Article pubs.acs.org/IECR
Phase Inversion of Agitated Liquid−Liquid Dispersions in the Presence of Micrometer-Sized Particles Ramana Reddy,‡ S. Prakash,¶ and Sanjeev Kumar* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India ABSTRACT: We have investigated the impact of partially wetting particles of tens of micrometers on inversion instability of agitated liquid−liquid dispersions. Particles of this size can be easily separated from the exit streams to avoid downstream processing-related issues. The results show that the presence of hydrophilic particles in small quantities (volume fraction range of 2 × 10−4 to 1.25 × 10−2) significantly decreases the dispersed phase fraction at which water-in-oil (w/o) dispersions invert but leaves the inversion of oil-in-water (o/w) dispersions nearly unaffected. The addition of the same particles after they are hydrophobized decreases the dispersed phase fraction at which o/w dispersions invert but leaves the inversion of w/o dispersions unaffected. These findings suggest an increased rate of coalescence of drops when particles wet drops preferentially and a marginal decrease when they wet the continuous phase preferentially. High-speed conductivity measurements on w/o dispersion show transient conduction of a few hundred milliseconds duration through voltage pulses. Close to the inversion point, voltage pulses appear at high frequency for even 7 cm separation between the electrodes. The presence of hydrophilic particles produces a nearly identical signal at a significantly lower dispersed phase fraction itself, close to the new lowered inversion point in the presence of particles. We propose formation of elongated domains of the conducting dispersed phase through a rapid coalescence−deformation−breakup process to explain the new observations. The voltage signal appears as a forerunner of inversion instability.
■
INTRODUCTION Dispersal of one liquid into another in the form of small drops reduces the domain size of one phase and increases the contact area between the two phases. A number of applications in the chemical industry use one or both of these features to achieve desired objectives. Because the coalescence of drops is accompanied by a decrease in interfacial energy, the tendency of a dispersion is to phase separate. Two approaches are used in the literature to keep one phase dispersed in the other. In one approach, the coalescence of drops is retarded by using surfactants. In the other approach, external energy is supplied continuously to balance the rate of coalescence of drops with their breakup. Under the conditions of dynamic equilibrium, a steady drop size distribution is reached. The interfacial area corresponding to it and the state of mixing in the internal and the external phases control the rate of interfacial transport processes. The latter is the preferred route for providing intimate contact between two immiscible phases of a short duration to exchange solute and/or energy between them. The discontinuation of energy supply leads to separation of phases through buoyancy driven coalescence of drops. Mixer−settlers, multistage extraction columns, multiphase reactors, etc. are some of the widely used equipment1,2 based on agitated dispersions. An increase in dispersed phase fraction to increase interfacial area and processing capacity results in nearly catastrophic inversion of one type of dispersion into another at a critical value. Because of the existence of an ambivalent range3 of dispersed phase fraction in which both oil-in-water (o/w) and water-in-oil (w/o) dispersions can exist stably (also known as hysteresis), the inverted dispersion cannot be restored to the original type by a differential decrease in dispersed phase fraction; a large change in dispersed phase fraction is required © XXXX American Chemical Society
for most systems. The cascading effect of an inverted dispersion on downstream operations degrades process performance substantially. The large-scale operations are therefore carried out in safe range, away from the critical conditions. Because the critical dispersed phase fraction for intensely agitated dispersions depends only on the physical properties of a liquid−liquid system,4 it can be increased advantageously only by altering the nature of the liquids used, for example by adding surfactants or electrolytes,5 both of which can adversely impact the downstream processing. Partially wetting particles are known to impart exceptionally high stability to (Pickering) emulsions. A partially wetting particle locates itself on an oil−water interface in such a way that a larger part of it remains in the liquid it wets preferentially. Thus, the bigger part of a partially hydrophilic particle remains in the water phase and the smaller part in the oil phase. The particles provide stabilization against coalescence by providing steric hindrance to two approaching drop surfaces. In general, hydrophilic particles stabilize o/w emulsions and hydrophobic particles stabilize w/o emulsions. The presence of such particles can be expected to retard coalescence of drops in agitated dispersions as well; they thereby increase the value of critical dispersed phase fraction and the range of stable operation. A recent study6 used hydrophilic silica nanoparticles of 12 nm mean size. The study finds that in the presence of these particles, the critical dispersed phase fraction is increased for Special Issue: Doraiswami Ramkrishna Festschrift Received: May 14, 2015 Revised: June 29, 2015 Accepted: June 29, 2015
