Reconfigurable Microfluidic Droplets Stabilized by Nanoparticle

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Reconfigurable Microfluidic Droplets Stabilized by Nanoparticle Surfactants Anju Toor, Sean Lamb, Brett Helms, and Thomas P. Russell ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07635 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Nano

Reconfigurable Microfluidic Droplets Stabilized by Nanoparticle Surfactants Anju Toor,1 Sean Lamb,2 Brett Helms,1,3 and Thomas P. Russell1,4,5* 1

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

94720, USA 2

Department of Chemistry, University of California, 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]

KEYWORDS: Microfluidics, surfactants, interfacial assembly, nanoparticles, droplet formation ABSTRACT – Interfacial assemblies of nanoparticles can stabilize liquid-liquid interfaces. 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 well-defined number of polymer chains onto the nanoparticles leads to the formation of NP-surfactants that assemble at the interface and reduce the interfacial energy. We have developed droplet interfaces covered with elastic, responsive monolayers of NP- surfactants. Due to the presence of an elastic layer at the interface, the droplets offer a greater resistance to

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coalescence and can prevent the exchange of materials across interfaces. Our results show the successful encapsulation of nanoparticles, dyes and proteins with diameters in the 2.4–30 nm range. Further, we show that stable water-in-oil droplets can be generated for various combinations of polymer ligands and nanoparticles bearing complementary functionalities. These NP-surfactant stabilized microfluidic emulsions would enable applications requiring liquid-liquid interfaces that can adapt and respond to external stimuli, and whose mechanical properties can be easily tailored. NP-surfactants consist of nanoparticles (NP) dispersed in one liquid phase and functionalized surface-active polymers dispersed in a second, immiscible liquid phase.1,2 Complementary (e.g., acid-base) chemistry between the particles and ligands binds the assembly at the liquid-liquid interface. These systems are versatile, since NP-surfactants can be formed regardless of the choice of NP and polymer, and the interfacial segregation relies solely on the interactions between their complementary functionalities. Furthermore, by controlling the strength of interactions between the polymer and NP, assemblies of NP-surfactants can be made responsive to the local chemical environment such as pH.3 Moreover, formation of NP-surfactants is not limited by the constraint that particles need to be partially wetted by both the water and the oil phases, as is the case with particle-stabilized (Pickering)4–6 emulsions. Self-assembled NP-surfactants at the interface between two immiscible liquids can stabilize microfluidic emulsions of one liquid in another, suggesting self-assembled NPsurfactants hold significant potential for drug delivery applications. Many important drug delivery techniques leverage microfluidics for the encapsulation and on-demand release of drugs from the processed droplets. One of the major challenges is the leakage of molecules from the droplet phase into the surrounding phase. For example, molecule retention has been a challenge in assays using fluorophore probes as their readout, since hydrophobic molecules partition out of droplets into the

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continuous phase. According to previous studies, the reverse micelles formed by the surfactants can result in transport of small hydrophobic molecules in the droplets.7–9 On the contrary, the NPsurfactant assemblies are rigid in nature and could prevent the molecular exchange between the droplets. Microfluidics offer a continuous, high-throughput method for the generation of monodispersed droplets stabilized by NP-surfactants. Droplet formation is facilitated in a microfluidic device with a T-junction geometry. The liquid phase to be dispersed is injected into a microchannel by a pressure-driven flow and the second immiscible liquid phase is pumped independently into another orthogonal microchannel. The two liquid phases meet at the junction, where the interface is deformed by the local flow field resulting in necking and eventual break-off of the droplets from the dispersed phase. The size of the droplets depends on the dimensions of the microchannels,10 flow rates11 and viscosities of the liquid phases,12 and the concentration of surfactants.13 The breakup of droplets is dictated by the balance between the local viscous shear stresses acting to deform the interface and the capillary pressure acting to resist the deformation. In a T-junction, three main droplet formation regimes are observed: squeezing, dripping and parallel flowing stream. At low capillary number (𝐶# = 𝜇𝑈/𝜎, where 𝜇 is the viscosity of the most viscous liquid phase in the two-phase system, 𝑈 is the velocity of that phase, and 𝜎 is the interfacial tension), as the dispersed phase penetrates the channel filled with the continuous phase a pressure gradient builds up upstream squeezing the dispersed phase, such that it breaks up into droplets. This droplet formation regime is called squeezing. The size of the droplets scales with the ratio of the flow rates of the dispersed and continuous phase, 𝑄, /𝑄- .10,14 With the increase in capillary number, shear stresses dominate the droplet formation, and the droplet breakup occurs when the viscous shear stresses overcome the interfacial tension. This high capillary number

