Article pubs.acs.org/crystal
Nonphotochemical Laser-Induced Nucleation in Levitated Supersaturated Aqueous Potassium Chloride Microdroplets Ke Fang, Stephen Arnold, and Bruce A. Garetz* Department of Chemical & Biomolecular Engineering, NYU Polytechnic School of Engineering, New York University, Brooklyn, New York 11201, United States ABSTRACT: We have observed nonphotochemical laser-induced nucleation (NPLIN) in levitated 8 pL microdroplets of supersaturated aqueous potassium chloride irradiated with nanosecond pulses of 532 nm light. A much higher supersaturation ratio S of 1.20 was required to observe NPLIN in a microdroplet, compared to the value of 1.06 for bulk solutions, attributed to a 40-million-fold lower nucleation rate in a microdroplet at S = 1.06 as a result of its much smaller volume. This finding has application to high spatial and temporal resolution pump−probe studies of nucleation in a containerless environment.
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INTRODUCTION Nucleation from solution, the first step in crystallization, is a rare, stochastic, symmetry-breaking event that is difficult to study experimentally and to model theoretically, and that is still incompletely understood.1 It is an essential tool for purification and control of crystal structure in the pharmaceutical industry,2 and it is the first step in biomineralization, a process essential for many living organisms.3 Not only must solute molecules cluster to form a separate phase, they also must organize to form an ordered crystalline structure, and there is evidence that these two steps can occur sequentially.4,5 In 1996, our group reported that intense near-infrared nanosecond laser pulses could induce a supersaturated aqueous urea solution to nucleate, which we attributed to the optical Kerr effect, in which the optical electric field induces the alignment of solute molecules in a liquid-like cluster, helping the molecules to organize into a critical nucleus. Although photochemical light-induced nucleation had been known since the 19th century work of Tyndall,6 we ruled that out because the peak light intensities and the near-infrared photon energies were too low to induce photochemistry, and we called the phenomenon “nonphotochemical laser-induced nucleation” (NPLIN).7 In 2002, we reported that, by simply switching the polarization state of the light between linear and circular, we could induce aqueous glycine to nucleate into different polymorphs.8,9 Such control over crystal structure had previously been possible only with chemical additives or modifying the physical conditions. Since these initial reports, NPLIN has been observed in a wide variety of materials, including supersaturated solutions of aqueous L-histidine,10 hen egg white lysozyme,11 potassium chloride and bromide12,13 and carbon dioxide,14 as well as supercooled melts of sodium chlorate,15 glacial acetic acid,16 and the liquid crystal 5CB.17 © 2014 American Chemical Society
Simulations by Peters and co-workers suggest that optical Kerr interaction energies are too small to account for NPLIN.18 While several alternative mechanisms have been proposed to explain some of these observations, including isotropic electronic polarization12 and nanocavitation,14 no single mechanism appears to fit all of the materials in which NPLIN has been observed. Because spontaneous nucleation is a random process, an experimenter has little control over when or where a nucleation event will occur. We noted in ref 7 that NPLIN had the potential to allow an experimenter to control precisely the time and place that nucleation occurs.7 In 2009, Alexander and coworkers demonstrated macroscopic 3D spatial control of potassium chloride (KCl) nucleation in agarose gels using NPLIN.19 In this paper, we report a realization of this goal on the microscopic scale: the observation of NPLIN in 8 pL levitated supersaturated solution microdroplets of KCl in water. Alexander and co-workers have characterized NPLIN in bulk aqueous solutions of KCl.12,13 They proposed that the interaction of the optical electric field with the isotropic electronic polarizability of subcritical solute clusters decreases their free energy, making some of them become supercritical. This system exhibits a relatively low laser intensity threshold (5.6 MW/cm2) at a wavelength of 532 nm and with a minimum supersaturation ratio S of 1.06. (S = c/csat, where c is the concentration of a given solution and csat is the concentration of a saturated solution.) We chose to study KCl because of its low intensity threshold. Received: March 27, 2014 Published: April 2, 2014 2685