A
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
both o/w/o and w/o/w type large structures are formed and multiple types of dispersions coexist. Some of their images show quite deformed drops surrounded by smaller drops in close proximity. A mechanistic understanding of how phase inversion is triggered is not yet available. Minimization of surface energy is not supported by the observations of Luhining and Sawistowski.3 Imbalance between breakup, predominantly near the impeller, and coalescence, in regions away from the impeller, is not supported by the finding4 that vessel size does not alter critical values for intensely agitated dispersions. Local imbalance between breakup and coalescence of drops is the widely advocated hypothesis,4,10,18,19 but this hypothesis still needs to be quantitatively established with realistic breakup and coalescence kernels.20
both o/w and w/o dispersions. Because nano- and micrometer sized particles cannot be easily separated from the exit stream in continuous mode of operation, their addition is likely to affect downstream processing, similar to the addition of surfactants and electrolytes. The particles in the size range of 50 μm can be easily separated from the exit stream using various means, leaving the downstream processing unaffected. In this work, we have studied the effect of addition of such large particles on phase inversion in stirred vessels. We begin the next section with a brief review of the present understanding of inversion phenomenon in stirred vessels, followed by experimental details. The results obtained bring out an interesting feature of agitated dispersions containing large particles, which potentially can be used to intensify transport rates in agitated dispersions. Phase Inversion in Liquid−Liquid Dispersions. A typical experiment to study phase inversion3,4,7−11 is started with a known type of agitated dispersion. Next, the dispersed phase fraction is increased in small steps at a fixed stirrer speed. At a critical fraction, the dispersion inverts rather suddenly, in about 1−2 s.11 The same can be realized at high enough dispersed phase fractions by increasing the stirrer speed for o/w dispersion and by decreasing it for w/o dispersion. The appearance of dispersion and its conductivity are monitored to detect phase inversion.7 A number of interesting characteristics of agitated dispersions and inversion phenomenon are discussed in detail by Kumar,10 Yeo et al.,12 and Deshpande and Kumar.4 Briefly, Quinn and Sigloh13 found that the critical fraction attains an asymptotic value at high intensity of agitation. Luhning and Sawistowski,3 who were among the first to carry out a detailed study of phase inversion, found that matching viscosities and densities of the two phases (interfacial tension being a common property) does not render the behavior of o/w and w/o dispersions identical. Near the inversion point, they reported, the dispersion inverts locally in a small domain and quickly reinverts. They also established that inversion is not necessarily accompanied by a reduction in surface energy. The authors hypothesized an unknown interfacial property to be the cause of the asymmetric behavior. Guilinger et al.14 and Kumar et al.8 have altered the wetting character of the surfaces present in the vessel to introduce additional8 mechanisms for breakup and coalescence of drops. The effect of the additional mechanisms, quite substantial at low intensity of turbulence, diminishes nearly completely at high intensity of turbulence. Pacek et al.9 found through in situ photography droplets of water trapped in oil drops in o/w dispersions; no such structures were found for w/o dispersions. However, w/o dispersions alone have a delay time before phase inversion.15 Both Kato et al.16 and Pacek et al.9 reported that o/w dispersions take longer to settle than their w/o counterpart. Kumar10 has invoked latent charge present on oil−water interface and the widely different dielectric constants for oil and water phases to reconcile a number of the known asymmetric observations. Recently, Deshpande and Kumar4 have shown, also supported by the earlier findings,17 that critical fractions for sufficiently intensely agitated o/w and w/o dispersions are properties of a liquid−liquid system, independent of the turbulence-related parameters such as impeller type, impeller size, vessel size, and distributed versus point source of energy input to the vessel. Liu et al.11 have combined high-speed photography and laser-induced fluorescence to expand on the findings of Pacek et al.9 The authors find that close to inversion,
■
EXPERIMENTAL SECTION Materials. All the experiments were conducted with deionized water as aqueous phase and toluene as organic (oil) phase. A small amount (0.3 g/L) of sodium chloride was added to the aqueous phase to enable reliable detection of the phase inversion point using conductivity measurements. Toluene (≥99% purity) was obtained from Merck. The silica particles, obtained from Hindustan Glass Beeds, Mumbai, India, had an average diameter of 50 μm and density 2.4 × 103 kg/m3. The size distribution of these particles, measured by Mastersizer from Malvern Instruments Ltd., U.K., is shown in Figure 1. Octadecyltrichlorosilane of 95% purity, used for silanation of silica particles to make them hydrophobic, was obtained from Sigma-Aldrich.