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regime is referred to as the dripping regime. As the capillary number of the continuous phase increases further, the transition from droplet formation to parallel flows occurs. 15

Figure 1 (a) The formation of water-in-oil droplets in a microfluidic T-junction. The water phase has acid-treated silica (Si-COOH) NP, and the amine-terminated polymer ligands are dissolved in the oil phase. The water phase is sheared by the oil phase resulting in the elongation of the water phase in the outlet channel, followed by necking and eventual breakage of the water phase into droplets. At high polymer and NP concentrations, polymer chains and NP interact at a suitable time-scale to form NP-surfactants at the droplet interfaces. (b) Micrograph of the microfluidic device showing the droplet generation in a T-junction geometry. Shown in Figure 1 (a) is a schematic diagram of droplet formation in a T-junction in the dripping regime where the dispersed phase is an aqueous dispersion of acid-functionalized NP and silicone oil containing an amine-terminated polymer constitutes the continuous phase. As water droplets are formed, NP and polymer can interact at the droplet interface forming NP-surfactants. The extent of coverage of the droplet interface by the NP-surfactants depends on the concentrations of the NP and polymer ligands in the respective phases and the flow rates of the dispersed and continuous phase. In our previous studies 2,16–18, a water droplet was equilibrated in the oil phase for extended periods of time, permitting adequate time to facilitate the interaction between the acid-treated NP dispersed in water and amine-terminated polymer chains dissolved in oil, leading to the formation and the assembly of NP-surfactants. In microfluidics, the timescales involved are much faster as the fluids are in motion and the droplet formation occurs rapidly. Hence, high

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concentrations of the NP and polymers are likely required to allow the formation of the NPsurfactants effectively. We were successful in generating NP-surfactant stabilized aqueous droplets in a T-junction glass microfluidic device (Figure 1 (b)) with a hydrophobic coating. Such well-defined droplets provide platforms for encapsulation in the drug and food industries, high-throughput screening and diagnostics

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and environments for chemical reactions22. The size of the droplets and their

stability is investigated as a function of the size and concentration of the acid-functionalized NP, type of the amine-functionalized polymer, the viscosities and flow rates of the fluids. These studies rely on the understanding of the kinetics of the formation and assembly of NP-surfactants at the water/oil interface that is studied using a pendant drop tensiometer. The in-situ formation of the NP-surfactant assemblies on the surface of the droplets yields stable and robust water-in-oil microfluidic emulsions. Results and Discussion The kinetics of the NP-surfactant formation at the water-oil interface was studied using pendant drop tensiometry. The interfacial tension (g) between silicone oil and water is 25 mN/m (a in Figure 2A). When a water droplet was placed in the oil phase containing PDMS-NH2, the amine-terminated PDMS behaves as a surfactant, assembling at the water-oil interface to reduce the interfacial energy.

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(A)

Water Silicone Oil

(B) pH = 6

Volume Reduction

pH = 4.2

Volume Reduction

Figure 2: (A) Dynamic interfacial tension of the water/silicone oil interface at different concentrations of PDMS-NH2, (a): water against silicone oil, (b): 4 mg/mL Si-COOH NP (30 nm)/water against 1% w/w PDMS-NH2/silicone oil and (c): 4 mg/mL Si-COOH NP /water against 5% w/w PDMS-NH2/silicone oil. (B) Interfacial jamming of the NP-surfactants at the oil-water interface for a system having 4 mg/mL Si-COOH NP dispersed in water and a 1% w/w of PDMSNH2 in silicone oil for a NP dispersion at pH 6 or 4.2. 6