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voltage was adjusted manually using a DC power supply (HY10010E, Mastech, San Jose, CA, USA) to keep the particle centered at the null point of the trap. Lowering and raising the relative humidity in the EDB vacuum chamber takes trapped particles through hygroscopic cycles in which they alternately undergo efflorescence and deliquescence, respectively.26 Droplets can easily reach supersaturation ratios of 1.80 without spontaneously nucleating. The ratio of the balancing voltages for a dry solid particle and a solution droplet is equal to the concentration (mass fraction) of the solute in the solution droplet. The stainless steel vacuum chamber was connected to a vacuum pump. Mounted in the chamber wall were pressure, temperature, and humidity gauges, monitored using a USB data acquisition device (National Instruments model USB-6289, Austin, TX, USA) and recorded using LabView (National Instruments, Austin, TX, USA). The temperature in the chamber was held at 21−23 °C for all experiments, with temperature fluctuations of no more than 1 °C. A side arm containing water was connected to the chamber to provide a source of water vapor, and gas tanks connected to the chamber provide sources of purified gases. After a droplet was injected into the chamber and captured in the trap, the chamber was sealed and pumped out, leaving a levitated solid particle of KCl. The side arm containing water was opened, allowing the vapor pressure of water to increase until the particle deliquesced, leaving a saturated microdroplet. At that point, the side arm was closed off, and air or another gas was added to bring the total pressure up to atmospheric pressure. By opening an escape valve slightly, additional gas could flow through and out of the chamber to reduce the relative humidity to the desired value while maintaining a constant total pressure. The reduced humidity caused water to evaporate from the droplet, creating the desired supersaturation ratio. Maintaining the droplet at atmospheric pressure allowed one to keep the particle firmly trapped with a high AC voltage while avoiding breakdown and parametric instability.22 Laser Sources. A linearly polarized, diode-pumped, frequencydoubled TEM00 Nd:YAG laser (Nanolase model NG-2111−000, Grenoble, France), producing a train of 1 ns green pulses at a wavelength of 532 nm, with a repetition rate of 11 kHz and an average output power of 5 mW, was used to irradiate the microdroplets. The laser was collimated and focused to a beam waist of 25 μm with a pair of plano-convex lenses. The measured average power at the focal point was 3 mW, corresponding to a peak intensity of 55 MW/cm2. Since light incident on a spherical droplet was further focused by the curvature of the droplet, the near-field intensity could exceed the incident intensity by a factor of a hundred in small regions inside the droplet and a factor of a thousand in small regions outside the droplet.27 A Glan-Laser polarizer in the path of the laser beam was rotated to attenuate and control the laser intensity. The laser beam entered the chamber though a glass window in one side wall of the chamber, and the lens pair was positioned so that the beam waist coincided with a microdroplet centered at the null point of the trap. A counterpropagating 50 mW unfocused continuous-wave blue diode laser beam at 473 nm (Laserglow model LRS-473-TM-50-5, Toronto, ON, Canada) entered the chamber through a glass window in the opposite side wall of the chamber and was used to image the centered droplet. The droplet could be viewed through a glass window in a third side wall of the chamber, by eye or with a CCD camera, using a microscope placed at right angles to the two laser beams.