Figure 1. Size distribution of silica particles mesured using Mastersizer of Malvern Instruments Ltd., U.K. The Sauter mean particle diameter is measured to be 49.3 μm.
Wetting Character of Particles. To study the effect of wetting character of particles on phase inversion, the same particles with changed wetting characteristics were used in the experiments. The protocol of Wasserman et al.21 was followed to silanate partially hydrophilic silica particles to render them partially hydrophobic. Briefly, the hydrophilic particles were first washed with concentrated sulfuric acid, hydrogen peroxide, and finally with copious amounts of deionized water. The dried particles were next reacted with octadecyltrichlorosilane in toluene medium at room temperature for about 30 h. The treated particles were again washed with copious amounts of toluene and ethyl alcohol and finally dried at 60 °C. Figure 2 B
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. Close view of a hydrophobized silica particle, as seen in SEM.
Figure 2. SEM image of hydrophilic silica particles.
shows a scanning electron microscopy (SEM) picture of the untreated particles. Images of the same particles after they were silanated are shown in Figure 3. A close-up view of a treated
measured on four plates was averaged to obtain a representative value. The average contact angle measured from the water side was 30° for water−toluene−hydrophilic glass plate and 134° for the silanated glass plates. We will refer to these partially hydrophilic and partially hydrophobic particles as hydrophilic and hydrophobic particles, with the understanding that these are partial characteristics. Procedure. Experiments were carried out in batch mode in a glass vessel of diameter 105 mm and height 110 mm, containing four stainless steel baffles and a stainless steel top cover. The stirrer was connected to a dc motor. The speed of the motor was controlled to 600 ± 10 rpm using input voltage. The experiments were carried out at high intensity of turbulence, in the regime in which, as shown earlier by Deshpande and Kumar,4 the dispersed phase fraction at inversion becomes independent of turbulent flow-related parameters such as stirrer speed and impeller diameter. Sixbladed standard Rushton turbines of about 5 cm diameter was used. Phase inversion, accompanied by a change in appearance, was also detected by monitoring conductivity of a dispersion. The conductivity undergoes a nearly abrupt change, from that of the continuous phase before inversion to that of dispersed phase after inversion. A Wheatstone bridge was used in this work along with National Instrument’s high-speed data acquisition card 6013 to convert electrical resistance of dispersion between two electrodes to voltage data. To follow a uniform procedure for studying phase inversion with and without the presence of particles, a semibatch protocol3 was adopted. Starting with an initial dispersion of known volume and composition, the dispersed phase was added slowly until inversion occurred. Close to the inversion point, the dispersed phase was added slowly, less than 1 mL at a time. The dispersion was allowed to reach new dynamic equilibrium before the next addition. Deshpande and Kumar4 showed that the dispersed phase fraction at inversion remains unchanged for 2-fold increase in dispersion volume at high intensity of agitation. The initial volume of dispersion and its composition were varied to ensure that the final total volume of dispersion at phase inversion remained in a narrow range of 10% variation around 800 mL. The experiments in the presence of particles are carried out by adding measured quantities of particles initially itself to the
Figure 3. SEM image of hydrophobic silica particles, obtained by silanation of hydrophilic silica particles, shown in Figure 2.
particle is shown in Figure 4. These figures together show that the particles used in this work have smooth surfaces and spherical shape and that the silanation process does not result in aggregation, floc formation, or roughening of particle surfaces. The equilibrium contact angle for particles was determined by using a tensiometer (Dataphysics, Germany). The experimental procedure followed is detailed by Teipel and Mikonsaari.22 Identical thin glass tubes were filled with particles and brought in contact with the liquid preferentially wetted by them. Hexane was used as the reference liquid. Contact angle for air−water−hydrophilic particle and air−toluene−hydrophobic particle were measured to be 13.6° and 43.6°, respectively. Water did not climb into a tube filled with hydrophobic particles. The three-phase contact angle on a glass plate, treated in the same way as the silica particles (ensured by adding a couple of glass plates to the silica particles taken for silanation), was measured with a goniometer. The equilibrium contact angle C
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
in an identical manner, in just a couple of seconds, after agitation was stopped. The above set of experiments was repeated with the same particles silanated to alter their wetting characteristics from hydrophilic to hydrophobic. The results obtained are shown in Table 3. The data show that a change in wetting character
vessel. The experiments carried out by adding particles initially to water phase or oil phase showed that the critical fractions were independent of this variable. Determination of Volume Fraction at Inversion. The dispersed phase fraction at inversion is estimated by measuring dispersion volume and the amount of water present in the phase-separated dispersion, using a 1 L measuring cylinder with 10 mL graduation marks. The expected uncertainty in the measurement of volume fraction is 0.0125 (=10 mL/800 mL).
