ARTICLE pubs.acs.org/IECR
Vertically-Oriented-Capillary Video-Microscopy: Drops Levitated by a (Reacting) Fluid Miguel Garcia-Bermudes,† Riccardo Rausa,‡ and Kyriakos Papadopoulos†,* † ‡
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana, United States Eni S.p. A., Division of Refining & Marketing, Centro Ricerche di S. Donato, Milanese, Italy
bS Supporting Information ABSTRACT: Microscopy in a vertical capillary provided the ability to continuously observe the dynamic particle phenomena on microscopic objects levitated by an imposed flow. Such a technique was developed and used to monitor flow-levitated droplets, by manually regulating the imposed flow so as to keep the droplets suspended in the area of view. Local sudden increases and decreases in temperature were achieved with an external heating filament, which produced rapid changes in the fluids’ properties (viscosity and density). Even in such cases it was possible to control the levitated drop’s position in the microscope’s field of vision by adjusting the levitating fluid’s vertical flow. The shrinkage and alteration of levitated acid drops that react with the basic nanoparticles of levitating oil, verified that, when compared to static conditions, fluid flow significantly accelerated the neutralization of the acid drop by the oil’s basic nanoparticles. Allowing droplets to levitate and rotate due to the flow of another fluid in high-temperature regions, this technique may be used as an improved way to mix contents in suspended microscopic droplets.
1. INTRODUCTION While there have been numerous developments in microfluidic devices with vast arrays of applications, what has been singularly missing is a class of such devices that would involve vertical cylindrical microconduits, which can take advantage of gravity and the levitating action of a flowing fluid. The inherent complexity of lab-on-chip design on one hand, and the relative ease associated with horizontally oriented flow on the other, may have limited these devices’ usefulness and applications. To reach commercial-scale production of processing systems based on microfluidics, a significant number of microfluidic chips should work in parallel.1,2 This is known as “numbering up” as opposed to “scaling up,” since the number of chips would be significant, but the chips’ design would remain the same as that of one operating alone. Even the most efficient microfluidics devices for droplet and particle generation should require at least hundreds of chips to produce commercial quantities. Therefore, also in the case of vertically oriented microscopic flow, spatially efficient equipment based on microfluidics should take advantage of a compact three-dimensional design. Vertical flow is similar to horizontal flow in some simple conditions such as those encountered in isothermal, single-phase flow. When more complex flow conditions are present, such as multiphasic flow (i.e., particles in a liquid), reactive systems, or spatial temperature gradients, complex phenomena can arise in vertical flow, due to resulting interfacial tension gradients and/or density gradients. Recently, potential advantages were discussed in using natural convection mixers for highly efficient mixing of components in microfluidic chips when carrying out biochemical reactions.3,4 The work of Kurabayashi4 et al. maybe the only published study of a vertical microfluidic device, which focused on the mixing of two reacting streams that constituted a singlephase system. Significant differences existing between horizontal r 2011 American Chemical Society
and vertical flows of single- and multiphase systems can be expected to lead to novel designs and uses of future microfluidic devices. To study vertical microfluidic phenomena in this paper, we modified a capillary video microscopy technique developed in our lab in the early 1990s.5 8 The focus was to obtain an experimental setup that could produce vertical microflow while keeping a microparticle or microdroplet “levitated” in the area of view of the video acquisition system long enough to allow the recording of its fate (i.e., dissolution and/or reaction with the flowing liquid phase). Mathematical models on droplet-levitation by flow inside a restricted channel have made assumptions such as rigid spherical shape, absence of rotational motion, and lateral translation among others.9 12 While vertical microfluidic studies on multiphase systems have not been conducted before, there have been fluid-mechanical studies on the flow of droplets and bubbles in vertical capillaries, where models for different isothermal buoyancy-driven-flow conditions were developed.13 20 Even though such experimental studies were performed using inner-tube diameters on the millimeter scale, the models that were developed may be applicable to the micrometer (microfluidic) scale. The current experimental technique levitates microparticles in vertically oriented microcapillaries by simply exploiting the drag force and feedback of the levitated droplet’s position to regulate the needed flow of the levitating fluid. Though air-stream levitation has been well-known, our device may be the first one where the levitating fluid is a liquid, even outside the realm of microfluidics. Numerous processes can be conducted through Received: July 1, 2011 Accepted: October 19, 2011 Revised: October 11, 2011 Published: October 19, 2011 14142
dx.doi.org/10.1021/ie201409e | Ind. Eng. Chem. Res. 2011, 50, 14142–14147
Industrial & Engineering Chemistry Research
Figure 1. Schematic of vertically oriented capillary video microscopy.