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Si-COOH NP 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 is evident in b-c in Figure 2A. The blue curve (b in Figure 2A) corresponds to a system with 4 mg/mL Si-COOH NP (30 nm diameter) in water and 1% w/w PDMS-NH2 in silicone oil, while the red curve (c in Figure 2A) corresponds to 4 mg/mL silica in water and 5% w/w PDMS-NH2 in silicone oil. As can be seen in Figure 2A, the equilibrium interfacial tension decreases with increasing concentration of PDMS-NH2 at a fixed Si-COOH NP concentration. Figure S1 in the Supporting Information shows the time evolution of the water/silicone oil interfacial tension for the NP only and polymer only cases. After the interfacial tension equilibrated the volume of the drop was reduced by withdrawing the aqueous phase into the syringe. A wrinkling on the surface of the droplet was observed which is characteristic of an elastic film at the oil-water interface. Figure 2B shows the droplet morphology and buckling behavior for different NP solution pH values. In both cases, the water droplet was equilibrated for an hour in the surrounding oil solution containing 1% w/w PDMS-NH2 before withdrawing the silica NP dispersion into the syringe. At a pH = 6 (i.e., pH > pKa of carboxylic acid groups) the surfaces of NP are deprotonated. With the layer of PDMS-NH2 at the oil-water interface, negatively charged NP diffuse to the interface and interact with the protonated amines, forming NP-surfactants. These NP-surfactant assemblies develop wrinkles upon compression due to the jamming of the NP (Figure 2B). Furthermore, the wrinkles do not relax suggesting an irreversible adsorption of the NP-surfactants at the interfaces. It is worth noting that the wrinkling was observed only when both NP and PDMS-

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NH2 were present in the respective water and silicone oil phases (Figure S2 in the Supporting Information). At a pH = 4.2 < pKa of carboxylic acid, no wrinkles appeared. At pH < pKa, the carboxylic acid groups on the NP are in the protonated state (COOH), and hence, electrostatic interactions between the NP and PDMS-NH3+ ligands are weakened leading to an ejection of the NP-surfactants from the interface when the assemblies are compressed. These results show that the interfacial packing of the NP-surfactants can be manipulated by changing the pH. By precisely controlling the flow rates of the water and oil phases, and the NP and polymer concentrations, conditions can be established allowing the NP and polymers to interact and form NP-surfactants at the droplet interfaces on a time-scale commensurate with droplet production. Studying the effect of particle concentration on the timescale of interfacial jamming yielded the range of NP concentration to be employed for droplet generation experiments. Figure S3 in the Supporting Information shows that under static conditions, at [NP] ≥ 2mg/mL, wrinkles develop almost immediately i.e., even under very small strains, suggesting a more “solid-like” nature of the assembly. Wrinkling occurs due to the jamming of NP-surfactants suggesting that the droplet surface was sufficiently covered with NP-surfactant assemblies. To understand the effect of NP concentration on the stability of microfluidic droplets, droplets were generated by injecting aqueous dispersions of carboxylic acid-functionalized silica NP (30 nm diameter) into a flowing solution of amine-terminated PDMS in silicone oil. The concentration of PDMS-NH2 was kept constant at 20% w/w, and the NP concentration varied from 2-4 mg/mL. Figure 3 (a) and (b) show the optical microscope images of the droplets generated at [NP] = 2 mg/mL and 4 mg/mL, respectively. It can be observed in Figure 3 (a) that some of the droplets have coalesced yielding non-spherical structures. Partial coalescence of the water droplets lacking sufficient coverage by the NP-surfactants gives rise to such structures. At increased NP

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concentration, as shown in Figure 3 (b), monodisperse, stable droplets are obtained. By increasing the NP concentration, the density of NP-surfactants assembling and forming on the droplet interfaces is increased, which is responsible for the increased stability against coalescence. Figure S4 in the Supporting Information shows the confocal micrographs of water-in-oil emulsions coated with interfacial assemblies of fluorescent Si-COOH (30 nm diameter) and PDMS-NH2.

