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J. Phys. Chem. B 2008, 112, 11258–11263
Local Control over Phase Transitions in Microgel Assemblies Ashlee N. St. John and L. Andrew Lyon* School of Chemistry and Biochemistry & Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: May 1, 2008; ReVised Manuscript ReceiVed: July 8, 2008
Poly(N-isopropylacrylamide) microgels were coassembled with Au nanoparticles into disordered glassy phases and irradiated with a tightly focused laser (λ ) 532 nm) to study crystallization dynamics following a localized photothermal annealing process. The degree of crystallization produced by the annealing process is dependent upon heat flux into the sample at the site of irradiation, the length of irradiation time, and the temperature of the surrounding bulk assembly that functions as a quenching bath. Control over these sample conditions provides a method by which to probe the dynamics of crystallization over a set of microgel concentrations. The mobility, and thus the crystallization, of particles is shown to be frustrated as the microgel concentration is increased. This is in contrast with equilibrium experiments that have shown an increase in particle mobility with microgel concentration that is manifest as an increase in the freezing transition of the bulk assembly with increasing packing density. Introduction Crystallineassembliesofpoly(N-isopropylacrylamide)(pNIPAm) microgels can be tuned using simple thermal modulation due to the thermoresponsivity of that polymer.1-3 In pNIPAm, the thermoresponsivity is manifest as a temperature dependence of polymer solvation; the pNIPAm microgel radius will decrease due to deswelling near the lower critical solution temperature (LCST) of pNIPAm, which is 31 °C in water.3 The ability to alter microgel size with temperature provides a method for changing the volume fraction of the colloidal dispersion in a closed system.4-8 Thus, the particle mobility and assembly modality can be tuned in a single sample without adding particles or solvent. This allows the study of phenomena that are otherwise difficult to probe. Thermoresponsivity has been exploited in optical devices where the Bragg diffraction of the device is tunable with temperature.9 More recently, it has been utilized as a means to study premelting at defects,4 and to investigate the effects of concentration on the phase behavior of soft repulsive particles.6 In the following article we take advantage of this thermal “tuning knob” to exert control over the colloidal phase in a very small region of a microgel assembly. This permits investigations of the local disruption and subsequent healing of soft colloidal materials. To locally heat the microgel assemblies, we employ the “nanoheater” properties of Au nanoparticles. Such nanoparticles function as heaters when irradiated with the proper wavelength of light, providing localized control over the mobility and phase behavior of the system.10,11 When the microgel/Au nanoparticle assembly is irradiated at a wavelength that is close to the Au nanoparticle absorption maximum (∼520 nm), the nanoparticles absorb the light via plasmon absorption and then relax back to the ground-state through nonradiative processes,12 giving off heat to the surrounding environment. This increase in temperature causes deswelling of the microgels and the area around the site of irradiation becomes a fluid. When irradiation is stopped the sample cools, and under the correct conditions can * To whom correspondence should be addressed. E-mail: lyon@ chemistry.gatech.edu.
Figure 1. Scheme depicting the photothermal melting and annealing process for pNIPAm microgel/Au nanoparticle coassemblies.