microfluidic drop levitation, for example, liquid liquid extraction, particle or droplet dissolution and dispersion, emulsification/ coalescence of drops and aggregation of particles, droplets or particles reacting with a fluid under flow conditions, response and movement of cells in microcirculation, etc. The basic setup used here may be easily modified in the future to induce levitation by other means such as acoustic or magnetic.
2. MATERIALS AND METHODS The design of the capillary video microscopy technique5 8 was modified by introducing major changes, the main one being the vertical orientation of the capillary’s axis so as to take advantage of gravity. Pressure-driven flow in the capillary was imposed against gravity, and an external heating filament was used to create asymmetric local heating in order to create natural convection. The use of a heating filament highlights the simplicity of the setup, by not requiring more sophisticated heating equipment, such as an IR laser, which could be implemented in future modifications of the device. Heating was on/off and the position of the filament remained fixed with respect to the glass capillary. The core of the setup was a vertical microscope, which was built using a microforge (model MF-9, Narisinge, Japan) connected to a digital camera (Lynx, Imperx, USA). Video was sent to a computer with acquisition image software (XCAP-Std 3.0, EPIX Inc., USA). The heating platinum filament of the microforge was fixed to the objective and always located on the right side of the area of view in order to cause localized heating of the observable region of the capillary as detailed in Figure 1. Pressure-driven flow was achieved by connecting the capillary’s lower end to either a syringe pump (model 220, KD Scientific, USA) or a pneumatic microinjector (model IM-200, Narishige, Greenvale, USA). The pneumatic injection pressure was regulated manually, and changes were required in order to adjust continually the droplet’s position while the droplet’s fate was monitored. This possible use of this setup in studying the effects of dispersion, dissolution, and/or reaction on diverse liquid liquid systems allows the continuous monitoring of the phenomena and the ability to extract not only quantitative data as shrinking, but also relevant qualitative characteristics of the process.
ARTICLE
One important requirement for this technique is that the density of the droplet and the density of the external fluid must be different, so as to take advantage of gravity as a noncontact force. Two different fluid fluid systems were studied. The first system was perfluorodecalin droplets in deionized water. With a density of 1.92 g/cm3, perfluorodecalin was chosen also because it is insoluble and inert in water. This system was used to address simple relations among the system’s geometry and the flow variables required to control the droplet’s position in the area of view of the microscope. Experiments were performed at room temperature (25 °C) and a syringe pump was used to impose upward water flow. Perfluorodecalin was obtained from Oakwood Products. The second system consisted of sulfuric-acid droplets suspended in and reacting with a commercial marine-cylinderengine lubricant that incorporated basic nanoparticles. Marine engine lubricant oil samples were provided by Eni R&M, Italy. Using glass micropipets especially pulled for injection, the suspended droplets were produced by directly injecting them into vertical capillaries, which had been previously filled with the suspending medium. In both the inert and the reacting systems, the droplet diameter was around 100 μm, and perfluorodecalin was injected as received, whereas sulfuric acid was diluted to 50 vol % in deionized water prior to injection. Pressure-driven imposed flow was created using pneumatic microinjection, whose pressure drop was regulated in such a way that the observed shrinking microdroplet remained levitated in the observable region. In the absence of the flow-imposed pressure drop and the natural convection due to thermal gradients, the droplet simply sedimented within the capillary. The fate of the droplets during observation was recorded. The system was spatially calibrated via a microruler.
