Ind. Eng. Chem. Res. 2005, 44, 1199-1203
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Optical Microscopy inside a Heating Capillary Jianzhong Fu,† Yunfeng Lu,† Curt B. Campbell,‡ and Kyriakos D. Papadopoulos*,† Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118, and Chevron Chemical Company, Oronite Global Technology, 100 Chevron Way, Richmond, California 94802
This paper presents a heating-capillary video-microscopy system that allows visual observation and quantification of phenomena involving biphasic dispersions with interfaces at high temperatures. The cylindrical capillaries were made to have inside diameters of ∼200 µm, while their exterior was coated with a transparent tin-doped indium oxide film that acted as an electrically heating jacket. The produced capillaries achieved temperatures ranging from ambient to at least 287 °C, the boiling temperature of n-hexadecane, a high boiling point hydrocarbon used in the temperature-calibration experiments. The generated temperatures may oscillate, with maximum deviations of about 3 °C from ambient to 80 °C, 9 °C from 80 to 170 °C, and 45 °C from 170 to 265 °C. In the range of 100-265 °C, the desired temperatures were attained at a rate from 75 to 198 °C/s and could be easily adjusted by changing the applied ac voltage with a variable transformer. Two examples of using the heating-capillary technique for interfacial studies at high temperatures are illustrated. 1. Introduction Optical microscopy has historically been used to directly examine colloids and interfacial phenomena.1-3 In our laboratory, a thin-wall capillary, which may have an inside diameter ranging from 2.5 to 200 µm, has been developed for a video-microscopy system since 1992 to observe phenomena involving objects with dimensions ranging from 1 µm (Escherichia coli cells) to over 100 µm (double-emulsion globules). The capillary’s confined space causes microscopic objects that are suspended in a medium to lose to a great extent the range of motion they possess in bulk. The objects under observation can therefore be micromanipulated in unique ways that in our laboratory have led to observation and quantification of phenomena that could not have been studied using conventional microscopy. The first such studies were conducted on the coalescence among oil droplets and hetero-aggregation between droplets and solid particles as functions of pH and salinity4-6 and on E. coli chemotaxis, random motility, and electrokinetic transport in the cylindrically confined space of a capillary.7-10 Another investigation visualized and quantified the neutralization of acids produced during the combustion of high-sulfur diesel fuels inside large marine engines. Such neutralization is achieved by “overbased” marine cylinder lubricants (MCLs) containing CaCO3 reverse-micellar particles. In our study, the lubricant formulation was correlated to the rate of neutralization.11-13 The thin-wall capillary was also used to make single W1/O/W2 and O1/W/O2 doubleemulsion globules and study the stability and transport of material between the aqueous W1 and W2 phases through different mechanisms.14-19 The current paper presents the next step in the development of our technique, namely, the conduction of microscopy inside cylindrical capillaries at high temperatures. Several challenges existed: one of them * To whom correspondence should be addressed. Tel.: +1504-865-5826. Fax: +1-504-865-6744. E-mail:
[email protected]. † Tulane University. ‡ Chevron Chemical Co.
