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Thermo-Responsive Microcarriers for Smart Release of Hydrate Inhibitors under Shear Flow Sang Seok Lee, Juwoon Park, Yutaek Seo, and Shin-Hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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ACS Applied Materials & Interfaces

Thermo-Responsive Microcarriers for Smart Release of Hydrate Inhibitors under Shear Flow Sang Seok Lee,†,‡, Juwoon Park,§, ‡ Yutaek Seo,§,* and Shin-Hyun Kim†,* †

Department of Chemical and Biomolecular Engineering (BK21+ Program) KAIST, Daejeon, 305-701 Republic of Korea

§

Department of Naval Architecture and Ocean Engineering, Seoul National University, Seoul, 151 - 744 Republic of Korea

KEYWORDS: Hydrate, Microcarriers, Smart release, Microfluidics, Flow assurance

ACS Paragon Plus Environment

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ABSTRACT

The hydrate formation in subsea pipelines can cause oil and gas well blowout. To avoid the disasters, various chemical inhibitors have been developed to prevent or delay the hydrate formation and growth. Nevertheless, direct injection of the inhibitors results in environmental contamination and cross-suppression of inhibition performance in the presence of other inhibitors against corrosion and/or formation of scale, paraffin, and asphaltene. Here, we suggest a new class of microcarriers that encapsulate hydrate inhibitors at high concentration and release them on demand without active external triggering. The key to the success in microcarrier design is lying in the temperature dependence of polymer brittleness. The microcarriers are microfluidically created to have inhibitor-laden water core and polymer shell by employing water-in-oil-in-water (W/O/W) double-emulsion drops as a template. As the polymeric shell becomes more brittle at a lower temperature, there is an optimum range of shell thickness that renders the shell unstable at temperature responsible for hydrate formation under a constant shear flow. We precisely control the shell thickness relative to the radius by microfluidics and figure out the optimum range. The microcarriers with the optimum shell thickness are selectively ruptured by shear flow only at hydrate formation temperature and release the hydrate inhibitors. We prove that the released inhibitors effectively retard the hydrate formation without reduction of their performance. The microcarriers that do not experience the hydration formation temperature retain the inhibitors, which can be easily separated from ruptured ones for recycling by exploiting the density difference. Therefore, the use of microcarriers potentially minimize the environmental damages.


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1. INTRODUCTION Smart microcarriers have been designed to fulfill on-demand release function of bioactive substances required for various biomedical applications.1-3 The microcarriers protect the actives from the external environment to prevent deactivation and release them at the desired moment and location by sensing the environmental conditions to maximize the efficacy of the actives.4-7 Various polymeric materials have been used to compose carrier membranes, which sense conditions such as pH,8-10 temperature,11,12 and concentration of specific molecules13,14 and selectively release encapsulants in a response to the stimuli. Recent advances in droplet-based microfluidics enable the exquisite control over carrier size, structure, and composition, improving the performance of smart release and expanding the applicable fields of smart carriers.15 The smart release of actives can benefit deepwater oil and gas processes, despite no single study has been reported. In subsea pipelines, multiple phases of gas, water, and oil flow at high pressure and low temperature frequently lead to the formation of solid crystalline compounds, so-called gas hydrates, ending up being large chunks to block the flow;16,17 deepwater oil and gas well blowout may result in the uncontrolled burst of oil to the environment.18-22 Gas hydrates are composed of hydrogen-bonded water cages, in which light hydrocarbon molecules occupy the cages through van der Waals interaction with water molecules.16 They form at high pressure (e.g. higher than 5 MPa) and low temperature (e.g. lower than 17oC), thus the deepwater and ambient seafloor temperature are typical conditions at which gas hydrates are thermodynamically stable. Prevention of the hydrate formation has been achieved by mixing the aqueous phase with alcohol- or glycol-based hydrate inhibitors because they shift the thermodynamic conditions for