3. RESULTS AND DISCUSSION 3.1. Nonreactive System and Dimensionless Analysis. Using the syringe pump, water flow was adjusted by trial and error, so as to keep droplets with different sizes levitated. This was achieved through a vertical zero force balance of the buoyancy force, drag force, and the drop’s weight. For levitation to occur, the mean recorded velocity of the rising water stream should be approximately equal to a free-falling drop’s terminal velocity in a stagnant liquid. The relation between the perfluorodecalin droplet size and the water flow needed to keep the droplet in a controlled position was obtained via the recording. As perfluorodecalin is insoluble in water, the droplet did not shrink over time, and therefore it was possible to keep the droplets levitated with a constant fluid flow. As shown in Figure 2, perfluorodecalin droplets with the same size, but with different capillary diameters, needed different water flows in order to remain in the area of view. For a capillary diameter of 207 μm, a water flow of 0.4 μL/min was needed, whereas for a bigger diameter (266 μm), a larger flow (2.0 μL/min) was required. Following the analysis of Borhan et al.,13,17 relevant dimensionless numbers were used for this system. Reynolds’ number is defined as RVFex/μex, with V, μex and Fex denoting the surrounding fluid’s mean velocity, viscosity, and density, and R being the radius of the droplet. The Bond number is Bo = ΔFgR2/σ, with ΔF and σ being the density difference and interfacial tension between the two phases, and g being the magnitude of the gravitational acceleration. Finally, the capillary number is defined 14143
dx.doi.org/10.1021/ie201409e |Ind. Eng. Chem. Res. 2011, 50, 14142–14147
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Perfluorodecalin droplets levitated in the area of view using DI water flowing upward. (a) Droplet diameter, 117 μm; capillary diameter, 207 μm; water flow, 0.4 μL/min. (b) Droplet diameter, 119 μm; capillary diameter, 266 μm; water flow, 2.0 μL/min.
by Ca = μexV/σ. For the system shown in Figure 2, the values of these numbers were: Re ≈ 0.02, Bo ≈ 2 10 3, and Ca ≈ 1 10 5, and they supported the fact that the droplet shape remained nearly spherical during levitation. The fact that droplets appear ellipsoidal may be because of gravity as well as a slight optical distortion due to the curvature of the cylindrical capillary. When Bo and Ca , 1, the interfacial energy overcomes the drop-deformation action of both the buoyancy force and imposed water flow,10 and since Re , 1, the inertial forces were negligible, a fact that allowed the droplet position to be controlled immediately upon changing the imposed flow rate. The density ratio of the levitated droplets with respect to the imposed circulating fluid, was γ = 1.92 for perfluorodecalin water. The direction of the imposed flow required to levitate droplets or particles naturally depends on the relative densities of the levitated particles with respect to the imposed fluid. Happel and Brenner developed a model of a sphere undergoing steady axial motion along a cylindrical tube with Poiseuille flow.9 In our experiments, the spherical droplet was levitated in the area of the viewing field of the microscope, and its position was observed to fluctuate slightly as the droplet traveled small distances vertically and horizontally, and also rotated. Using the Happel and Brenner model, calculations were performed for a droplet that is fixed in space and does not rotate, in order to observe how close the experimental data fitted this model. Apparently because key assumptions of the model were not met by the experimental observations, the discrepancies between the model’s predictions and experimental results were too great to allow discussion and therefore are not presented here. 3.2. Reactive System: Sulfuric Acid Droplets in MarineEngine Lubricants. In previous studies, the neutralization reaction of acidic droplets by marine-engine lubricants was monitored and studied microscopically in a horizontally oriented capillary in our lab.21 27 Therefore, using vertically orientedcapillary video-microscopy on this system should illustrate the utility of this technique and its relevance to the particular industry. The marine-engine oil typically contains a significant amount of finely dispersed alkaline nanoparticles, stabilized as rigid reverse micelles. The main purpose of this high-alkalinecontent oil additive, known as “overbased detergent,” is to neutralize the acids (mainly sulfuric acid) that are formed upon combustion inside the diesel engine of marine ships, thereby preventing corrosion.28 The alkaline core of the nanoparticles is typically made of calcium carbonate and calcium hydroxide. The neutralization reaction between these alkaline nanoparticles and
Figure 3. Image taken from vertically oriented glass capillary. A sulfuric acid droplet is levitated by flowing marine-engine lubricant. The outer diameter of capillary is 200 μm.