EXPERIMENTAL SECTION
Electrodynamic Balance (EDB). Electrically charged microdroplets were levitated in air using an electrodynamic balance (EDB),20 which combines the balancing DC electric field of the Millikan oil drop experiment21 with the trapping AC electric field of the Paul quadrupole ion trap.22,23 By irradiating 25 μm diameter droplets with 1 ns laser pulses at a wavelength of 532 nm, we have induced supersaturated aqueous KCl solutions to nucleate, forming KCl crystals. The minimum light intensity required was comparable to that needed in bulk solutions, but, remarkably, the minimum supersaturation ratio was considerably higher, 1.20. We attribute our inability to induce nucleation in microdroplets at a low supersaturation ratio to the much lower rate of nucleation in a picoliter microdroplet compared to the rate in the milliliter sample volume of bulk experiments. This study also represents the first observation of NLPIN in a containerless environment, thus eliminating the effects of container walls. During these experiments, we also observed an unexpected phenomenon in which the laser irradiation of the charged microdroplets caused them to lose on the order of 1−10% of their charge. The incident laser intensity required for charge loss was comparable to that for NPLIN, and charge loss occurred in undersaturated as well as supersaturated solution droplets. We attribute this phenomenon to the laser-induced generation of gaseous ions in near-field hotspots just outside of the shaded face of the microdroplet, which neutralize some of the droplet charge. Materials and Methods. The solubility of potassium chloride in water is 34.6 g/100 g of water at 22 °C.24 Nearly saturated solutions of potassium chloride in water (25% w/w) were prepared from 99% KCl (Fisher, ACS reagent grade) and 18 MΩ water (Fisher, Environmental grade). Microdroplets were created using a piezoelectric picopipette microparticle generator.25 A microdroplet was ejected from the tip orifice upon application of a voltage pulse, and the microdroplet was subsequently charged upon passing through a charging ring, to which a voltage of 40−100 V was applied, depending on the particle (see Figure 1). The droplet then entered the EDB through an opening in the upper electrode of the electrodynamic levitator trap. The DC
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RESULTS AND DISCUSSION Microdroplet Charge Loss. When focused laser pulses with intensities in the range of 2−20 MW/cm2 irradiated a levitated particle, the particle sometimes suddenly became unbalanced and moved down, away from the center of the levitator. A higher DC voltage was required to rebalance the particle, indicating that the particle had either lost charge or gained mass. Since such a mass gain in a short time is unlikely, the net charge on the droplet must have decreased. This charge loss could be observed in undersaturated as well as super-
Figure 1. Cross-sectional view of the electrodynamic levitator trap. Levitated microdroplet in the center of the trap is irradiated from the left with green laser pulses to induce nucleation. 2686
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polarization mechanism proposed by Alexander and coworkers, the rate of NPLIN should be dependent on the rate of formation of subcritical solute clusters in the irradiated sample volume, which should have the same supersaturation ratio dependence as the spontaneous nucleation rate, i.e., the spontaneous rate of formation of critical solute clusters. According to classical nucleation theory (CNT), the nucleation rate density J(S) is given by the equation
saturated solution droplets and in solid particles, but was not observed when the incident laser intensity was below a threshold of about 2 MW/cm2. This charge loss, which was in the range of 103 to 104 electron charges, appeared to be independent of droplet size, solution concentration, and total charge on the droplet, although the amount of charge lost was greater for negatively charged particles than for positively charged ones. The illuminated microdroplets that lost charge were exposed to a green laser pulse train for approximately 0.1 s before moving out of the beam path. As mentioned above, spherical microparticles are known to focus incident light into near-field hotspots. At a given wavelength, the precise light intensity in these hotspots is a sensitive function of the sphere size, and it depends on how close the laser wavelength is to matching a morphological resonance of the sphere.28 Accounting for this focusing, the peak intensity could exceed 10 GW/cm2 just outside the shaded face of the droplet when a droplet is irradiated with an intensity of 10 MW/cm2. Although this is well below the ∼100 GW/cm2 threshold for the laser-induced breakdown of air at atmospheric pressure for nanosecond pulses, subthreshold intensities can generate sufficient ions to equal the charge loss (103 to 104 electron charges) seen in our experiments for a 0.1 s exposure to the pulse train (1000 pulses).29 With positively charged droplets, typically carrying 5 × 104 to 1 × 105 electron charges, the electric potential generated near the surface of the droplet would be sufficiently strong to capture any negative counterions generated by the laser-induced ionization. Laser-Induced Nucleation. Levitated supersaturated aqueous KCl microdroplets were first irradiated with laser pulses at low intensity. The laser intensity was slowly increased until charge loss and/or nucleation occurred. If only charge loss occurred, the droplet moved down. At that point, the green laser beam path was blocked, and the particle