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hydrate formation toward lower temperature region.16,23 However, this conventional approach involves the handling of a large amount of the hydrate inhibitors as it requires dosage as high as 10-40 v/v% to the water for high efficacy, therefore massive infrastructure is required for storage and injection of the inhibitors, which is not appropriate for deepwater hydrate prevention.16,17 To overcome the disadvantages of the thermodynamic hydrate inhibitors, kinetic hydrate inhibitors (KHIs) based on water-soluble polymers have been developed to delay the hydrate nucleation for the desired duration with a low dosage of 0.25-3.0 v/v%. The KHIs can delay hydrate formation by increasing the energy barrier to form hydrate nuclei, or by adsorbing to the surface of the hydrate crystals, instead of shifting equilibrium conditions for hydrate formation.23,24 These include the homo- and co-polymers of n-vinyl caprolactam (VCap)25-27 and n-isopropyl acrylamide (NIPAM).28-30 Although the KHIs have reduced expenditures, they may cause a severe pollution of water due to its active lactam rings in its polymer structure which interact with cells of a living organism, regulating or even banning the use of KHIs in offshore fields.24,31 Furthermore, the KHIs partially lose their efficacy when used with corrosion inhibitors which are also essential components in the flowlines; therefore, hydrate and corrosion inhibitors should be separately supplied using umbilical.32,33 There are many additional inhibitors used to prevent the formation of scale, paraffin, and asphaltene, of which cross-suppression of efficacies is difficult to predict. We believe smart carriers that encapsulate own inhibitors and release them on demand are promising for addressing all the aforementioned problems. In this work, we design core-shell microcarriers that encapsulate low dosage KHIs in the core and selectively release them only at temperature responsible for hydrate formation under shear flow. The microcarriers are microfluidically created by employing water-in-oil-in-water (W/O/W) double-emulsion drops as a template. The KHIs dissolved in the core at a high


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concentration are encapsulated by polymerizing photocurable resin in the oil shell. In general, polymers are more brittle at a lower temperature. Therefore, the polymeric shells can be delicately designed by controlling their thickness to be ruptured by shear flow only at hydrate formation temperature. The KHIs released from the microcarriers are diluted to a proper concentration, retarding hydrate formation. The microcarriers that do not experience the low temperature retain the KHIs and can be recovered for reuse, thereby minimizing the environmental damages; the intact microcarriers can be easily separated from ruptured ones by exploiting their density difference. As the microcarriers obviate the cross-suppression, various inhibitors encapsulated by their own microcarriers can be potentially injected through a single flowline, while maintaining their inhibition efficacy. 2. RESULTS AND DISCUSSION 2.1. Microfluidic production of microcarriers. Microcarriers are designed to have KHI-laden water core and polymeric shell using W/O/W double-emulsion drops as a template.34-37 To precisely control the sizes and compositions of core and shell, we use a capillary microfluidic device.38-41 The device is comprised of two tapered cylindrical capillaries that are coaxially aligned in a square capillary, as shown in Figure 1a. One of the tapered capillaries has an orifice with an inner diameter of 350 µm, which is rendered to hydrophobic. The other tapered capillary has an orifice with an inner diameter of 450 µm, which is rendered to hydrophilic. These two capillaries are assembled to have tip-to-tip separation of 300 µm. To compose a core of microcarriers, poly(n-vinyl caprolactam) (PVCap) is typically dissolved at the concentration of 20 w/w% and poly(vinyl alcohol) (PVA) is dissolved as a surfactant at the concentration of 2 w/w%. The maximum loading of PVCap in the core solution is 25 w/w%; above the concentration, the solution is highly viscous, which causes the transition of a drop breakup mode


