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Effect of Nanoparticle Surfactants on the Break-Up of Free-Falling Water Jets During Continuous Processing of Reconfigurable Structured Liquid Droplets Anju Toor, Brett A. Helms, and Thomas P. Russell Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00556 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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Effect of Nanoparticle Surfactants on the Break-Up of Free-Falling Water Jets During Continuous Processing of Reconfigurable Structured Liquid Droplets Anju Toor1,2, Brett A. Helms2,3, and Thomas P. Russell2,4,5* 1

Department of Mechanical Engineering, 6141 Etcheverry Hall, University of California, Berkeley, CA

94720, USA 2

Materials Sciences Division, Lawrence Berkeley National Lab, One Cyclotron Road, Berkeley, CA

94720, USA 3

The Molecular Foundry, Lawrence Berkeley National Lab, One Cyclotron Road, Berkeley, CA 94720,

USA 4

Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors

Drive, Amherst, MA 01003, USA 5

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, China *E-mail: [email protected]

ABSTRACT – Structured liquids, whose 3-D morphology can adapt and respond to external stimuli, represent a revolutionary materials platform for next-generation energy technologies, such as batteries, photovoltaics, and thermoelectrics. Structured liquids can be crafted by the jamming of interfacial assemblies of nanoparticle (NP) surfactants. Due to the interactions between functional groups on nanoparticles dispersed in one liquid and polymers having complementary end-functionality dissolved in a second immiscible fluid, the anchoring of a

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well-defined number of polymer chains onto the NPs leads to the formation of NP-surfactants that assemble at the interface and reduce the interfacial energy. Microfluidic techniques provide a simple and versatile route to produce one liquid phase in a second where the shape of the dispersed liquid phase can range from droplets to tubules depending on the flow conditions and the interfacial energies. In this study, the effect of NP-surfactants on Plateau-Rayleigh (PR) instabilities of a free-falling jet of an aqueous dispersion of carboxylic acid functionalized silica NPs into a toluene phase containing amine-terminated polydimethylsiloxane (PDMS-NH2), is investigated. NP-surfactants were found to significantly affect the breakup of laminar liquid jets, resulting in longer jet breakup lengths and dripping to jetting flow transitions. KEYWORDS: Plateau-Rayleigh instability, surfactants, nanoparticles, structured liquids, jetting The assembly of nanoparticles at liquid-liquid interfaces has been investigated for stabilizing emulsions and foams1–6, but studies on the generation of structured liquids7–9 are quite limited. Recently, Cui et al.10 showed that two immiscible liquids can be shaped on demand when the imposed non-equilibrium structure is preserved by the interfacial jamming of the NPsurfactants. A droplet of an aqueous dispersion of functionalized nanoparticles was first created in an oil phase containing end-functionalized polymer, allowing the interaction of NPs and polymers to form NP-surfactants at the water-oil interface. An electric field was applied across the droplet, deforming it into an ellipsoid, increasing the interfacial area, allowing more NPsurfactants to form and assemble at the interface. Upon removal of the electric field, the droplet attempts to revert to a spherical shape to decrease the interfacial area and, therefore, the interfacial energy, compressing the assembled NP-surfactants and causing them to jam, arresting further change in the interfacial area, thereby kinetically trapping the water in a highly nonequilibrium shape. 2