was achieving a relatively stable temperature in such a tiny volume exposed to ambient air; another was that the heating of the capillary on the stage of a microscope had to be done without interferring with its optical access. All available optical microscopes with heating features and all available heating devices for optical microscopy are designed for slide microscopy, and no such available accessory could be applied to heat the capillary without impairing visual observation during experiments. Methods of heating a sample on a microscope stage vary with the sample itself and the purpose of the specific experiment. In 1983, Yokoyama et al.20 had to rebuild a microscope to construct an oven enclosing the microscope’s objective lens, stage, and mirror. The contraption consisted of several layers of copper and insulators, and the part of the innermost copper layer encircling the stage was electrically heated. Bead-type thermistors (1.8 mm in diameter, 3 kΩ at 100 °C and 150 kΩ at 0 °C) were used to control and monitor the temperatures. In 1990, Lo´pez-Iglesias and Baro´n21 blew warm air instead of submerging the sample into a bath and controlled the temperature through two very thin thermistors, 1 cm length and 0.02 mm o.d. Recently, Gluch et al.22 conducted high-temperature microscopy in a rectangular channel with a height of 200 µm. The apparatus consisted of two parts, each containing a Peltier element, 30 × 30 mm in size with a 1-mm-thick copper surface. Both Peltier elements were connected to a circulating water bath that kept one side at constant working temperature. A hardboard plate connected the chamber to the microscope stage and kept it thermally isolated. The upper part was prevented from tilting to ensure a good thermal conductance. The temperature was measured with a Pt(100) foil-resistant element, which was sealed to the surface of the lower Peltier element. Temperatures of up to 100 °C were attained at a heating rate of 2.4 °C/s and a cooling rate of 2.0 °C/s. In the technique that we present in this paper, the cylindrical capillary and the contained sample are heated without rebuilding the microscope or adding
10.1021/ie040083i CCC: $30.25 © 2005 American Chemical Society Published on Web 07/21/2004
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Figure 1. Transparent ITO heating capillary reactor (TIHCR): (1) heating capillary; (2) plastic holder; (3) copper foil; (4) cushion; (5) screw bolts.
specially designed heating devices. We simply use solgel processing to coat a transparent conducting film on a thin-wall glass capillary, thus obtaining an electrically conductive and visibly transparent cylindrical tube inside which we can conduct microscopy.23 Tin-doped indium oxide (ITO) is one of the most widely used transparent conducting oxides. It is a nonstoichiometric oxide, engineered through introducing tin dopant to indium oxide, which has a wide band gap (g3 eV). Consequently, ITO is transparent in visible light and has a good electrical conductivity. Sol-gel processing involves the formation of a colloidal suspension (sol) and the gelation of the sol to form a network in a continuous phase (gel). Through this process, an ITO film with desirable properties of conductivity, optical transparency, and chemical durability can be produced at room temperature. Typically, coated substrates are annealed in air or a reduction environment to improve the properties of ITO films. Sol-gel processing also makes it possible to uniformly coat axially symmetric substrates, such as pipes, tubes, rods, and fibers, not easily handled by other conventional techniques.24 ITO thin films are extensively used as transparent electrodes in display, optoelectronic, and electrochromatic devices, solar cells, sensors, etc.,25-31 exhibiting excellent adherence, hardness, and chemical inertness. As heaters, ITO films are mainly used for the defrosting of airplane and automobile windows.32,33 It should be especially noted here that commercially available capillary tubes have also been successfully coated by Friedman and Meldrum34 with ITO films for the purpose of thermal cycling in a polymerase chain reaction. These investigators achieved temperatures higher than 800 °C and very fast heating and cooling rates. 2. Experimental Section 2.1. Heating-Capillary Preparation. The heating capillary was prepared by first pulling a clean glass tube with a micropipet puller, then dip-coating the pulled capillary in an ITO precursor solution, and finally annealing the coated capillary in a programmable furnace. As long as the inside of the capillary could be clearly seen, we kept coating additional ITO layers until the resistance of the capillary got lower than 100 kΩ. The resistance was directly measured by means of a multimeter between the two ends of the capillary via two copper foils described below. 2.2. Assembly. As shown in Figure 1, the heating capillary (1) is fixed on a plastic holder (2), which also provides the connection between the capillary and output cable of a variable transformer via two copper
Figure 2. Temperature-voltage working curve (for a heating capillary with ∼6 mm heating length and 52.2-63.8 kΩ resistance).