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from stable dripping to unstable jetting, resulting in polydisperse double-emulsion drops. In addition, either red- or green-colored dye is dissolved in the aqueous solution to inspect the leakage and release of encapsulants.42 As a middle oil phase, a photocurable resin of ethoxylated trimethylolpropane triacylate (ETPTA) containing 1 w/w% photoinitiator is used, which forms a highly-crosslinked polymer through photopolymerization; Young’s modulus of polymerized ETPTA (pETPTA) is 82 MPa at room temperature.43,44 As a continuous phase, an aqueous solution of 10 w/w% PVA is used. The inner solution is injected through the hydrophobic capillary and the middle oil is injected through the interstices between the hydrophobic and square capillaries. At the same time, the continuous phase is injected through the interstices between the hydrophilic and square capillaries as a counter-flow to the inner and middle flows, as illustrated in Figure 1a. The aqueous inner solution only wets inner wall of the hydrophobic capillary, whereas the middle oil wets the outer and tip walls, thereby enclosing the core, as shown in Figure 1b. The continuous phase wets the whole surfaces of the hydrophilic capillary, preventing a contact of the inner and middle phases. Therefore, the inner and middle phases are coaxially emulsified to the continuous phase, producing monodisperse W/O/W double-emulsion drops, as shown in Figure 1b and Movie S1 of the Supporting Information. The flow rates of the inner, middle, and continuous phases are typically set to 1000, 450, and 3250 µL/h, respectively, which produces 5.84 × 104 drops with an average diameter of 361.4 µm per hour in a dripping mode. The drops flow through the hydrophilic capillary, which are collected in an aqueous solution of 5 w/w% PVA. During the collection, drops are irradiated by ultraviolet (UV) to polymerize ETPTA shell. For each operation, drops are collected for 4 hours, which provides 2.35 × 105 microcarriers with a yield of 95%; a photograph of microcarriers is shown in Figure S1.


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2.2. Control of shell thickness. Polymers become brittle at low temperature in general. Therefore, the pETPTA shell has a higher chance to be ruptured at a lower temperature under constant shear flow. We hypothesize that the temperature responsible for the shell rupture depends on the shell thickness as higher mechanical stability is expected for the thicker shell. We figure out the optimum thickness for the shell rupture at the temperature responsible for the hydrate formation. The microfluidics enables us to precisely control the shell thickness relative to the outer radius, t/R, through adjusting flow rates. As the inner and middle phases are coemulsified to form double-emulsion drops, a simple mass balance equation predicts the dependence of t/R on the volumetric flow rates of the inner and middle phases, Qi and Qm:45,46 ௧

ோ

= 1 − ቀ1 +

భ

ொ೘ ିయ ቁ . ொ೔

(1)

The outer radius is predominately influenced by the flow rate of the continuous phase, Qc, in a dripping mode. Therefore, we adjust Qi and Qm, while slightly varying Qc to maintain dripping mode to prepare monodisperse microcarriers with different values of t and comparable values of R; details are summarized in Table S1 of the Supporting Information. For example, microcarriers with t = 31.7 µm and R = 182.0 µm (t/R = 0.174) are obtained by setting Qi = 1000 µL/h, Qm = 750 µL/h, and Qc = 3250 µL/h (Qm/Qi = 0.75), as shown in top panels of Figure 1c; the shell thicknesses and radii are measured with scanning electron microscope (SEM) and optical microscope (OM) images, respectively. In addition, microcarriers with t = 20.9 µm and R = 180.7 µm (t/R = 0.116) are prepared with Qi = 1000 µL/h, Qm = 450 µL/h, and Qc = 3250 µL/h (Qm/Qi = 0.45), as shown in bottom panels of Figure 1c. The influence of Qm/Qi on the value of t/R for distinct microcarriers is in good agreement with equation (1), as shown in Figure 1d, implying the microfluidic method highly controllable and reproducible.


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We first validate the hypothesis that larger value of t/R provides higher mechanical stability through two different methods: buckling and compression. Microcarriers with four different values of t/R are subjected to hypertonic conditions and the fraction of buckled microcarriers is characterized as a function of the pressure difference, as shown in Figure S2. In hypertonic condition, the positive pressure difference pumps water out through the shell, while the shell rigidity resists it.47,48 The threshold pressure difference, Π*, that causes the microcarriers buckled has a quadratic relation with t/R, indicating higher stability for larger t/R; this relation is consistent with classical shell theory.49 Microcarriers with four different values of t/R are compressed with a pair of two parallel glass plates to study the maximum forces the microcarriers endure, as shown in Figure S3. As compressed, the microcarriers are deformed to a disk while maintaining the shell integrity, which finally fracture to several pieces. The maximum force at the fracture is linearly proportional to t/R as shown in Figure S3, confirming higher stability for larger t/R. 2.3. Influence of temperature on the shell rupture under shear flow. Carriers are stable and remain intact even under shear flow if the ultimate stress of the shell is larger than the shear stress exerted by the flow. When the shear stress overwhelms the ultimate stress, the carriers are broken and all encapsulants are released to the surrounding. As temperature decreases, so does the ultimate stress of polymer shell. Because the ultimate stress increases with shell thickness, there is an optimum range of shell thickness that renders the shell unstable below a certain temperature. To study the influence of temperature on the shell rupture under shear flow and figure out the optimum value of t/R for the smart release of hydrate inhibitors, we use an autoclave with an inner diameter of 4 cm and height of 8 cm, equipped with an impeller with a diameter of 3.8 cm. The microcarriers with t/R = 0.142, 0.110, 0.099, and 0.068 suspended in