Table 3. Dispersed Phase Fraction at Inversion of o/w and w/o Dispersions without Particles, with (10 g) Hydrophobic Particles, and after Filtering off Particles
■
RESULTS AND DISCUSSION Experiments were first carried out in the absence of particles to determine the critical dispersed phase fraction for inversion of o/w and w/o dispersions. These are denoted by ϕo/w → w/o and ϕw/o → o/w , respectively. Table 1 shows experimental results for
before particle addition with hydrophobic particles after filtration of particles
o/w → w/o
(volume of water)/ (volume of dispersion)
ϕw/o → o/w
(volume of oil)/ (volume of dispersion)
ϕo/w → w/o
390/740 400/750 410/760 400/800
0.53 0.54 0.54 0.51
510/770 560/810 540/810 580/870
0.66 0.69 0.67 0.67
the toluene−water system. The experimental data for repeat experiments, in which the volume of dispersion at inversion was deliberately varied, are also shown. The measurements show that w/o and o/w dispersions invert at near constant values of 0.52 ± 0.02 and 0.67 ± 0.02, respectively, for different dispersion volumes. There is no trend in values of critical fractions with increase in volume of dispersions at inversion, as expected.4 It is therefore safe to conclude that the reported values have an experimental error of ±0.02. Effect of Nature of Particles. The next set of experiments was carried out with the addition of 10 g of hydrophilic silica particles, with no other change in the system. The volume fraction of particles corresponding to this loading is ∼0.005. As the hydrophilic particles are known to stabilize o/w emulsions by preventing coalescence of oil drops in water even at low surface coverage,23 the value of ϕo/w → w/o is expected to increase. The results presented in Table 2 show that the value
without particles with hydrophilic particles
ϕo/w → w/o
0.53 0.39
0.67 0.69
0.67 0.51 0.65
Table 4. Dispersed Phase Fractions at Inversiona
Table 2. Dispersed Phase Fraction at Inversion o/w and w/o Dispersions without and with (10 gm) Hydrophilic Particles ϕw/o → o/w
ϕo/w → w/o
0.53 0.57 0.56
completely alters the effect of particles on critical fractions. Now, the value of ϕw/o → o/w remains unchanged but the value of ϕo/w → w/o reduces substantially, from 0.67 to 0.50. The settling behavior of both o/w and w/o dispersions once again remains unaffected in the presence of these particles as well. To establish that the observed effects are exclusively due to the presence of particles, the above experiments were repeated with the same liquids after filtering off particles. The results, presented in Table 3, show that the values of ϕo/w → w/o and ϕw/o → o/w revert to the values obtained for these liquids before they were brought in contact with the particles. To see if the observed significant impact of particles is not confined to only high interfacial tension systems, experiments were carried out at reduced interfacial tension. We reduced the interfacial tension of the toluene−water system used in the earlier experiments from 29.5 to 11.5 mN/m by adding ethanol to the water phase (25 vol %). A nearly 3-fold decrease in interfacial tension expectedly3 increased ϕw/o → o/w from 0.53 to 0.83 and ϕo/w → w/o from 0.67 to 0.77. The addition of hydrophobic particles (10 g) decreased the value of ϕo/w → w/o from 0.77 to 0.58, while leaving the inversion of w/o dispersion unaffected, as observed before. These findings are summarized in Table 4. The decrease in critical dispersed phase fraction from 0.77 to 0.58 for a poorly coalescing system, due to low interfacial tension, is quite substantial.