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These structured liquids represent a unique state of matter where all components are liquid, since the jammed NP-surfactants assemblies are liquid on a local level with a percolated pathway of NP-surfactants bears the compressive load, locking in the shape of the liquid. Subsequent external stimuli, for example an electric or shear field, can break the jam, imparting fluidity to the interfacial NP-surfactant assembly, allowing the liquid to be re-shaped to spatially re-define distribution of the fluids10,11. Such systems can be achieved by using emulsification processes, electrospinning, or microfluidic techniques based on PR-instabilities. PR-instabilities offer a continuous, high-throughput method for the generation of structured liquids. If one of the fluids is an aqueous phase containing carboxylic acid functionalized NPs, while the second is a solution of an amine-terminated polymer in oil, then, as water is drawn from the orifice into the surrounding oil phase, NP-surfactants will form at the water-oil interface to reduce and eventually minimize the interfacial energy. From an array of droplets with well-defined diameters to tubules of one liquid in another immiscible liquid can be formed, depending on the rate of the formation of the NP-surfactants, the concentration of the components, and the viscosities of the fluids. If the rate is sufficiently rapid, then the instabilities can be arrested, since the reduction in the interfacial area will cause a jamming of the NPsurfactants, freezing in the non-equilibrium tubular shape of one liquid in the second, forming a unique platform for additive manufacturing. In previous studies10–14, the timescale of NP-surfactant formation was not important, since a droplet of one liquid was equilibrated in another immiscible liquid; this allows sufficient time for the formation and assembly of NP-surfactants. Here, on the other hand, an approach based on PR-instabilities relies heavily on the rate of NP-surfactant formation and assembly, since jet formation occurs very rapidly. The formation of NP-surfactants necessitates a more

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rapid diffusion of the polymer chains to the water-oil interface, which can be achieved at high concentrations of the polymer ligands. Therefore, to enable the continuous generation of structured liquids, it is critical to investigate the effect of NP-surfactants on the break-up dynamics of jets of one liquid in a second immiscible liquid.

Figure 1 (A) The formation of liquid droplets from a stream of one liquid in a second immiscible liquid by PR-instabilities. Lb is the breakup length, djet is the initial jet diameter, OD is the outer diameter of the capillary, and d is the droplet diameter. (B) A collection of the droplets generated using an approach exploiting PR-instabilities. At high polymer and nanoparticle concentrations, polymer chains and NPs interact at a suitable time-scale in the processes to form NP-surfactants at the interface. The bottom portion of (B) shows a schematic of the NP-surfactant assembly at the water-oil interface. Plateau15 and Rayleigh16 pioneered studies on the instabilities of fluid cylinders where, to minimize interfacial area (to reduce the interfacial energy), the free surface of the liquid cylinder undulates with a wavelength λ, where λ is greater than the circumference of the cylinder. These undulations grow with time leading, eventually, to the break-up of the cylinder into well-defined droplets with a characteristic size and separation distance17. A similar behavior is seen in microfluidic devices where one fluid emerges from an orifice into a second fluid where the

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density of the second fluid is lower than the first

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. Shown in Figure 1A is a schematic

diagram where, as the first liquid entrains into the second, the diameter of the stream or jet (djet) thins and capillary instabilities begin to grow eventually leading to droplet formation. Minimization of the surface area or interfacial energy drives the instability and the formation of the droplets. Figure 1B shows a collection of droplets generated using PR-instabilities. A schematic of the NP-surfactants assembly at the water-oil interface is shown in the bottom part of Figure 1B. Such well-defined droplets provide unique platforms for encapsulation in the drug and food industries, unique environments for chemical reactions and the “lab on a chip” technology, and for ink-jet printing. Several theoretical and numerical studies have been performed to understand the breakup of liquid jets. Rayleigh16 and Tomotika20 developed the linear breakup theory, which is a powerful tool to understand jet breakup. But, the linear theory is limited to systems where the viscosity of the surrounding fluid is negligible, and cannot predict the satellite drop size. Also, it does not take into account the effect of jet velocity, implying that the jet breakup lengths and droplet sizes are independent of the velocity. However, the direct numerical simulation (DNS) studies of Homma et al.21 showed that the value of breakup length increased with the Reynold’s number ( = / where is the density of water, is the jet velocity and is the dynamic viscosity of water) within the axisymmetric flow regime. All these models are valid only for surfactant-free systems where the liquid-liquid interface is bare. Studies performed on the breakup of surfactant-laden jets with or without the surrounding fluid being present22–27 showed that surfactants slow down the instability. Breakup lengths were observed to increase for the surfactant covered jets, and the increase was reported to be dependent on the surfactant concentration. According to the linear theory, the predicted droplet