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water are loaded in the autoclave respectively. The impeller is set in rotational motion at 600 rpm to reflect shear stress at typical subsea flowlines with Reynolds number of 30,000 and temperature is set to be 28, 23, 20, and 15ºC respectively, as illustrated in Figure 2a. The fractions of ruptured microcarriers are analyzed with optical microscope images after 30 minutes of stirring at each temperature, as shown in Figure 2b. The microcarriers with t/R = 0.068 and 0.099 rupture more than 95% and 57% at 28ºC as shown in Figure 2b and S4. The hydrate formation temperature is typically in the range of 15-25 ºC despite the temperature varies with the composition and pressure. Therefore, the microcarriers with t/R < 0.1 are inappropriate for smart microcarriers because they rupture and release the encapsulants even above the temperature range of hydrate formation. The microcarriers with t/R = 0.110 and 0.142 rupture only 18% and 12% at 28ºC as they are more stable than those with smaller t/R. The fraction of the ruptured microcarriers increases to 46%, 77%, and 88% as temperature decreases to 23ºC, 20ºC, and 15ºC respectively for the microcarriers with t/R = 0.110, as shown in Figure 2b and c. The microcarriers with t/R = 0.142 rupture 16%, 34%, and 69% for the same set of temperature change, as shown in Figure 2b and d. Therefore, the appropriate range of t/R for the smart release of KHIs is between 0.110 and 0.142. 2.4. The smart release of KHIs from microcarriers and retardation of hydrate formation. To verify the smart release of KHIs from microcarriers and study inhibition effect, we perform hydrate formation experiment in the presence of microcarriers using the autoclave, as illustrated in Figure 3a. In detail, 0.23 g microcarriers with t/R = 0.120 containing 20 w/w% PVCap in the core are suspended in a mixture of 12 ml water and 18 ml decane (40 % water-cut); 0.23 g is equivalent to 9,300 microcarriers. As the weight fraction of the core to the microcarrier is 0.66, a full release of PVCap can lead to the concentration of 0.25 w/w% in 12 ml water. In the


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autoclave, the suspension mixture is loaded and a natural gas composed of 90 mol% of methane, 6 mol% of ethane, 3 mol% of propane, and 1 mol% of n-butane is applied at 120 bars. The temperature of the reactor is set to decrease from 24ºC to 4ºC with a rate of 0.25ºC/min, while maintaining the rotation speed of the impeller at 600 rpm. In this condition of composition and pressure, the equilibrium temperature responsible for the hydrate formation is measured as 21ºC in our previous works.50-51 The temperature and pressure are continuously monitored during the cooling, as shown in Figure 3b and S5, respectively. The temperature linearly decreases from 24ºC to 9.2ºC with a constant slope, which then begins to fluctuate at 67.5 minutes as indicated by a red arrow in Figure 3b. The fluctuation is caused by exothermic hydrate formation. Therefore, subcooling temperature, ∆Tsub, defined as a temperature drop at the onset of hydrate formation from equilibrium point is estimated as 11.8ºC. Onset time, tonset, defined as the interval between the equilibrium and the onset of hydration formation is estimated as 46.5 minutes. The former, ∆Tsub, can be used to determine the temperature condition to prevent the hydrate formation, while the latter, tonset, can be used to determine the duration for hydrate avoidance. To further confirm that there is no hydrate formation before tonset, the quantity of gas consumed by hydrate formation, ∆nH,t, is calculated from the difference between the pressure experimentally recorded and pressure calculated from temperature in absence of hydrate formation, as shown in the right y-axis of Figure 3b; see Supporting Information for details. The value of ∆nH,t remains zero until the fluctuation point and increases over time. During the temperature decrease, almost 100% of the microcarriers rupture and fully release the PVCap. To confirm this, the mixture is recovered from the autoclave and observed, as shown in Figure 3c and S6. There are only fractured shells and entire water phase becomes green as the dye is released. To further confirm that the PVCap released from the microcarriers is fully