Table 1. Dispersed Phase Fractions at Inversion of Oil-inWater (o/w) and Water-in-Oil (w/o) Dispersions for Different Total Dispersion Volumes, in the Absence of Any Particles w/o → o/w
ϕw/o → o/w
case
system
ϕw/o → o/w
ϕo/w → w/o
A B C
base system + ethanol + hydrophobic particles
0.53 0.83 0.82
0.67 0.77 0.58
a
A: Base system consists of toluene and water phases. B: Ethanol is added to water phase (25% by volume) to reduce interfacial tension from 29.5 to 11.5 mN/m. C: Addition of 10 g of hydrophobic particles to case B.
of ϕo/w → w/o remains the same. The value of ϕw/o → o/w on the other hand reduces quite substantially, from 0.53 to 0.39. Assuming that the presence of particles at such loading does not affect breakup of drops, which are much larger in size, the decreased value of ϕw/o → o/w suggests that the presence of hydrophilic particles instead increases coalescence of water drops in agitated w/o dispersions. Interestingly, both types of dispersions, with and without particles in them, phase separated
Effect of Loading of Particles. To further explore the destabilization brought about by the addition of particles, the amount of particles added to dispersions was varied. A change in particle loading is expected to change the fraction of drop surface area covered with the particles, and thereby influence critical fractions. Tables 5 and 6 show the effect of adding hydrophilic and hydrophobic particles, respectively, on ϕo/w → w/o and ϕw/o → o/w . The measurements show that D
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 5. Variation of Critical Fractions at Inversion for Addition of Different Amount of Hydrophilic Particles to Toluene−Water System weight of particles (g)
ϕw/o → o/w
ϕo/w → w/o
0 1.5 3.0 6.0 12.0 24.0
0.53 0.39 0.40 0.38 0.38 0.37
0.67 0.69 0.70 0.71 0.72 0.72
Table 7. Fractional Surface Area of Drops Covered by 1.6 g Particles at Various Dispersed Phase Fractions, for Mean Drop Diameter of 5 mm (Column 2)a ϕd
surface area coverage fraction Af, d = 5 mm μm
size of drop with full coverage (m)
0.3
6.52 × 10−2
7.3 × 10−2
0.4
−2
9.8 × 10−2
0.5 0.6 0.7
4.88 × 10
−2
3.9 × 10
−2
3.24 × 10
−2
2.8 × 10
1.2 × 10−1 1.5 × 10−1
1.7 × 10−1
a
The third column shows the mean size of drops whose surfaces will be covered completely by 1.6 g of particles for mean particle size of 50 μm and particle density of 2.4 × 103 kg/m3.
Table 6. Variation of Critical Fractions at Inversion for Addition of Different Amount of Hydrophobic Particles to Toluene-Water System weight of particles (g)
ϕw/o → o/w
ϕo/w → w/o
0 0.5 1.0 2.0 5.0 10.0 20.0 30.0
0.53 0.57 0.62 0.59 0.57 0.57 0.58 0.59
0.67 0.50 0.53 0.52 0.50 0.51 0.50 0.48
o/w and w/o dispersions can be expected to be stabilized by hydrophilic and hydrophobic particles, respectively. However, the experimental data show only a marginal stabilization, which is also rather insensitive to particle loading. When the same particles preferentially wet the dispersed phase, the surface coverage in the range of 1.25−75% leads to significant and nearly coverage-independent extent of destabilization. Nienow et al.26 studied coalescence of sunflower oil drops (viscosity 72 cP) in water in a stirred vessel, in the presence of PMMA particles with Sauter mean diameter of 7 μm. The dispersed phase fraction in their step-down experiments was 0.05. They found that the presence of partially hydrophobic PMMA particles significantly increases the rate of coalescence of oil drops (or destabilizing role of particles) for surface coverage up to about 5%. At higher surface coverage corresponding to the addition of larger amount of PMMA particles, the rate of coalescence goes though a maximum and decreases nearly to that in the absence of particles altogether. The authors explained these findings by invoking a bridging mechanism. The variation of critical fraction with particle loading does not support a maximum in the extent of destabilization, even though the maximum estimated surface coverage is quite high. We have probed this issue further by examining the structure of agitated dispersions using conductivity measurements. High-Speed Conductivity Measurements. High-speed conductivity measurements of agitated dispersions were made at different separation distance between the electrodes to understand the morphology of dispersed phase and how the presence of particles affects it. A Wheatstone bridge was used to