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diameter increases with an increase in the Ohnesorge number (ℎ = / where is the dynamic viscosity of water, is the density of water, and is the interfacial tension). The addition of surfactants results in a reduction of the interfacial tension and, hence, an increase in the magnitude of the Ohnesorge number, implying an increase in the droplet diameter. In this contribution, the effect of the NP surfactants formed by the electrostatic interactions between carboxyl functionalized silica (SiO2) nanoparticles (50 nm in size), dispersed in an aqueous phase, and PDMS-NH2 dissolved in toluene, on the breakup of a laminar water jet in oil phase is investigated. The self-assembly and the formation of the nanoparticle surfactants at the water-oil interface is studied using pendant drop tensiometry. We present results demonstrating that the water-oil interfacial tension undergoes a significant reduction, due to the formation of nanoparticle surfactants. An extensive series of PR-instabilities experiments for the water-oil systems with and without nanoparticle surfactants were performed, varying fluid flow rate, jet diameter, and the nanoparticle surfactant concentration. NP-surfactants were found to significantly affect the breakup of laminar liquid jets resulting in longer jet breakup lengths and dripping to jetting phase transitions. Carboxylic acid-functionalized silica nanoparticles (50 nm in diameter) were purchased from Microspheres-Nanospheres Inc. as aqueous solutions (dispersions) and used as received. Monoamine-end-terminated polydimethylsiloxane (PDMS-NH2) was purchased from Gelest Inc. (Mn = 1,000 g/mol, 2000 g/mol). Silica nanoparticles with carboxylic acid functionalization (SiO2-COOH) were dispersed in water at a pH of 7 and PDMS-NH2 was dissolved in toluene. The interfacial assembly of the silica nanoparticles associated with PDMS-NH2 in toluene was assessed by the pendant drop tensiometry for concentrations ranging from 1 to 4 mg/mL. An interface was made by injecting a pendant drop of the aqueous nanoparticle solution into a

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toluene solution containing amine-terminated polydimethylsiloxane. A tensiometer (Kruss, DSA30) provided the time evolution of interfacial tension (γ) by fitting the axisymmetric profile of the droplet to the Young-Laplace equation. Figure 2 shows the time evolution of the water-toluene interfacial tension. The interfacial tension (γ) between toluene and water is 35 mN/m14, though with the PDMS-NH2 (Mw = 2000 g/mol) at 5% v/v concentration, the equilibrium γ = 25 mN/m (b in Figure 2). When a water droplet was placed in the toluene phase containing PDMS-NH2, the amine-terminated PDMS behaves as a surfactant, assembling at the water-toluene interface to reduce the interfacial energy. The carboxylic acid-functionalized silica nanoparticles in the aqueous phase are not interfacially active, but, in contact with a solution of the amine-terminated polymers in the oil phase, a well-defined number of polymer chains anchor to the NP, producing NP-surfactants, that remain and assemble at the interface. This further reduces the interfacial energy as evident in Figures 2c and 2d. The blue curve Figure 2c corresponds to a system with 1 mg/mL silica NP in water and 0.5% v/v 2k PDMS-NH2 in the toluene, while Figure 2d corresponds to 1 mg/mL silica in water and 5% v/v 2k PDMS-NH2 in toluene system is illustrated. As can be seen in Figure 2, the equilibrium interfacial tension decreases with increasing concentration of PDMSNH2 at a fixed silica nanoparticle concentration. Figure 3A shows the dynamic interfacial tension for different molecular weights (Mw = 1000, 2000 g/mol) of PDMS-NH2 at the same nanoparticle and polymer concentrations. The interfacial tension decreased with time for both 1k and 2k PDMS-NH2, but the decrease was much slower in the case of 2k suggesting that the interfacial adsorption is limited by the diffusion of the chains to the interface. Furthermore, by withdrawing the aqueous phase into the syringe, decreasing the area of the water-toluene interface, the self-assembled NP surfactant

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monolayer was compressed and visible wrinkles developed, implying the formation of a solidlike layer at the interface. Upon expansion to its original volume or further, the wrinkles were removed and the original interfacial tension was recovered. Since the assembly did not crack, there was a rapid return to a fluid-like state, suggesting that the assemblies jammed, as opposed to forming a glassy or crystalline layer when the wrinkles were observed.