Figure 3: Effect of Si-COOH NP (30 nm) concentration on the stability against coalescence for the droplets generated in a microfluidic device. (a) [NP] = 2 mg/mL and (b) [NP] = 4mg/mL. The concentration of the PDMS-NH2 was kept constant at 20% w/w in silicone oil. Qw = 60 µL/h, Qo = 1892 µL/h. The effect of flow rate of the continuous phase on the droplet size is illustrated in Figure 4. The flow rate of the water phase (Qw) was kept constant at 60 µL/h, and the oil phase flow rate (Qo) was varied over a range of 1800–2600 µL/h. Figures 4 (a), (b) and (c) correspond to the optical microscope images of the droplets collected at Qo = 1892, 2208 and 2398 µL/h respectively. Figure 4 (d) shows a plot of the measured average droplet diameter as a function of the oil phase flow rate. The droplet diameter decreases with an increase in flow rate of the oil phase. This is in agreement with theoretical predictions23 and previous experimental studies

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formation in T-shaped microchannels. The diameter of the generated droplets can be varied from 77 µm to 100 µm by varying Qo in the 1800–2600 µL/h range.

Figure 4: Optical microscope images of the water droplets generated for varied flow rates of the oil phase. (a) Qw = 60 µL/h, Qo = 1892 µL/h, (b) Qw = 60 µL/h, Qo = 2208 µL/h, (c) Qw = 60 µL/h, Qo = 2398 µL/h. (d) Average droplet diameter as a function of the flow rate of the oil phase (error bars represent one standard deviation of the mean). [Si-COOH] = 4mg/mL, [PDMS-NH2] = 20% w/w in silicone oil. In addition to PDMS-NH2, we examined different amine-terminated polymer ligands, such as poly[dimethylsiloxane-co-(3-aminopropyl)-methylsiloxane] copolymer (CP) and a–w diamino terminated polystyrene (PS). An aqueous dispersion of Si-COOH NP ([NP] = 4 mg/mL, pH = 6) was used as the dispersed phase for the droplet generation experiments discussed in Figure 5 (ac). In the optical microscope images shown in Figure 5 (a) and (b), the continuous phase (CP)

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consists of 30% w/w of CP in toluene and silicone oil respectively. The average diameter of the droplets shown in Figure 5 (a) and (b) is 95 ± 1 µm and 98 ± 1.5 µm respectively. Droplet generation experiments were also carried out using a toluene dispersion of PS ligands (5% w/w). The optical microscope image of the generated droplets is shown in Figure 5 (c). The droplets have an average diameter of 78 ± 1 µm. These results confirm that a wide range of functional polymers can be used, provided that the functional groups on the surface of the NP are complementary to those of the functional polymer. Therefore, interfaces can be engineered by tuning the type and molecular weight of the functionalized polymer to obtain tailored mechanical properties. This implies that the rigidity of the droplets interfaces can be manipulated. The extent of versatility is not limited to the choice of polymer ligands, a gamut of NP of varying size, shape or material can be used to form NPsurfactant assemblies. PS-COOH NP (24.5 nm in diameter) dispersed in water and PDMS-NH2 dissolved in silicone oil were considered as the NP-surfactant system for the droplet generation experiments. Figure 5 (d) shows the resulting droplets having an average diameter of 115 ± 3 µm. It should be noted that, independently, the NP and polymer ligands are not effective in stabilizing droplets. For the results shown in Figure S5 (a-e), no NP were added to the dispersed phase. Figure S5 (a-b) in the Supporting Information shows the microscope images of the droplets obtained using water as the dispersed phase and a 20% w/w of PDMS-NH2 in silicone oil. The image shown in S5 (a) was captured immediately after the droplet generation and (b) was taken after a few minutes. It can be seen that the droplets were not stable against coalescence. The micrographs of the water in oil droplets stabilized by amine-functionalized PS and CP is shown in Figure S5 (c) and S5 (d) respectively. A similar behavior was observed when the droplets were generated using an aqueous dispersion of silica nanoparticles ([NP] = 4 mg/mL, pH = 6) as the

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dispersed phase and silicone oil as the continuous phase (i.e., no Polymer-NH2 was added to the oil phase) (Figure S5 (f) in the Supporting Information). This result would be expected, since the silica NP are not interfacially active when used independently.