was rebalanced. The supersaturation ratio was increased, the beam block was removed, and the laser intensity was again slowly increased until charge loss and/or nucleation occurred. If both charge loss and nucleation occurred, the droplet first moved down from the charge loss, and then moved up as the KCl crystal grew and the water in the droplet evaporated. Eleven positively charged microdroplets were studied in a series of 34 irradiation experiments. Ten nucleation events were observed. The lowest supersaturation ratio that resulted in a nucleation event was 1.20, and the lowest light intensity incident on the microdroplet that gave rise to a nucleation event at this supersaturation ratio was 3 MW/cm2. As mentioned above, refraction by the droplet focuses the incident light intensity up to about a factor of 100 inside the shaded face of the droplet. Thus, a threshold intensity of 3 MW/cm2 incident on the droplet results in an intensity inside the droplet ranging between 3 and 300 MW/cm2 at different locations within the droplet, which is consistent with the reported bulk intensity threshold for NPLIN of 5.6 MW/cm2.13 On the other hand, we have never observed nucleation at any intensity in droplets with a supersaturation ratio below 1.20, which is substantially higher than the minimum of 1.06 reported in bulk solutions. We suggest that the extremely small volume of the microdroplet, compared to the volume in a bulk experiment, is responsible for our inability to observe NPLIN at S < 1.20. Volume and Supersaturation Dependence of Nucleation Rate. On the basis of the isotropic electronic
J(S) = AS exp[−B /(ln S)2 ]
(1)
where A is the kinetic pre-exponential factor, S is the supersaturation ratio, and B is the thermodynamic factor, given by B = 16πγ 3ν 2/[3(kBT )3 ]
(2)
where γ is the crystal−solution interfacial tension, v is the molecular volume of a solute molecule, kB is the Boltzmann constant, and T is the temperature.30 The average number of nucleation events N occurring in volume V irradiated by the laser for time t, is given by N = JVt. To observe, on average, the same number of nucleation events under the conditions of the bulk vs microdroplet experiment, we require J1V1t1 = J2 V2t 2
(3)
where the subscripts 1 and 2 refer to bulk and microdroplet experiments, respectively. The exposure times in bulk and microdroplet experiments were 5 ns and 1 μs (assuming a 0.1 s exposure to the pulse train, i.e., exposure to 1000 1 ns pulses), respectively. Since the microdroplet volume (8.18 × 10−9 cm3) is approximately 40 million times smaller than the irradiated volume in bulk experiments (0.345 cm3), but the irradiation time in the microdroplet experiment is 200 times longer than the time in bulk experiments, the resulting ratio of nucleation rate densities J2/J1 would have to be ∼200 000 to see the same number of nucleation events in microdroplet and bulk experiments. If one tried to observe one nucleation event in a microdroplet at the minimum supersaturation ratio of 1.06 of the bulk experiments, then J2 would be equal to J1, and one would have to irradiate a microdroplet with the Nd:YAG laser pulse train for 40 million times 5 ns (the exposure time in the bulk experiment) divided by the duty cycle of the Nd:YAG laser (1.1 × 10−5) or ∼5 h. The only way to compensate for the smaller volume and short time exposure in the microdroplet is to increase J by increasing the supersaturation ratio. For an increased supersaturation ratio to supply the required increase in nucleation rate density, the value of B must be consistent with the following equation, obtained from eq 1: ln(J2 S1/J1S2) = B[(ln S1)−2 − (ln S2)−2 ]
(4)
Using the values J2/J1 = 2.1 × 105, S1 = 1.06, and S2 = 1.20 yields a value of B = 0.046. The density of a KCl crystal is 1.984 g/cm3,24 making the molecular volume 6.24 × 10−29 m3. Therefore, the interfacial tension would need to be 3.6 mJ/m2. This value is in good agreement with the values reported in ref 12 (2.19 mJ/m2) and ref 13 (5.28 mJ/m2). This is strong evidence that we have observed the same NPLIN phenomenon in levitated micrometer-sized droplets as was observed by Alexander and co-workers in bulk solutions,13 but under dramatically different volume and supersaturation conditions. 2687
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(16) Ward, M. R.; McHugh, S.; Alexander, A. J. Phys. Chem. Chem. Phys. 2012, 14, 90−93. (17) Sun, X.; Garetz, B. A.; Moreira, M. F.; Palffy-Muhoray, P. Phys. Rev. E 2009, 79, 021701. (18) Knott, B. C.; Doherty, M. F.; Peters, B. J. Chem. Phys. 2011, 134, 154501. (19) Duffus, C.; Camp, P. J.; Alexander, A. J. J. Am. Chem. Soc. 2009, 131, 11676−11677. (20) Davis, E. J.; Schweiger, G. The Airborne Microparticle: Its Physics, Chemistry, Optics, and Transport Phenomena; Springer: New York, 2002. (21) Millikan, R. A. Science 1910, 32, 436−443. (22) Paul, W.; Raether, M. Z. Phys. 1955, 140, 262−273. (23) Wuerker, R. F.; Shelton, H.; Langmuir, R. V. J. Appl. Phys. 1959, 30, 441. (24) Haynes, W. M., Ed. CRC Handbook of Chemistry and Physics, 94th ed.; CRC Press: Boca Raton, FL, 2013. (25) Arnold, S.; Folan, L. M. Rev. Sci. Instrum. 1986, 57, 2250−2253. (26) Ehre, D.; Fang, K.; Aber, J. E.; Arnold, S.; Ward, M. D.; Garetz, B. A. Cryst. Growth Des. 2011, 11, 4572−4580. (27) Benincasa, D. S.; Barber, P. W.; Zhang, J.-Z.; Hsieh, W.-F.; Chang, R. K. Appl. Opt. 1986, 26, 1348−1356. (28) Chylek, P.; Pendleton, J. D.; Pinnick, R. G. Appl. Opt. 1985, 24, 3940−3942. (29) Raizer, Yu. P. Laser-Induced Discharge Phenomena; Consultants Bureau: New York, 1977. (30) Kashchiev, D.; van Rosmalen, G. M. Cryst. Res. Technol. 2003, 38, 555−574.