Figure 2: Dynamic interfacial tension of the water/toluene interface at different concentrations of 2k PDMS-NH2, a: water against toluene, b: water against 5% v/v 2k PDMS-NH2/toluene, c: 1 mg/ml silica/water against 0.5% v/v 2k PDMS-NH2/toluene, and d: 1 mg/mL silica/water against 5% v/v 2k PDMS-NH2/toluene.

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Figure 3: (A) Effect of polymer molecular weight on the interfacial tension at 1mg/ml particle concentration and 5% v/v polymer concentration. (B) Interfacial jamming of the nanoparticle surfactants at the water-toluene interface. B(i): 4 mg/mL carboxylic acid functionalized silica nanoparticles (50 nm, pH 7) in water and a 10% v/v of 1k PDMS-NH2 in toluene, and B(ii): 4 mg/mL carboxylic acid functionalized silica in water and a 10% v/v of 2k PDMS-NH2 in toluene. 9


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Interfacial jamming was investigated for two systems, 4 mg/mL carboxylic acid treated silica nanoparticles in water and a 10% v/v of 1k PDMS- NH2 in toluene, and 4 mg/mL silica nanoparticles in water and a 10% v/v of 2k PDMS-NH2 in toluene. For both systems, a water drop equilibrated for 5 mins, was subjected to compression by reducing the volume of the silica nanoparticle dispersion. Figure 3B(i) shows that wrinkles establish at the water-toluene interface only at large areal compressions, underscoring the liquid-like nature of the assembly and that compressive force is sufficient to cause a detachment of the 1k PDMS-NH2 ligands from the NPs or is sufficient to generate instabilities at the interface causing the NP to be drawn into either the oil or water phase. For 2k PDMS-NH2 ligands, on the other hand, wrinkles developed immediately, even under very small strains, suggesting a more “solid-like” nature of the assembly. This is illustrated in Figure 3B(ii). Video S1 in the Supporting Information shows the buckling of a water droplet immediately after the water-toluene interface is formed for the 4 mg/mL silica nanoparticles in water and a 10% v/v of 2k PDMS-NH2 in toluene system. It should be noted that wrinkling was observed only when both nanoparticles and PDMSNH2 were added to the water and toluene phases, respectively. The compression of the water droplet with the PDMS-NH2 alone being present in the toluene phase shows no wrinkling (Figure S1 in the Supporting Information). As discussed earlier, the structuring of liquids relies on the interfacial jamming of the NP surfactants. Since the water-toluene systems with 2k PDMS-NH2 ligands can be jammed with relative ease, in comparison to the 1k PDMS-NH2, therefore for PRinstabilities studies silica nanoparticle/water - 2k PDMS-NH2 toluene system is considered. To understand the effect of nanoparticle surfactants on the breakup of water jets, jets were formed by forcing aqueous NP dispersions through a narrow capillary using a syringe pump. A high frame rate (up to 650,000 fps) video camera was used to record the breakup of the

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water jets in a toluene solution containing PDMS-NH2. A variety of Nikon lenses were used in conjunction with the high-speed camera. Three capillaries were used to form the jets in this study, having inner diameters of 0.25 mm, 0.41 mm, and 0.69 mm, respectively. Flow rates of the water jet were varied from 1–7 mL/min corresponding to Reynolds numbers from 50–250, depending on the capillary in use. Videos of the breakup of water jets were recorded with and without the nanoparticle surfactants. Figure 4(A) shows the sequence of the images for the breakup of a clean water jet in toluene. It can be seen that primary and satellite droplets are formed from the water jet. Figure 4(B) shows a similar set of images of the breakup of a water jet containing 4 mg/ml carboxylic acid treated silica (pH 7) falling in a toluene solution of amine-terminated PDMS (Mw = 2000 g/mol, 10% v/v). Unlike the case with no surfactant, satellite droplets were not formed with the primary droplet.