the sulfuric acid droplet produces calcium sulfate, carbon dioxide, and water; hence this reaction is heterogeneous due to the presence of multiple phases. The actual acid neutralization that takes place inside marine engines can occur in a thin, vertical, lubricant-oil film on the engine-cylinder linear, while the reciprocating piston moves vertically. The acid invades the enginecylinder linear in the form of droplets of varying sizes.29,30 Therefore a vertical lubricant convective flow is present during neutralization, a process that helps to disperse the reaction’s products. The vertically oriented capillary video-microscopy technique presented here, can introduce a convective oil flow around the acid droplet while allowing continuous monitoring of the fate of the reacting acidic droplet. The observation of the neutralization reaction of the acidic droplet was successfully performed. Image resolution was good as can be observed in Figure 3, which depicts a freshly produced acid drop in a lubricant-filled capillary. As a consequence of the neutralization reaction between the acid drop and the basic nanoparticles of the levitating oil, the droplet size diminished. Adjusting the pressure of the pneumatic microinjector could control the droplet’s position, and such regulation addressed the temporal change of the flow geometry stemming from the shrinkage of the acid droplet. Whereas in the case of the water perfluorodecalin system, with no significant geometrical changes, a constant flow rate of the oil resulted in a fairly stable position for the droplet, this was not the case in the less stable, heated, acid oil reactive system. The heating and shrinking required continuous regulation of oil flow via the pneumatic microinjector, in order to keep the droplet’s position inside the microscope’s area of view. Whereas reproducibility was good in the water perfluorodecalin system, at this stage it has not been possible to obtain good reproducibility when both heating and drop reacting/shrinking occurred. For such reproducibility to be feasible, future work on reactive systems will necessitate the production of droplets of exactly the same diameter in the exactly same position within the capillary. Of special interest was the case when the external heating filament caused a fast temperature gradient to the capillary’s contents in the area of view, rapidly causing both a lower oil 14144
dx.doi.org/10.1021/ie201409e |Ind. Eng. Chem. Res. 2011, 50, 14142–14147
Industrial & Engineering Chemistry Research
ARTICLE
Figure 4. Reaction of 50 vol % H2SO4 droplet in lubricant sample A containing only alkaline nanoparticles. Reaction times are (a) 0, (b) 5, (c) 7, (d) 10, (e) 18 min. Capillary diameter is about 200 μm.
Figure 5. Reaction of 50 vol % H2SO4 droplet in lubricant sample B containing alkaline nanoparticles and other additives. Reaction times are (a) 0, (b) 4, (c) 9, (d) 13, (e) 18, (f) 23 min. Capillary diameter is 200 μm.