convert resistance offered by dispersion to voltage signal. The Wheatstone bridge was excited by using ±2 V square pulses at a frequency of 5000 Hz. These pulses were generated using the NI card itself. The absolute value of the output voltage across the mid points was recorded at a sampling rate of 10 000 Hz. Figure 5 shows voltage signal for w/o dispersion in the absence of particles at ϕd = 0.33 and 2 cm distance between the electrodes. The x-axis represents a real time window. Nearly constant voltage of 1.8 V in the output signal indicates no conduction. A normal low-speed conductivity measurement would also produce the same signal. When ϕd is increased to 0.38, the voltage signal shows a number of pulses (Figure 6). These pulses are missed by a low-speed conductivity measurement. The duration of the pulses ranges from 50 ms to hundreds of milliseconds. An increase in frequency of excitation voltage, or replacement of square pulses by other waveforms, led to the same conclusion about the voltage pulses in the output signal. Each pulse corresponds to the formation of a
hydrophilic particles destabilize w/o dispersions significantly, nearly to the same extent at all particle loadings ranging from 1.5 to 24 g. Similarly, hydrophobic particles destabilize o/w dispersions significantly, nearly to the same extent at all particle loadings ranging from 0.5 to 30 g. It is interesting to note that increasing the particle loading by a factor of 60 in this case does not increase the impact of hydrophobic particles on the stability of o/w dispersions, while the presence of just 0.5 g of particles to 800 mL of dispersion decreases the critical fraction from 0.67 to 0.50. There appears to be a marginal stabilization of o/w dispersions by hydrophilic particles and w/o dispersions by hydrophobic particles. The extent of stabilization is however quite small. Discussion. The energy required to remove a particle of π diameter dp from interface, of the order of 4 d p2σ(1 ± cos θ ), is several orders of magnitude larger than the thermal energy (kBT). It is also about an order of magnitude larger than the energy associated with turbulent fluctuations24,25 of the same length scale ( ρc ε 2/3d p2/3(1 + 4ϕd)−2 (πd p3/6), where ε is energy dissipation rate and ϕd is dispersed phase fraction). Assuming that all the particles of diameter dp are present on drop surfaces, the fractional surface coverage of drops of Sauter mean diameter dd is Af =
1 ϕp dd 4 ϕd d p
(1)
11
Liu et al. have recorded high-speed images during inversion. The mean drop size inferred from these images is in the range of 5 mm. The second column of Table 7 shows the estimated values of Af for different values of ϕd, for particle loading of 2 g/ L (1.6 g). The third column shows the mean drop size required for their surface to be fully covered with particles. At ϕd = 0.4, the particles are expected to cover 5% of the surface of drops. At the highest particle loading of 37.5 g/L (30 g), the surface coverage is predicted to increase to 75%. At such high coverage, E
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 7. Conductivity signal for w/o dispersion. All the conditions are the same as those for the Figure 6. The volume fraction of water is further raised to 0.50, and the distance between the probes is increased to 7 cm.
Figure 5. Conductivity signal for w/o dispersion in the absence of particles. Here, the y-axis shows the voltage measured across two opposite nodes of an unbalanced Wheatstone bridge in which one of the resistances is that between two probes immersed in the agitated dispersion, under the impeller. The x-axis represents real time. The volume fraction of water is 0.33, and the distance between two probes is 2 cm.
time that the connected water domains are still longer in length. Our measurement also bring out the high rate at which these are formed and disrupted, with their lifetime in the range of a few hundred milliseconds. The w/o dispersion at ϕd = 0.38, which did not produce voltage pulses at 7 cm distance between the probes, shows a large number of pulses when hydrophilic silica particles are added to it. The voltage signal obtained under these conditions is shown in Figure 8. The voltage signal is similar to that shown
Figure 6. Conductivity signal for w/o dispersion. All the conditions are the same as those for Figure 5. The volume fraction of water is raised to 0.38.