A similar suppression in the

formation of satellite droplets has been reported for surfactant covered jets25,28,29. Figure S2 in the Supporting Information shows the breakup of a water jet in a toluene solution containing 10% v/v PDMS-NH2 (Mw = 2000 g/mol). The jetting behavior with PDMS-NH2 alone, i.e., in absence of nanoparticles in the water phase, is similar to the silica nanoparticles in water/ PDMS-NH2 in toluene system (Figure 4(B)). However, it is worth noting that, to lock the shape of liquid jet, jamming of the NP-surfactant assemblies at the water-toluene interface is required, a condition that cannot be satisfied by the polymer alone.

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Figure 4: (A) High speed photography of a clean water jet falling in toluene. The interval between consecutive images is 200 µs. (B) High speed photography of an aqueous nanoparticle solution with 4 mg/mL silica nanoparticles (50 nm, pH 7) falling in toluene solution containing 10% v/v of 2k PDMS-NH2. The interval between consecutive images is 1 ms. Initial jet diameter is 0.25 mm, flow rate is 2 mL/min.

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Figure 5: Phase diagram showing the approximate locations of the transitions between dripping and jetting flow for jets of (A) water in toluene, and (B) 4 mg/mL aqueous silica nanoparticle solution in toluene containing 10% v/v of 2k PDMS-NH2. 13


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Surfactants are reported to affect the transitional boundaries between the dripping and jetting flow regimes29,30. In particular, Clanet and Lasheras30 studied the different flow transitions observed upon increasing the flow rate of a Newtonian liquid. A first “dripping” transition is reached when, instead of the periodic droplet formation with constant volume, a chaotic behavior is observed where the mass of the droplets could vary from one to the next. A “jetting” transition is characterized by the shift of the droplet detachment point downstream, away from the capillary, and a continuous jet is formed. On further increasing the flow rate, longer jets are observed. The jetting transition is defined as the flow rate required to obtain a jet ten times longer than its diameter. To understand the effect of nanoparticle surfactants on the nature of the jetting regime transitions, phase diagrams were constructed for A: water/toluene and B: 4 mg/mL silica NPs (pH 7) in water/toluene with 10% v/v of 2k PDMS-NH2 systems. Figure 5 shows the approximate locations of the two-phase regime (dripping flow, and jetting flow) on the jet velocity as a function of the capillary diameter. As evident from Figure 5(B), with the nanoparticle surfactants, the transitional boundary between the dripping-jetting regimes shifts to the smaller jet velocities, resulting in experimental conditions at the same velocity and same diameter to occur within different flow regimes. For example, for the 0.25 mm (internal diameter) capillary, at a jet velocity of 0.60 m/s, dripping flow was observed for the watertoluene system, i.e., with no surfactant present. (Figure 5A). However, the nanoparticle surfactant system exhibited a jetting flow at the same jet velocity (Figure 5B). This drippingjetting flow transition was less pronounced for capillaries with a larger diameter. For instance, for the 0.69 mm capillary, the dripping-jetting transition was observed at a jet velocity 0.33 m/s for the water-toluene system in comparison to 0.29 m/s for the nanoparticle surfactant system. A