Figure 5: Optical microscope images of the NP-surfactant stabilized water droplets. (a) Dispersed phase: 4 mg/mL, Si-COOH (30 nm) NP dispersed in water, continuous phase: 30% w/w CP dissolved in toluene. (b) Dispersed phase: 4 mg/mL, Si-COOH (30 nm) NP dispersed in water, continuous phase: 30% w/w CP dissolved in silicone oil. (c) Dispersed phase: 4 mg/mL, Si-COOH (30 nm) NP dispersed in water, continuous phase: 5% w/w PS dissolved in toluene. (d) Dispersed phase: 4 mg/mL, PS-COOH (24.5 nm) NP dispersed in water, continuous phase: 20% w/w PDMSNH2 dissolved in silicone oil.

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Figure 6 (a) shows the water droplets stabilized by the NP-surfactant assemblies consisting of 20 nm Si-COOH NP and PDMS-NH2 ligands. The micrograph in Figure 6 (b) shows the droplets after ~30 min of generation. As evident from Figure 6 (b), a solid film is formed at the droplet interfaces due to the jamming of the NP. With time, as water evaporates from the droplets, the NP-surfactant assemblies are subjected to a compressive force which results in the jamming of the NP. This behavior is analogous to the pendant drop results where wrinkles developed on the droplet surface upon compression (shown in Figure 2(b)).

Figure 6: Optical microscope images of the water droplets stabilized by the NP-surfactant assemblies consisting of 20 nm Si-COOH NP and PDMS-NH2 ligands. [NP] = 4 mg/mL, [PDMSNH2] = 20% w/w. (a) right after droplet generation (b) after 30 min of generation. Confocal microscope images of water droplets containing (1) fluorescent silica particles (30 nm diameter) and (2) 6 nm diameter FITC-Dextran are shown in Figure 7 (a) and (b) respectively. It is evident that the fluorescent particles are confined to the droplets. In Figure 7 (a) dispersed phase is an aqueous dispersion of 4 mg/mL Si-COOH NP (20 nm diameter) and 2 mg/mL ‘plain’ fluorescent silica NP (30 nm diameter). The particle clusters seem to be forming due to the interactions between the acid-functionalized silica and plain fluorescent silica. Figure S6 (a) in the Supporting Information shows an emulsion comprising two populations of water droplets 13


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containing either 1 or 2 mg/mL of FITC-Dextran as represented by dark and bright green color respectively. In Figure S6 (b), we compared the fluorescence intensity of bright droplets with the dark ones which happen to be in the vicinity of each other, as depicted by the white circles in Figure S6 (a). The difference in the magnitudes of the FITC-Dextran fluorescence signals of the two populations remained constant over time, and the two droplet populations were distinguishable.

Figure 7: Confocal laser scanning microscopy images of the NP-surfactant stabilized water droplets. Encapsulation of (a) 30 nm diameter, ‘plain’, fluorescent silica NP and (b) ~ 6 nm diameter FITC-Dextran. (a) Droplet phase: 4 mg/mL, 20 nm Si-COOH NP + 2 mg/mL 30 nm ‘plain’ silica NP. (b) Droplet phase: 4 mg/mL, 20 nm Si-COOH NP + 2 mg/mL FITC-Dextran. Surrounding phase: 20% w/w PDMS-NH2 dissolved in 5 cSt silicone oil.

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Figure 8: (a) Schematic of a NP-surfactant stabilized water droplet containing fluorescent proteins. Confocal laser scanning microscopy images showing encapsulation of EGFP and YFP. (b) Droplet phase: 4 mg/mL, 20 nm Si-COOH NP + 40 µg/mL EGFP dispersed in PBS buffer. (c) Droplet phase: 4 mg/mL, 20 nm Si-COOH NP + 40 µg/mL YFP dispersed in PBS buffer. Surrounding phase: 20% w/w PDMS-NH2 dissolved in 5 cSt silicone oil. We were interested in understanding how the processing of NP-surfactant stabilized liquid droplets might influence the stability of macromolecular guests, like proteins. As model guests, we chose fluorescent proteins, which require a precisely folded structure to maintain their luminescence. In the absence of luminescence, we could reason that the folded structure of the protein was substantially disrupted. Figure S5 (f) in the Supporting Information shows the droplet micrograph for the case when droplet phase is PBS buffer solution (i.e. no acid-functionalized NP) and surrounding phase is amine-terminated PDMS dissolved in silicone oil. To investigate the encapsulation of proteins, NP (20 nm Silica-COOH) dispersion was prepared in PBS buffer with a EGFP concentration of 40 µg/mL. Similarly, silica NP PBS dispersion containing YFP (40 µg/mL) was prepared. To encapsulate proteins, aqueous droplets were generated in a silicone oil phase containing PDMS-NH2 ligands. Figure 8 (a) shows a schematic of a water droplet containing EGFP and the corresponding confocal microscope image is shown in Figure 8 (b). Encapsulation of YFP in the NP-surfactant stabilized droplets is shown in Figure 8 (c). These