viscosity and a oil density. The viscosity change is much more dramatic than the density change, and about a 10-fold decrease in kinematic viscosity, ν = μ/F, is typical in marine-engine lubricants when the temperature changes from room temperature to 100 °C.28 Temperature was estimated in the absence of the acid droplet as follows. A microthermocouple (300 series, Paul Beckman Company, PA, USA) was inserted inside the capillary’s region where the droplet was to be observed, and, in the absence the droplet, only oil flowed at similar rates as when acid neutralization was observed under the heating of the filament. Thus a temperature calibration was obtained. Like in the case of the nonreactive system, the levitation of the microscopic sulfuric acid droplet in a marine-engine oil, was a phenomenon that occurred at very low Reynolds numbers. A typical value of kinematic viscosity for this type of marine-engine lubricant is 22 cSt at 100°C (viscosity index, 100). The largest Re was in experiments where the initial acid droplet diameter was about the size of its host capillary diameter, 300 μm. Maximum velocity of oil flow in the capillary was about 300 μm/s, hence Re ≈ 0.004. Besides the fact that Re , 1, the rest of the conditions did not meet the assumptions in Stoke’s Law or its extensions to flow past a sphere-in-a-cylindrical system. Owing to the anisotropic heating of the capillary’s contents, the oil closer to the heat filament was at higher temperature when
compared to regions further from the filament. Therefore it was less dense and viscous, and moved upward more rapidly compared to the net oil flow. Conversely, the relatively cooler oil on the capillary’s side furthest from the heating filament had a relatively lower velocity, and the net natural convection produced a fast rotational flow within the acid droplet (see Supporting Information, video 2). This observed rotation may have been also influenced by temperature-gradient-driven Marangoni flows, and caused a significantly faster neutralization rate and consequent shrinkage of acid droplets. Such rotational droplet motion was absent in symmetrically heated, horizontally oriented capillaries, as will also be discussed later. A reaction of an acidic droplet in “lubricant sample A” containing only alkaline-salt basic nanoparticles is shown in Figure 4. In panel a the droplet has just been injected and is transparent since no crystalline reaction products have formed, whereas at a later time, in in panel c, the droplet is dark due to the production of micrometer-sized crystals and bubbles formed upon the neutralization between sulfuric acid and the calcium carbonate of the alkaline nanoparticles. The solid reaction products were formed and remained inside the acid droplet. However, during the reaction, tiny droplets were seen to form and grow on the convex (oil) side of the interface, and then detach and get carried away upward by the lubricant oil. Finally in 14145
dx.doi.org/10.1021/ie201409e |Ind. Eng. Chem. Res. 2011, 50, 14142–14147
Industrial & Engineering Chemistry Research
ARTICLE
Figure 6. Horizontal capillary without convective lubricant flow. Reaction of 50 vol % H2SO4 in lubricant sample B. Reaction times are (a) 0 min, (b) 17 min, (c) 32 min. Capillary diameter is 200 μm.
panel e, the droplet has lost its previous spherical shape, and has been reduced to a solid aggregate of small, visible crystals and bubbles that collapsed. This shows that in the heterogeneous reaction that occurred on the acid-oil interface of sample A, most of the neutralization products (calcium sulfate micrometer-sized crystals and carbon dioxide) were formed inside the aqueous phase, where they remained trapped until all the aqueous phase was effectively dispersed into the oil. In the case of lubricant sample B containing alkaline nanoparticles and other additives, a more homogeneous shrinkage occurred, as can be observed in Figure 5. In panel a the droplet is transparent since the reaction has just started, in panel e the droplet diameter is diminished by 50% and the droplet contained some particulate matter but remained nevertheless spherical and relatively clear, and finally in panel f the remaining was a small aggregate when compared to its initial size. The video shows the final, remaining particle to be a solid, resulting from the agglomeration of micrometer-size particulates on the acid-oil interface during the length of the reaction. The reaction occurred on the acid oil interface, and some amphiphilic particulates remained adsorbed on the interface, whereas most of the submicrometer-sized neutralization products were finely dispersed into the lubricant oil phase (since no crystals were observed microscopically), and were presumably carried away with the upward flow. Although in both cases the alkaline nanoparticles neutralized effectively the acid droplet, the more complex sample B allowed the droplet to remain spherical and liquid, and to shrink to a much smaller size when compared to sample A. In this case, the neutralization products (calcium sulfate micrometer-sized crystals and carbon dioxide) of the heterogeneous reaction on the acid oil interface of sample B, were formed mostly in the oil phase and were dispersed effectively by the oil flow, minimizing agglomeration of calcium sulfate crystals. Using sample B in a horizontal capillary, Figure 6, can provide some comparison between the rates of acid neutralization in the absence and the presence of flow. The 150 μm-diameter droplet in the horizontal capillary had a volume of 14.1 nL as opposed to 33.5 nL in the vertical capillary. Nevertheless, in the horizontal capillary neutralization was achieved in 35 min, whereas the droplet in the vertical capillary (Figure 5) was neutralized in 23 min. In comparing results on the speed of neutralization between a vertical and a horizontal capillary setup, it should be noted that heating in the latter is almost homogeneous, and the temperatures on the surface of drops are not likely equal. As expected, by replenishing additives and basic nanoparticles in the proximity of the acid-oil interface, and by removing the reaction products from it, flow conditions affected significantly the speed of neutralization.