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qualitatively similar behavior for the dripping-jetting transition has been reported in the literature for the air-water jets30, although the precise location of the transition varies under different experimental conditions. The effect of the NP-surfactant concentration on the primary droplet diameter and the jet breakup length was investigated. At a constant flow rate, experiments were performed for water jets with a 4mg/mL silica nanoparticle concentration and the concentration of 2k PDMS-NH2 in the surrounding toluene phase was varied from 5–30% v/v. The videos for the jet breakup events were recorded and the resulting images were analyzed using the ImageJ software to estimate the primary droplet diameter and breakup length. Figure 6(A) shows the effect of the polymer concentration on the measured average droplet diameter, the error bars represent one standard deviation of the mean. The influence of ligand concentration on the droplet diameter was not discernible up to a polymer concentration of 20% v/v. However, at 30% polymer concentration, an increase in the droplet diameter was observed, the measured average droplet diameter being 736 ± 64 µm. The increase in the droplet diameter is in keeping with the linear theory proposed by Rayleigh that predicted an increase in the primary droplet diameter with the increase in Ohnesorge number. An increase in the NP surfactant concentration results in a further reduction in the interfacial tension and, therefore, an increased Ohnesorge number.

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Figure 6: Effect of the polymer concentration on (A) the droplet diameter and (B) the breakup length for a jet of an aqueous silica nanoparticle solution (4 mg/mL) into toluene solution containing amine-terminated PDMS. The insets corresponding to the marked PDMS-NH2 concentration in (B) illustrate the jet breakup length.

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As shown in Figure 6(B), NP surfactants significantly influence jet breakup lengths. In general, breakup lengths increased in the presence of NP surfactants, in comparison to clean jets, i.e., with no SiO2-COOH nanoparticles or PDMS-NH2 in the water or toluene phases, respectively. The increase in breakup length was slight at low polymer concentrations, but more pronounced at high concentrations. At a polymer concentration of 15% v/v, breakup lengths increased by approximately 45% above values observed for a surfactant-free water-toluene system. It is not surprising that low polymer concentrations do not affect the jet breakup length since the polymer chains have insufficient time to diffuse to the surface of jet to form nanoparticle surfactants through their electrostatic interaction with the carboxylic acid treated nanoparticles. By increasing the polymer concentration, we increase the rate at which polymer chains arrive at and anchor to the interface. Therefore, the jet is stabilized by more NP surfactants and it lasts for longer times, allowing the formation of more NP surfactants and jet stabilization. Thus, significantly longer jet breakup lengths at high NP-surfactant concentrations are observed, allowing the interfacial jamming of the NPs. In future, experiments will be performed with jets of sufficiently high viscosities to suppress the breakup of the jets into droplets, enabling the formation of NP-surfactant stabilized tubules (or ligaments) of one liquid in another. In summary, NP-surfactants provide a simple yet reliable route to generate structured liquids via interfacial jamming. A detailed study of the assembly and jamming behavior of NPsurfactants at the interface between two immiscible fluids was performed using 50 nm diameter carboxylic acid functionalized silica nanoparticles and amine terminated PDMS ligands. Microfluidic techniques based on PR-instabilities hold significant promise for the generation of structured liquids. A comprehensive investigation of the effect of the nanoparticle surfactants on

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the breakup of a liquid jet in a surrounding immiscible liquid phase was performed. Nanoparticle surfactants were found to significantly affect the breakup of laminar liquid jets resulting in longer jet breakup lengths, suppression of the formation of satellite drops, and dripping to jetting flow transitions towards lower jet velocities. These findings provide valuable guidance for the selection of concentration of nanoparticles, concentration and molecular weight of the end-functionalized polymer and the jet flow rates for enabling the generation of structured liquids. Although, completely stable jets or tubules were not obtained, due to the high flow rates used in our studies, NP-surfactants will tend to stabilize a jet at reduced flow rates, conventional for additive manufacturing techniques such as 3D printing. The results of these studies constitute a unique platform for printing liquids into structures such as tubules, stabilized by the NPsurfactants. SUPPORTING INFORMATION Droplet morphology and the breakup of water jet in the absence of nanoparticles. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award under Contract No. DE-AC02-05-CH11231 within the Adaptive Interfacial Assemblies Towards Structuring Liquids program (KCTR16). B.A.H. acknowledges additional support from The Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DEAC02-05CH11231.

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