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images imply that fluorescent proteins can be encapsulated in the NP-surfactant stabilized droplets, and the structure of proteins is not adversely affected during the droplet generation and collection process. Conclusions Elastic droplet interfaces consisting of NP-polymer assemblies are developed and employed for encapsulation applications. In contrast to the surfactant stabilized microfluidic emulsions where typically the materials can diffuse either from one droplet to another or from the droplet phase to the surrounding oil phase, the molecular exchange/transport was restricted in the droplets coated with NP-surfactant assemblies. The NP and polymer concentrations play a major role in governing the extent of coverage of the droplet surfaces by the NP-surfactant assemblies at time scales relevant to microfluidics. A range of polymer ligands and NP were examined and the appropriate flow rate conditions allowing the generation of stable droplets were determined. These NP-surfactant stabilized microfluidic droplets can be useful as microreactors for droplet-based assays, diagnostics, and high-throughput screening.

Materials and Methods Fluorescent carboxylic acid-functionalized silica (Si-COOH) NP (30 nm in diameter) were obtained from Micromod Partikeltechnologie GmbH and used as received. Aqueous dispersions of Si-COOH NP, diameter 20 nm and 24.5 nm diameter carboxylated polystyrene (PS-COOH) NP were obtained from Microspheres Nanospheres. Aqueous dispersions of NP with a desired concentration and pH were prepared by diluting the as-received dispersions with de-ionized water and adjusting the pH using either 1.0 M HCl or 1.0 M NaOH. Based on our experiments, NP solution pH in the range 5–6.5 results in stronger interactions between the NP and the

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functionalized polymers. Mono-aminopropyl terminated polydimethylsiloxane (PDMS-NH2) was received

from

Gelest

Inc.

(Mw

=

2000

g/mol).

Poly[dimethylsiloxane-co-(3-

aminopropyl)methylsiloxane] copolymer (CP) was obtained from Sigma Aldrich and a-w diamino terminated polystyrene (‘PS’) with a molecular weight, Mw = 30,000 g/mol was received from Polymer Source. The ligand dispersions were prepared by dissolving the aminefunctionalized polymer in the oil phase. For encapsulation experiments, plain i.e., with no acid functionality, fluorescent silica particles (30 nm diameter) were obtained from Micromod Partikeltechnologie GmbH and fluorescein isothiocyanate-dextran (FITC-Dextran), Mw = 20,000 was received from Sigma Aldrich. Enhanced green fluorescent protein (EGFP) and yellow fluorescent protein (YFP) were obtained from Biovision Inc. A pendant drop tensiometer was used to probe the interfacial tension of Si-COOH NP in aqueous media associated with PDMS-NH2 in silicone oil. A drop of the aqueous NP dispersion was injected into silicone oil containing PDMS-NH2 to form the interface (and interfacial film). A tensiometer (Kruss, DSA30) provided the time-evolution of interfacial tension (g) by fitting the axisymmetric profile of the droplet to the Young-Laplace equation. ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Contract No. DE-AC02-05-CH11231 within the Adaptive Interfacial Assemblies Towards Structuring Liquids program (KCTR16).

SUPPORTING INFORMATION Droplet buckling and optical microscope images of the droplets generated in the absence of NP or functional polymers, and measurement of fluorescence intensity as a function of time. 17


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Reconfigurable Microfluidic Droplets Stabilized by Nanoparticle

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