foils (3). The plastic holder has three openings and a groove on the horizontal axis of symmetry of the rectangular holder, in which the capillary is secured. The big opening is in the center of the holder and provides enough space to let visible light go through, while the two small ones, with cushions (4) and screw bolts (5), allow fixing of the capillary in the groove. The soft copper foils also serve as buffers to reduce the moving forces caused by the alligator clips of the cable connecting to the variable transformer. 2.3. Temperature Calibration. A K-type fine-wire thermocouple was employed to measure the inside temperature produced in the capillary, which was filled with pure n-hexadecane and whose two ends were sealed with wax to avoid oil movements. To capture the values of temperatures corresponding to applied voltages as well as the response time after a step change in voltage implemented, a camcorder recorded simultaneously the thermocouple meter, multimeter, and clock. The entire heating period was recorded as the voltage was raised by 5 V increments until n-hexadecane boiled. 2.4. Visualization Examples at High Temperature. The acid-neutralizing behavior of a MCL, visualized and quantified in our previous studies11-13 at room temperature, was visualized in the present work in the range of 110-140 °C. The fate of the acid droplets and the surrounding oil phase was monitored and recorded. Another application consisted of the observation of airbubble coalescence in water, as the bubbles were forced to contact by volume expansion as a result of heating at temperatures of 60-80 °C. 2.5. Safety Precautions. Because the variable transformer can supply voltage of up to 140 V ac and the heating capillary, copper foils, and alligator clips of the cable are not protected with insulation, care has to be taken so as not to touch them during operation. Additionally, any contacts that cause a short circuit must be avoided. 3. Results and Discussion 3.1. Temperature-Voltage Working Curve. A heating capillary was fabricated and assembled on a holder as described in the Experimental Section. Its heating length was ∼6 mm, its outside diameter near 300 µm, and its resistance 52.2-63.8 kΩ. Figure 2 is its temperature-voltage working curve, where the data were collected from the recorded video during the temperature calibration. By applying proper voltages according to the temperature-voltage working curve,
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Figure 3. Heating and cooling rates (for a heating capillary with ∼6 mm heating length and 52.2-63.8 kΩ resistance): the average heating rates are 75, 100, and 198 °C/s when supplying selected working voltages of 55, 65, and 85 V, respectively.
one can easily control and adjust the temperatures with a variable transformer. At this stage of development of the technique, we could only know the range of the temperatures generated by a specific capillary at a specific voltage; exact temperatures could not be measured during experiments because the thermocouple wire would block part of the light source and also impact the interfacial activities under investigation. The generated temperature unfortunately oscillated with time, especially at the higher temperatures reached. As shown in Figure 2, the fabricated capillary produced 113.9119.1 °C when supplied with 55 V. This temperature oscillation may be attributed to temporal changes in the resistance of ITO films, which vary with the generated temperatures, and it is certainly a challenge that we will pursue in future studies. Nevertheless, it falls well within the working temperature range of 110-140 °C, which we selected to simulate the working temperature of lubricant oil, based on the temperature profile of internal combustion engines. The maximum deviation is about 3 °C from ambient to 80 °C, 9 °C from 80 to 170 °C, and 45 °C from 170 to 265 °C.