4. CONCLUSIONS Vertical capillary microscopy allows continuous observation of dynamic phenomena in multiphase systems, such as microscopic droplet interactions and their reaction and alteration, under levitation by flow of a second liquid. The position of levitated, changing drops was successfully controlled within the capillary by manually regulating the imposed upward flow of the surrounding bulk liquid. When heating the capillary’s contents with a filament, the droplet position required a more careful regulation since local temperature gradients produced local changes in fluid properties such as viscosity and density. In the instance where an acid droplet was levitated by the basic nanoparticles inside flowing oil, flow conditions affected significantly the droplet’s shrinkage, decreasing the time required for its neutralization and dispersion when compared to a horizontal setup and absence of flow. By levitating droplets, which also experience rotation at high temperature regions created by an external heating filament, this technique can potentially provide new ways to mix the droplets’ contents without incorporating in situ electric heaters. ’ ASSOCIATED CONTENT
bS
Supporting Information. Videos of the levitated droplets for nonreactive and reactive systems. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by a grant from Eni S.p. A. ’ REFERENCES (1) Nisisako, T.; Torii, T. Microfluidic Large-Scale Integration on a Chip for Mass Production of Monodisperse Droplets and Particles. Lab Chip 2008, 8, 287. (2) Li, W.; Greener, J.; Voicu, D.; Kumacheva, E. Multiple Modular Microfluidic (M3) Reactors for the Synthesis of Polymer Particles. Lab Chip 2009, 9, 2715. (3) Krishnan, M.; Agrawal, N.; Burns, M. A.; Ugaz, V. M. Reactions and Fluidics in Miniaturized Natural Convection Systems. Anal. Chem. 2004, 76, 6254. (4) Kim, S.-J.; Wang, F.; Burns, M. A.; Kurabayashi, K. TemperatureProgrammed Natural Convection for Micromixing and Biochemical Reaction in a Single Microfluidic Chamber. Anal. Chem. 2009, 81, 4510. 14146
dx.doi.org/10.1021/ie201409e |Ind. Eng. Chem. Res. 2011, 50, 14142–14147
Industrial & Engineering Chemistry Research (5) Deshiikan, S. R.; Papadopoulos, K. D. London vdW and EDL Effects in the Coalescence of Oil Drops. J. Colloid Interface Sci. 1995, 174, 302. (6) Deshiikan, S. R.; Papadopoulos, K. D. London van der Waals and EDL Effects in the Coalescence of Oil Drops: II. Ionic Strength and pH Effects. J. Colloid Interface Sci. 1995, 174, 313. (7) Kuo, C.-C.; Papadopoulos, K. D. Electrokinetic Movement of Settled Spherical Particles in Fine Capillaries. Environ. Sci. Technol. 1996, 30, 1176. (8) Liu, Z.; Papadopoulos, K. D. Unidirectional Motility of Escherichia coli in Restrictive Capillaries. Appl. Environ. Microbiol. 1995, 61, 3567. (9) Brenner, H.; Happel, J. Slow Viscous Flow Past a Sphere in a Cylindrical Tube. J. Fluid Mech. 1958, 4, 195. (10) Clift, R.; Grace, J. R.; Weber, M. E. Bubbles, Drops and Particles; Academic Press, Inc.: New York, 1978. (11) Koh, C. J.; Leal, L. G. An Experimental Investigation on the Stability of Viscous Drops Translating through a Quiscent Fluid. Phys. Fluids 1990, 2, 2103. (12) Ohta, M.; Yamaguchi, S.; Yoshida, Y.; Sussman, M. The Sensitivity of Drop Motion Due to the Density and Viscosity Ratio. Phys. Fluids 2010, 22, 072102. (13) Borhan, A.; Mao, C.