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utilized to inhibit the hydrate formation, the cooling and heating processes are repeatedly performed over 4 cycles, as shown in Figure S7. The values of ∆Tsub and tonset remain almost unchanged during the cycles of hydrate formation and dissociation, indicating that PVCap is fully released and utilized at the first cycle; the average values of ∆Tsub and tonset for 4 cycles are 11.3oC and 44.1 minutes, respectively. We further compare the inhibition effect with a bulk solution of PVCap, as illustrated in Figure 3d. The concentration of PVCap is set to be same with that for microcarriers, 0.25 w/w% and 4 cycles of cooling and heating are performed, in the exactly same condition. The bulk solution yields the average values of ∆Tsub and tonset as 10.8ºC and 45.9 minutes respectively as shown in Figure S8, which are comparable to those for microcarriers. Because PVCap is fully utilized for the microcarriers, there is no discernable difference from the bulk counterpart. In addition, we perform the same cycles with distilled water in absence of inhibitors PVCap, as illustrated in Figure 3e. The values of ∆Tsub and tonset are 7.5ºC and 29.7 minutes respectively for the aqueous phase without hydrate inhibitors, indicating fast hydrate formation as soon as the temperature passes through the hydrate equilibrium condition. Finally, we perform the same cycles with empty microcarriers without PVCap, as illustrated in Figure 3f. The values of ∆Tsub and tonset are not different from those for distilled water, indicating that microcarriers themselves do not have any inhibition effect. The values of ∆Tsub and tonset for all four experiments are summarized in Figure 3g. From the set of experiments, we conclude that PVCap released from microcarriers can increase ∆Tsub and tonset up to 11.3oC and 44.1 minutes respectively, indicating the hydrate formation was effectively delayed with 50% increased hydrate avoidance duration 1.5 times longer due to full utilization of PVCap molecules in the aqueous phase. It is noted that the duration for delaying hydrate formation can be controlled by adjusting the target concentration of


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KHI in released aqueous phase, thus via controlling the numbers of the microcarriers or the concentration of PVCap inside the microcarriers. Recent literature suggests the hydrate onset time could vary from several hours to several days depending on the types and concentrations of KHIs.

50-56

The use of microcarriers provides a flexible way to manage the risk of hydrate

formation in the deepwater environment. 2.5. Recovery of intact microcarriers. The microcarriers can selectively release the encapsulant only when they experience the temperature responsible for the hydrate formation. Therefore, the only portion of microcarriers can be used for hydrate inhibition, depending on the variable field condition; this smart release minimizes the pollution of water. The microcarriers that retain their encapsulant can be separated from the fractured ones by exploiting the density difference for recycling.57 The fractured microcarriers have density same with shell material of pETPTA, ρpETPTA ~ 1.11. By contrast, the intact microcarriers have a density averaged from water core and pETPTA shell weighted by respective volume fractions: ௧ ଷ

௧ ଷ

ߩ௖௔௥௥௜௘௥ = ቀ1 − ோቁ ߩ௖௢௥௘ + ൜1 − ቀ1 − ோቁ ൠ ߩ௦௛௘௟௟ .

(2)

The value of ρcarrier can be estimated as 1.04 for microcarriers with t/R = 0.12 from ρcore ~ 1 and ρshell ~ 1.11. Therefore, intact microcarriers can be separated from the fractured shells by suspending the mixture in the medium with intermediate density, as illustrated in Figure 4a. As the separating medium, we use an aqueous solution of 16 w/w% sucrose which has a density of 1.09. Therefore, the intact microcarriers can be easily separated as they float, whereas the fractured shells settle, as shown in Figure 4b and Movie S2 of the Supporting Information. The separation efficiency is as high as 95%.