3.2. Heating and Cooling Rates. The pulselike curves in Figure 3 are temperature vs time curves plotted at voltages of 55, 65, and 85 V, which can generate temperatures falling within the ranges of 110140, 140-170, and 200-265 °C, respectively. The average heating rates are 75, 100, and 198 °C/s, respectively. and the cooling rates are almost the same as the heating rates. Figure 3 illustrates that the heating capillary has very fast heating and cooling rates, which makes it possible to heat a sample to the desired temperatures and return it to its original state with little delay. Our rates are faster than those achieved in the ITO-covered tubes of Friedman and Meldrum34 and, to our knowledge, the fastest achieved in any microscopy setup that involves heating. 3.3 Visualization Examples at High Temperatures. Figure 4 shows eight snapshots captured from the video recording of acid-oil neutralization, performed at 55 V (generated temperatures falling within the range of 110-140 °C). From snapshots 00:05:08-00:05: 37, we clearly observed that needlelike crystals with small CO2 bubbles formed inside the acid droplet, while the crystals got bigger and more numerous as the reaction time elapsed. Snapshots 00:05:48-00:006:47 show the acid-oil interface deforming and the structure eventually breaking down. The time of acid-droplet “breakdown” is a characterizing index of the reaction rate that we developed in our previous research to rank the acid-neutralizing ability of overbased detergents.11 Acids are continuously formed in marine piston engines during fuel combustion and oil degradation, and they should be neutralized immediately so that they will not attack engine parts. In practice, the way these acids are neutralized is through the continuous addition of lubricant oil that contains overbased detergents. Knowing the rates and mechanisms of acid-oil neutralization is very important in the selection of proper detergents and the formulation of effective lubricants to prevent engine damage. Nevertheless, the testing of acid-oil
Figure 4. Visualization of an acid-oil reaction at 110-140 °C: a high-temperature reaction started at time 00:05:08 (hh:mm:ss), and the acid droplet structure was observed to break down at 00:06:47.
Figure 5. Visualization of air-bubble coalescence as a result of thermal expansion by heating to 60-80 °C.
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neutralization in real engines is impractical, and the results of such testing may not be known until a long time has elapsed. Additionally, acid-base reaction in a biphasic oil-aqueous system is entirely different from that in a single aqueous phase. Whereas our previous work described the microscopic visualization of the neutralization reaction in such systems, the sequence of photographs shown in Figure 4 demonstrates that the heating-capillary technique provides a new way to investigate acid-oil neutralization in biphasic systems at high temperatures. As a second example illustrating the usefulness of the heating-capillary technique, we chose to observe the coalescence of two air bubbles, brought into contact by thermal expansion. The bubbles appeared to have an oblong shape and at ambient temperature had the heights of 42 and 64 µm, respectively (see Figure 5, 00: 17:17). When the temperature was raised to 60-80 °C, the bubbles grew to the heights of 54 and 84 µm, respectively (Figure 5, 00:17:25), and then to the point where their expansion forced them to establish contact, 74 and 101 µm, respectively (Figure 5, 00:17:28). The two bubbles then coalesced and formed a bigger bubble with the height of 168 µm as seen in Figure 5, 00:17: 37. 4. Conclusion Our technique for conducting microscopy in cylindrical capillaries has been successfully upgraded to heating-capillary microscopy by incorporating ITO films on the outer surface of the capillary as a heating jacket. The capillary can be easily heated by supplying proper voltages with a variable transformer. It has a stunning feature of fast heating and cooling rates, 75-198 °C/s, which make possible the subjection of a sample to investigating temperatures for very short times. With the new system, we can investigate phenomena involving biphasic dispersions with interfaces at temperatures ranging from ambient to at least 287 °C. In the current stage of the fabricated capillary, the generated temperatures can only be known as falling within a rough range, with its maximum deviations being approximately 3 °C from ambient to 80 °C, 9 °C from 80 to 170 °C, and 45 °C from 200 to 265 °C. The current stage of the developed technique has two drawbacks: (i) only a rough range of temperature over a certain time period can be expected because the conductivity of ITO films deposited on the capillary is unstable; (ii) the exact temperatures during an experiment could not be measured because of the size of the capillary and the access of the light source. The newly developed technique will enhance the applicability of our capillary-video-microscopy system in the areas of hetero-aggregation, droplet coalescence, cell motility, electrokinetic transport, etc. It is being, and may be, used to investigate acid-oil neutralization, the behavior of extremophilic organisms, and the stability of and transport in double-emulsion systems. Acknowledgment The authors thank Vincent Leo, a Tulane chemical engineering undergraduate, for laboratory assistance in this project. This research was funded by LEQSF of the Louisiana Board of Regents under its ITRS program and Chevron-Oronite Company.
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Received for review March 22, 2004 Revised manuscript received June 3, 2004 Accepted June 9, 2004 IE040083I