-F. Effect of Surfactants on the Motion of Drops through Circular Tubes. Phys. Fluids A 1992, 4, 2628. (14) Borhan, A.; Pallinti, J. Buoyancy-Driven Motion of Viscous Drops through Cylindrical Capillaries at Small Reynolds Numbers. Ind. Eng. Chem. Res. 1995, 34, 2750. (15) Haj-Hariri, H.; Shi, Q.; Borhan, A. Thermocapillary Motion of Deformable Drops at Finite Reynolds and Marangoni Numbers. Phys. Fluids 1997, 9, 845. (16) Borhan, A.; Pallinti, J. Pressure-Driven Motion of Drops and Bubbles through Cylindrical Capillaries: Effect of Buoyancy. Ind. Eng. Chem. Res. 1998, 37, 3748. (17) Almatroushi, E.; Borhan, A. Surfactant Effect on the BuoyancyDriven Motion of Bubbles and Drops in a Tube. Ann. N.Y. Acad. Sci. 2004, 1027, 330. (18) Almatroushi, E.; Borhan, A. Interaction and Coalescence of Drops and Bubbles Rising through a Tube. Ind. Eng. Chem. Res. 2006, 45, 398. (19) Almatroushi, E.; Borhan, A. Coalescence of Bubbles Translating through a Tube. Ann. N.Y. Acad. Sci. 2006, 1077, 508. (20) Al-Matroushi, E.; Borhan, A. Coalescence of Drops and Bubbles Rising through a Non-Newtonian Fluid in a Tube. Ann. N.Y. Acad. Sci. 2009, 1161, 225. (21) Wu, R. C.; Campbell, C. B.; Papadopoulos, K. D. Visualization Test for Neutralization of Acids by Marine Cylinder Lubricants. AIChE 1999, 45, 2011. (22) Wu, R. C.; Campbell, C. B.; Papadopoulos, K. D. Acid Neutralizing of Marine Cylinder Lubricants: Effect of Nonionic Surfactants. Ind. Eng. Chem. Res. 2000, 39, 3926. (23) Wu, R. C.; Campbell, C. B.; Papadopoulos, K. D. Acid Neutralizing of Marine Cylinder Lubricants: Measurements and Effects of Dispersants. AIChE 2000, 46, 1471. (24) Fu, J.; Lu, Y.; Campbell, C. B.; Papadopoulos, K. D. Optical Microscopy inside a Heating Capillary. Ind. Eng. Chem. Res. 2005, 44, 1199. (25) Fu, J.; Lu, Y.; Campbell, C. B.; Papadopoulos, K. D. Temperature and Acid Droplet Size Effects in Acid Neutralization of Marine Cylinder Lubricants. Tribol. Lett. 2006, 22, 221. (26) Fu, J.; Papadopoulos, K. D.; Lu, Y.; Campbell, C. B. Ostwald Ripening: A Decisive Cause of Cylinder Corrosive Wear. Tribol. Lett. 2007, 27, 21. (27) Fu, J. Acid Neutralization by Marine Cylinder Lubricants inside a Heating Capillary Microreactor. Ph.D. Thesis, Tulane University, New Orleans, LA, 2006. (28) Mortier, R. M.; Fox, M. F.; Orszulik, S. T.; Carter, B. H.; Green, D. Marine Lubricants. In Chemistry and Technology of Lubricants, Springer: The Netherlands, 2010: pp 389.
ARTICLE
(29) Andreasen, A.; Mayer, S. Modelling of the Oxidation of Fuel Sulfur in Low Speed Two-Stroke Diesel Engines. In CIMAC Congress 2010; International Council on Combustion Engines: Bergen, Norway, 2010; Paper no. 39. (30) Huijbregts, W. M. M.; Leferink, R. G. I. Latest Advances in the Understanding of Acid Dewpoint Corrosion: Corrosion and Stress Corrosion Cracking in Combustion Gas Condensates. Anti-Corros. Methods Mater. 2004, 51, 173.
14147
dx.doi.org/10.1021/ie201409e |Ind. Eng. Chem. Res. 2011, 50, 14142–14147