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3. CONCLUSIONS In this work, we suggest a new class of microcarriers that provide smart release of hydrate inhibitors. The thickness of the polymeric shell is controlled by flow adjustment in microfluidics to tune the mechanical stability of the microcarriers. Through the tuning, the relative thickness to the radius is optimized to produce the microcarriers that release encapsulant only at temperature responsible for hydrate formation under shear flow. The smart microcarriers selectively release the hydrate inhibitors on demand without active external triggering by sensing the temperature. The released inhibitors effectively retard the hydrate formation without reduction of their performance. Moreover, intact microcarriers which do not undergo the hydrate formation temperature can be easily separated from the fractured ones, which are in turn recycled. This hydrate inhibitor microcarriers can be injected directly into the pipeline in the presence of other carriers containing other inhibitors as the inhibitors are separately stored, thereby preventing cross-suppression of inhibition efficacy. The temperature sensitivity can be further improved by designing the shell of microcarriers with phase-change materials (PCMs). When the shells are composed of crosslinked polymer networks containing PCMs, PCMs act as a plasticizer above melting point as they are homogeneously distributed in the crosslinked networks. By contrast, PCMs render the shells brittle below the melting point as they are frozen. Therefore, the shells are expected to show a sharp change of their brittleness at the melting point, improving the temperature sensitivity. We believe that the smart delivery system based on the microcarriers will provide new opportunities for minimization of pollution and reduction of the capital expenditure in various subsea processes involving multiphasic flows.


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4. MATERIALS AND METHODS 4. 1. Materials. As innermost phase, aqueous solution of 20 w/w% of poly(n-vinyl caprolactam) (PVCap, Mn 5100, BASF), 2 w/w% of PVA (Mw 13000-23000, Sigma-Aldrich), and 0.07 w/w% of red or green food colorant (Vidhi Dyestuffs Mfg. Ltd.) are used; red colorant is 2hydroxy-1-(2-methoxy-5methyl-4-sulfonatophenyl azo) naphthalene-6-sulfonate and green colorant is a mixture of 20 w/w% of disodium α-(4-(N-ethyl-3-sulfonatobenzylamino) phenyl)-α(4-N-ethyl-3-sulfonatobenzylamino) cyclohexa-2,5-dienylidene) toluene-2-sulfonate and 80 w/w%

of

trisodium-5-hydroxy-1-(4-sulfonatophenyl)-4-(4-sulfonatophenylazo)-Hpyrazole-3-

carboxylate. As a photocurable middle phase, ETPTA (Mn 428, Sigma-Aldrich) containing 1 w/w% of 2-Hydroxy-2-methylpropiophenone (97%, Sigma-Aldrich) is used. As the continuous phase, 10 w/w% aqueous solution of PVA is used. 4.2. Preparation and characterization of microcarriers. The microfluidic device comprised two tapered cylindrical capillaries aligned in a square capillary is used for the production of the microcarriers as shown in Figure 1a. The cylindrical capillaries are tapered by puller (P97, Sutter Instrument) and sanded to have desired orifice diameter. One tapered cylindrical capillary with a 350-µm-diameter orifice is treated with trimethoxy(octadecyl)silane (Sigma-Aldrich) to render it hydrophobic. The other tapered capillary with a 450-µm-diameter orifice is treated with 2[methoxy(polyethyleneoxy)propyl] trimethoxy silane (Gelest, Inc.) to render it hydrophilic. The innermost phase is injected through the hydrophobic capillary and middle phase of photocurable ETPTA is injected through the interstices between the hydrophobic and square capillaries. The continuous phase of PVA aqueous solution is injected through the interstices between the