transiently connected domain of water, spanning from one electrode to the other at a distance of 2 cm. An increase in distance between the probes to about 7 cm at the same conditions (ϕd = 0.38) resulted in a signal similar to that shown in Figure 5. This result indicates that the transiently connected domains of water, spanning 7 cm, are not formed at this dispersed phase fraction. An increase in ϕd to 0.50 for the same 7 cm distance between the electrode produced a voltage signal with a very large number of pulses, as shown in Figure 7. The voltage signal indicates formation of very long (7 cm or more in a vessel of 10.5 cm diameter) transiently connected domains of water. Insertion of a 2 cm × 2 cm nonwetting and nonconducting thin sheet of Teflon between the electrodes made the voltage pulses disappear completely. This lends further support to the voltage pulses being indicative of transiently connected domains of water. This dispersion, with no particles in it, inverts at a dispersed phase fraction of about 0.52. The results of Liu et al.,11 who have provided high-speed video clips of agitated dispersions near the inversion point, also contain a few elongated drops of about 2 cm length. The conductivity measurement presented here show for the first
Figure 8. Conductivity signal for w/o dispersion. All the conditions are same as those for Figure 6, except for the presence of hydrophilic particles.
in Figure 7. The similarity of signal also coincides with the inversion behavior. The dispersion with particles inverts at about ϕd = 0.39 itself. The use of the same measurement principle to probe o/w dispersions requires modifications as the continuous phase is conducting in this case. These efforts are presently underway. Liu et al.11 have however concluded that the inversion of both types of dispersions follows similar changes in dispersed phase morphologies. On the basis of the above findings, we hypothesize that the mechanism of phase inversion involves formation of highly elongated and deformed domains of dispersed phase which rapidly break because of the surface tension-driven instability. As the daughter fragments relax to spherical shape, driven by surface tension, they collide and coalesce with the drops in F
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Present Addresses
close proximity, become bigger, and simultaneously deform because of the superimposed flow. The elongated domains break once again, and the process continues. Fresh interface is continually created in this process at high rate. The 50 μm particles are unlikely to reach the new interface and get adsorbed in the available time to provide stabilization against coalescence, even if they preferentially wet the continuous phase. Therefore, although the expected surface converge is high based on a static picture, in view of the rapid breakup, coalescence, and deformation of drops, the approaching surfaces may not have enough particles on them to provide stabilization. The particles that wet the dispersed phase preferentially can mediate close approach of interfaces of two drops by bridging and bring about their coalescence at low surface coverage as well. In fact, highly deformed drops in close proximity of each other require just one particle in the contact zone to bring about their coalescence. A similar mechanism operates, for example, in destabilization of aqueous foams by addition of hydrophobic particles27 and destabilization of o/w food emulsions28 by hydrophobic fat crystals. To summarize, bridging across the thin film in the contact zone between two colliding drops by one particle is enough to convert an unsuccessful collision into a successful one. In comparison, a nearly closed packed layer of particles is required to prevent their coalescence.
‡
R.R.: Hindustan Unilver Research Center, Whitefield, Bangalore. ¶ S.P.: Center for Nano Science and Engineering, Indian Institute of Science, Bangalore. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
REFERENCES
While the special issue celebrates the contributions made by Prof. Ramkrishna to the diverse fields in Chemical Engineering, I (S.K.) wish to acknowledge the impact he has had on the thinking of generations of Ph.D. students and postdoctoral fellows (I am one of those) through his quest for mathematical rigor, elegance of execution, beauty in the final outcome, and “what is new here?”. The most cited work from his lab on discretization methods for solving population balance equations owes a great deal to the ideals he has sought and inspired in those around him. It is an honor to be able to contribute to this special issue, on a problem that fascinated him once.