SUMMARY We have observed nonphotochemical laser-induced nucleation in 8 pL microdroplets of supersaturated aqueous potassium chloride. This is the first study of the volume dependence of NPLIN, as well as the first study in the absence of container walls. The small volume also significantly reduces the number of impurity molecules in the sample, reducing the probability of heterogeneous nucleation. By studying NPLIN in microdroplets, we have demonstrated microscopic spatial control for inducing nucleation in a desired location, paving the way for pump−probe experiments in which a single pump laser pulse induces nucleation and a probe beam (pulsed or cw) monitors the time evolution of crystallization through, for example, angle- and polarization-resolved light scattering. Because the supersaturation ratio at which NPLIN occurs in microdroplets is so high, it is possible that the polymorphs formed from NPLIN in microdroplets will be different from those formed in bulk samples.26 The results of this study may also have application to protein crystallization11 and to crystallization-ondemand.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 718-260-3287. Fax: 718-260-3125. E-mail: bgaretz@ nyu.edu. Website: http://engineering.nyu.edu/people/brucegaretz. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge the generous support of the National Science Foundation through Award Number CBET-0932810. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the National Science Foundation.
(1) Kalikmanov, V. I. Nucleation Theory; Springer: Dordrecht, Netherlands, 2013. (2) Chen, J.; Sarma, B.; Evans, J. M. B.; Myerson, A. S. Cryst. Growth Des. 2011, 11, 887−895. (3) Weiner, S.; Addadi, L. Annu. Rev. Mater. Res. 2011, 41, 21−40. (4) Myerson, A. S.; Trout, B. L. Science 2013, 341, 855−856. (5) Vekilov, P. G. Cryst. Growth Des. 2010, 10, 5007−5019. (6) Tyndall, J. Philos. Mag. 1869, 37, 384−394. (7) Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young, R. G.; Myerson, A. S. Phys. Rev. Lett. 1996, 77, 3745−3746. (8) Garetz, B. A.; Matic, J.; Myerson, A. S. Phys. Rev. Lett. 2002, 89, 175501. (9) Oxtoby, D. W. Nature 2002, 420, 277−278. (10) Sun, X.; Garetz, B. A.; Myerson, A. S. Cryst. Growth Des. 2008, 8, 1720−1722. (11) Lee, I. S.; Evans, J. M. B.; Erdemir, D.; Lee, A. Y.; Garetz, B. A.; Myerson, A. S. Cryst. Growth Des. 2008, 8, 4255−4261. (12) Alexander, A. J.; Camp, P. J. Cryst. Growth Des. 2009, 9, 958− 963. (13) Ward, M. R.; Alexander, A. J. Cryst. Growth Des. 2012, 12, 4554−4561. (14) Knott, B. C.; LaRue, J. L.; Wodtke, A. M.; Doherty, M. F.; Peters, B. J. Chem. Phys. 2011, 134, 171102. (15) Ward, M. R.; Copeland, G. W.; Alexander, A. J. J. Chem. Phys. 2011, 135, 114508. 2688
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