be annealed to form a crystal. Figure 1 depicts this process schematically. Previously, our group has used this technique to pattern large crystalline regions into pNIPAm microgel assemblies.10 The patterned regions were characterized by imaging and spectral analysis of Bragg diffracted light, and were found to function as microlens, when formed under the proper conditions.11 In the present contribution, we are using this local heating technique in conjunction with imaging and video microscopy on the particle length scale to observe the effects of microgel concentration on the processes of crystal nucleation and growth. Experimental Section Materials. The monomer N-isopropylacrylamide (NIPAm, Aldrich) was recrystallized from hexane (Fisher Scientific) prior to use. N,N′-Methylenebis(Acrylamide) (BIS), ammonium persulfate (APS), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4), trisodium citrate dihydrate, and rhodamine 6G were purchased from Aldrich and used as received. All water was purified to 18 MΩ with a Barnstead E-pure system. Microgel Synthesis. Microgels were synthesized via precipitation polymerization13 without the addition of a surfactant stabilizer. The 100 mL aqueous synthesis contained 135.8 mM
10.1021/jp803848m CCC: $40.75 2008 American Chemical Society Published on Web 08/19/2008
Phase Transitions in Microgel Assemblies (1.5367 g) NIPAm, 4.2 mM (0.0649 g) BIS cross-linker, and was initiated with 0.0369 g of APS. Particles were purified by repeated centrifugation and resuspension. Dynamic Light Scattering (DLS). The average hydrodynamic radius (Rh) and polydispersity of the particles at 20 °C were characterized by DLS (Wyatt Technology). Data presented herein are an average of 60 measurements with 30 s acquisition times. Samples were equilibrated for 10 min at a 20 °C before measurements were taken. The average Rh of the particles was calculated from the measured diffusion coefficients using the Stokes-Einstein equation. Diffusion coefficients were determined from the autocorrelation decay functions using a regularization algorithm included in the manufacturer-supplied software (Dynamics v6.9.2.9, Wyatt Technology). Au Colloid Synthesis. Au nanoparticles were prepared via previously reported methods14 for citrate reduction of HAuCl4. Transmission electron microscopy imaging (JEOL 1210 Analytical TEM) indicates a particle diameter of 13 ( 1.7 nm. An absorption maximum for the particles was observed at 521 nm on a Shimadzu UV-1601 spectrometer. Sample Preparation. Homogeneous dispersions containing varying concentrations of microgels, and 6.7 nM Au nanoparticles was prepared and introduced into rectangular capillary tubes (VitroCom) that were then sealed to form a closed system as described previously.10,15 The inner dimension of these capillaries is 100 µm along the imaging axis, and all microscopic observations were made approximately 50 µm into the sample, thus removing contributions due to the glass interface. Samples were allowed to equilibrate for at least 3 days after preparation before any measurements were taken.6 Irradiation and Microscopy. A frequency-doubled Nd:YAG laser was aligned through the back port of an Olympus IX-71 microscope and focused through a 100X (1.3 N.A.) oil immersion objective for simultaneous sample irradiation and imaging. This illumination condition essentially produces a diffraction limited laser spot at the center of the sample. Images were recorded with Andor Luca EMCCD (image resolution: 100 µm/ pixel) and Cooke PixelFly black and white CCD (image resolution: 68 µm/pixel) cameras. The LUCA camera was used exclusively for all image time series due to its higher frame rate capabilities. In the case of still images, camera choice was dictated by laboratory availability. All images in this article, except those in Figure 2 were taken with the LUCA. Images in Figure 2 were taken with the PixelFly. The sample temperature was controlled with a Physitemp temperature stage, as well as a Bioptechs objective heater to within (0.1 °C. The exact location of the laser irradiation site was determined by casting a film of Rhodamine 6G onto a glass coverslip. The center of brightness of the fluorescence emission spot due to excitation from the laser was then determined and recorded. Results and Discussion In this experimental arrangement, the size of the melted region can be controlled by the heat flux into the sample, length of irradiation time, and effective bath temperature of the assembly. Heat flux is dependent on both the concentration of Au particles in the assembly and the incident laser power. In all of the experiments discussed in this contribution, the Au nanoparticle concentration was kept constant. It should also be noted that the Au nanoparticles are well dispersed among the microgels throughout the sample. At no point before, or after, irradiation do we observe any aggregation or phase separation of the Au nanoparticles anywhere in the sample. This is important to the homogeneity of heat flux across each sample and between
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Figure 2. Time-lapsed images of a 6.20 wt % sample during 10 min of irradiation at 11 mW/µm2. Scale bar ) 10 µm.