(1) Science and Practice of Liquid-Liquid Dispersion; Thornton, J. D., Ed.; Clarendon Press: Oxford, 1992. (2) Liquid-Liquid Extraction and Equipment; Godfrey, J. C., Slater, M. J., Eds.; Wiley: Chichester, U.K., 1994. (3) Luhning, R. W.; Sawistowski, H. Phase inversion in stirred liquidliquid systems. Proceedings of the International Solvent Extraction Conference, ISEC 71, the Hague, April 19−23, 1971; Society of Chemical Industry: London, 1971; pp 873−887. (4) Deshpande, K. B.; Kumar, S. Chem. Eng. Sci. 2003, 58, 3829− 3835. (5) Deshpande, K.; Kumar, S. Chem. Eng. Sci. 2012, 78, 33−37. (6) Asadabadi, M. R.; Abolghasemi, H.; Maragheh, M. G.; Nasab, P. D. Korean J. Chem. Eng. 2013, 30, 733−738. (7) Selker, A. H.; Sleicher, C. A. Can. J. Chem. Eng. 1965, 43, 298− 301. (8) Kumar, S.; Kumar, R.; Gandhi, K. S. Chem. Eng. Sci. 1991, 46, 2365−2367. (9) Pacek, A. W.; Nienow, A. W.; Moore, I. P. T. Chem. Eng. Sci. 1994, 49, 3485−3498. (10) Kumar, S. Chem. Eng. Sci. 1996, 51, 831−834. (11) Liu, L.; Matar, O. K.; de Ortiz, E. S. P.; Hewitt, G. F. Chem. Eng. Sci. 2005, 60, 85−94. (12) Yeo, L. Y.; Matar, O. K.; de Ortiz, E. S. P.; Hewitt, G. F. Multiphase Sci. Technol. 2000, 12, 51−116. (13) Quinn, J.; Sigloh, D. Can. J. Chem. Eng. 1963, 41, 15−18. (14) Guilinger, T. R.; Grislingas, A. K.; Erga, O. Ind. Eng. Chem. Res. 1988, 27, 978−982. (15) Gilchrist, A.; Dyster, K.; Moore, I.; Nienow, A.; Carpenter, K. Chem. Eng. Sci. 1989, 44, 2381−2384. (16) Kato, S.; Nakayama, E.; Kawasaki, J. Can. J. Chem. Eng. 1991, 69, 222−227. (17) Norato, M. A.; Tavlarides, L. L.; Tsouris, C. Can. J. Chem. Eng. 1998, 76, 486−494. (18) Vaessen, G. E. J.; Visschers, M.; Stein, H. N. Langmuir 1996, 12, 875−882. (19) Yeo, L. Y.; Matar, O. K.; de Ortiz, E. S. P.; Hewitt, G. F. Chem. Eng. Sci. 2002, 57, 1069−1072. (20) Deshpande, K. Studies on Phase Inversion. M.Sc. Thesis, Indian Institute of Science, Bangalore, 2001. (21) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074−1087. (22) Teipel, U.; Mikonsaari, I. Part. Part. Syst. Charact 2004, 21, 255−260.
■
CONCLUSIONS The experiments show that the addition of very small quantities of partially wetting 50 μm hydrophilic silica particles significantly decreases the dispersed phase fraction at which w/o dispersion inverts to o/w dispersion. An increase in the loading of particles (volume fraction range of 2 × 10−4 to 0.013) impacts their effect on inversion instability only marginally. The inversion of o/w dispersion remains largely unaffected. The same particles after their surfaces are silanated to render them partially hydrophobic cease to influence the inversion of w/o dispersions. Instead, the dispersed phase fraction required for inversion of o/w dispersions decreases substantially. The extent of the effect observed is the same for moderate and low interfacial tension systems. High-frequency conductivity measurements of w/o dispersion, recorded as voltage pulses using a Wheatstone bridge, show that with an increase in dispersed phase fraction transient conduction occurs between the electrodes at increasingly larger separations between them. In the proximity of inversion instability, transient conduction (voltage pulses of duration of a few hundred milliseconds) occurs at high frequency even at electrode separation of 7 cm, in a vessel of 10.5 cm diameter. Similar voltage pulses are obtained at a substantially decreased dispersed phase fraction in the presence of hydrophilic particles. This decrease correlates with the decreased dispersed phase fraction required for inversion instability in the presence of particles. We propose that transient conduction indicates formation of long transiently connected domains of dispersed phase, and that these are precursor to inversion instability. It is possible to use the voltage signal obtained at high sampling rate to determine the proximity of an agitated dispersion to inversion instability.
■
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. G
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research (23) Horozov, T. S.; Binks, B. P. Angew. Chem., Int. Ed. 2006, 45, 773−776. (24) Hinze, J. O. AIChE J. 1955, 1, 289−295. (25) Kumar, S.; Gandhi, K. S.; Kumar, R. Chem. Eng. Sci. 1991, 46, 2483−2489. (26) Nienow, A. W., Pacek, A. W., Nixon, A. J. In Proceedings of the Tenth European Mixing Conference; den Akker, H. E. A. V., Derksen, J. J., Eds.; Elsevier Science B. V., 2000; pp 157−164. (27) Aveyard, R.; Binks, B.; Fletcher, P.; Peck, T.; Rutherford, C. Adv. Colloid Interface Sci. 1994, 48, 93−120. (28) Boode, K.; Walstra, P. Colloids Surf., A 1993, 81, 121−137.
H
DOI: 10.1021/acs.iecr.5b01806 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
