to facilitate film rupturing and thus accelerate droplet coalescence (namely the bridging-dewetting effect).41 Similar phenomenon has been reported in some studies on the antifoam effect of solid particles with sharp edges or corners.44-45 It seems that for AR6 particles with acicular corner in our study, the bridging-dewetting effect is dominant over the steric hindrance effect as the droplet size grows up upon adding AR6 particles.
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Along with the change in RV, the droplet orientation style varies, as well (Figure 3b). In pure and short ellipsoid-filled emulsions (AR0, flow alignment region (white)). It is understandable since larger droplets with relatively higher capillary number prone to deform under flow more easily and receive stronger stabilization effect due to wall confinement.26 When the AR increases to 6.0, vorticity-aligned anisotropic droplets emerge (D* 1.2) and the shear walls will distort the velocity fields, which may lead to an attractive interaction of droplets to coalescence for the droplet-string transition.26 The stabilization of strings is reported to rely on the suppression of Rayleigh instability on account of confinement49 and flow which offers an additional stabilization of strings during shear.47 Resembling those vorticity-aligned droplets, these slender strings are capable to retain excellent stability after the cessation of flow and could not relax into spherical shape at rest for at least 9 h (Figure 10f''). The excellent stability of strings after shear
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may be ascribed to the confinement effect of walls and the greatly enhanced interfacial/bulk viscoelasticity of droplets upon addition of PS ellipsoids.
Shear rate. In the previous sections, the vorticity alignment behavior of droplets is investigated at a shear rate of 0.1 s-1. To explore the effect of flow intensity on the alignment behavior, the applied shear rate is changed from 0.05 s-1 up to 1.0 s-1 at a constant H of 300 µm and a strain unit of 2,000 (Figure 11). With the increase of shear rate, the RV of droplets decreases monotonously due to the enhancement of shear intensity (Figure 12a). According to the experimental results of Debruijn50 and the partial mobile interface (PMI) model,51 the breakup and coalescence limiting curves in double logarithmic coordinates can be obtained for our pure PIB/PDMS (10/90) blend as shown in the inset of Figure 12a. Comparing with the pure system, the addition of PS ellipsoids shifts the coalescence curve up due to particle-promoted coalescence because of “bridging-dewetting” mechanism and sharp corner effect. In detail, the size difference between pure and particle-filled emulsions is small at slow shear rates (0.05-0.1 s1
) but it is large and close to the breakup limiting curve at high shear rates (0.5-1.0 s-1).
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Figure 11. Effect of shear rate on the morphology of PIB/PDMS (10/90) emulsions ( φPS =0.5 wt.%, H=300 µm).
Considering the fact that the deformation and breakup of droplets under flow is closely related to the capillary number (Ca), the D* of different emulsions was plotted against Ca in Figure 12b. Although the droplet size reduces with increasing shear rate, the Ca of emulsions increases monotonously (see the inset in Figure 12b). In pure and short particle-filled systems, deformed droplets are orientated in the flow direction (Figure 11a-l) with a positive D* which grows with Ca (Figure 12b). For AR6-filled emulsion (Figure 11m-p), however, vorticityaligned droplets (D*