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hydrophilic and square capillaries. The flow rates of the innermost, middle, and continuous phases are independently controlled by syringe pumps (KdScientific, Inc.) and drop generation is observed with an optical microscope equipped with a high-speed camera (Motionscope M3, Redlake). The shell of double-emulsion drops is polymerized by UV light (Innocure 100N, Lichtzen Co.) in the collection vial. The microcarriers are observed using an optical microscope (Ti, Nikon) equipped with a camera (DS-Ri1, Nikon) and scanning electron microscope (SEM, S-4800, Hitachi). 4.3. Evaluation of hydrate inhibition performance. A 12 ml water suspension of microcarriers containing PVCap and a 16 ml decane are loaded into a 100 ml autoclave cell equipped with an impeller. The cell is immersed in a water-ethanol bath (Jeio Tech, RW-2025G) to control the temperature. The cell is pressurized with natural gas at 120 bars and temperature is set to 24oC. The stirrer is set in rotational motion at 600 rpm. The cell is cooled to 4oC at a constant rate of 0.25oC/min and maintained at 4oC for 10 hours. The temperature and pressure are measured using a temperature sensor PT 100 Ω (± 0.15oC) and a pressure transducer (0 to 200 bars, ± 0.1 bar), respectively while logging them continuously with a data acquisition system. After the hydrate formation is completed, the mixture is retrieved to inspect the fraction of ruptured microcarriers with an optical microscope. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Table containing volumetric flow rates for microfluidic operation, optical microscope images of intact, deformed, and fractured carriers, plots for osmotic resistance, strain-


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stress curves, fractions of fractured carriers under shear flow, pressure and temperature changes during hydrate formation experiments. (PDF) Movie S1 shows the generation of double-emulsion drops in microfluidic devices. (AVI) Movie S2 shows separation of intact microcarriers from fractured shells. (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Shin-Hyun Kim) *E-mail: [email protected] (Yutaek Seo)

Author Contributions S.S.L and J.P carried out all experiments and Y. Seo and S.-H.K. supervised the research. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Industrial Strategic Technology Development Program (No. 10045068) of the Korea Evaluation Institute of Industrial Technology (KEIT) funded by MOTIE.


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Figure 1. (a,b) Schematic of a capillary microfluidic device and optical microscope (OM) image showing the generation of monodisperse water-in-oil-in-water (W/O/W) double-emulsion drops. The drops are used as a template to produce hydrate inhibitor-laden microcarriers. (c) OM and scanning electron microscope (SEM) images showing microcarriers with two different relative thicknesses to radii, t/R, where t/R = 0.170 for two top panels and 0.116 for two bottom panels. (d) Relative thickness to the radius, t/R, as a function of the relative volumetric flow rate of middle phase to inner one, Qm/Qi.


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Figure 2. (a) Schematic showing experimental setup to evaluate temperature-dependent release from microcarriers under shear flow. (b) The fraction of ruptured microcarriers as a function of temperature, where microcarriers with t/R = 0.142, 0.110, 0.099, and 0.068 are used. (c, d) Series of OM images of microcarriers recovered after stirring at denoted temperature in each panel, where microcarriers with t/R = 0.110 (c) and 0.142 (d) are used. Insets show corresponding images of suspensions, of which color tone increases as more microcarriers rupture.


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Figure 3. (a) Schematic for experimental setup to evaluate inhibition effect of microcarriers. (b) Temperature change (blue line, left y-axis) and gas consumption (black line, right y-axis) as a function of time, where cooling rate is set to 0.25oC/min. The exothermic hydrate formation causes fluctuation of temperature at the point denoted by an arrow. Equilibrium temperature (Teq), subcooling temperature (∆Tsub), and onset time (tonset) are denoted in the plot. (c) OM image of suspension recovered after a cycle of cooling and heating. The right panel shows the release of encapsulant as a result of microcarrier fracturing. (d-f) Schematics for experimental setups to study inhibition effect of free PVCap (d), kinetic delay of hydrate formation without inhibitor (e), and influence of empty microcarriers without inhibitor (f). (g) The plot for ∆Tsub and tonset for four different experiments in (a, d-f).


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Figure 4. (a, b) Schematic and photograph showing spontaneous separation of intact microcarriers from fractured ones. OM images of recovered microcarriers from the top and fractures shells from the bottom are included in right panels of (b).


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Table of Contents Graphic


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