samples, and therefore the reproducibility of the observed results. Experimentally we have observed that the length of irradiation time has a marked effect on the size of melted region obtained. Since we are initially heating the system at a faster rate than it is able to dissipate the energy, the melted region grows over the time scale of minutes. This can be seen in Figure 2 where time-lapsed images of a sample during irradiation show virtually no initial perturbation. However, over a 10-min period, a melted region tens of microns in diameter has developed around the site of irradiation. We have optimized the irradiation times used in these experiments to allow for observable differences in the size of the resultant crystalline annealed region within the boundaries of the field of view imposed by the dimensions of our CCD camera. The effective bath temperature of the bulk assembly is also crucial in controlling the size of the melted region. It is an extremely important parameter in controlling the degree of order or crystallization obtained in the region after cooling as well. This largely arises from the influence that the temperature difference between the irradiation spot and the bath has on the rate at which the bath can equilibrate with the heated area. When the bath temperature is low, the melted region will quench more quickly, effectively freezing microgels in a disordered state. Higher bath temperatures result in a shallower quench; the microgel particles will crystallize before being locked into place. The impact these different control parameters have on the system is discussed below. Classically, in the case of purely repulsive colloids, the phase diagram is determined by a single volume fraction parameter (φ); higher values of φ dictate the transition from fluid to crystal or fluid to glass.16-18 For solvent swollen, deformable microgels with variable densities, this control parameter is often called the effective volume fraction (φeff). The effective volume fraction for microgels is obtained by fitting dilute solution viscosity data to Batchelor’s equation,19 thus obtaining a shift factor that relates the polymer wt % to a φeff value.20 Given the thermoresponsivity of the particles, we can also define an effective bath “temperature” or volume fraction (φeff,b); this term is essentially a temperature-corrected microgel volume fraction, taking into account the previously determined temperature dependence of the shift factor.6 The actual bath temperature is always kept below the LCST of pNIPAm, where the decrease in particle volume as a function of temperature is significant to the overall volume fraction of the assembly (between ∼20 and
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St. John and Lyon TABLE 1: Experimental Details for wt % Normalized Heating Conditions. sample wt %
ra/σb
bath temperature (°C)
φeff,bc
φeff,fd
irradiation time (min)
6.20 6.89 8.02
0.98 0.95 0.92
24.0 24.9 26.1
0.700 0.749 0.855
0.589 0.630 0.719
10 11 13
a Center-to-center distance between particles at 20 °C. b Fully swollen microgel diameter at 20 °C, as determined by dynamic light scattering. c Effective volume fraction of the sample at the bath temperature. d Effective volume fraction of freezing for the sample.
which by nature produces highly polycrystalline organization, especially in the early stages of growth. Because of our control over the heat flux into the sample and the rate at which it is quenched by the surrounding bath, this technique should allow us to probe the crystallization dynamics of microgel assemblies as a function of bath temperature and microgel concentration. We previously described the dependence of microgel phase transitions on initial volume fraction (or overpacking).6 Increases in the packing fraction shift the freezing transition (and presumably the rest of the transitions on the phase diagram), to higher φeff. Thus, even if all samples are observed at a the same φeff,b as extrapolated from the rheometry, the absolute location of each sample’s bulk melting point will still be a function of the polymer concentration. We must therefore take into account this effect when comparing observations from more than one microgel (polymer) concentration. To do so, we have calculated a term we call the quenching depth (QD) for all samples such that φeff,b is normalized to the independently determined freezing transition (φeff,f) of that sample:
QD ) φeff,b-1/φeff,f-1
Figure 3. Images taken of a 6.89 wt % sample irradiated for 5 min at 19 mW/µm2 and φeff,b ) 0.762 (T ) 24.5 °C), 0.746 (T ) 25.0 °C) and 0.730 (T ) 25.5 °C) from top to bottom. Ordered regions have been shaded in red and the green dot near the center of each image indicates the location of the laser spot. Scale bar ) 10 µm.
30 °C), but it is not the drastic volume change that is observed as the temperature approaches and exceeds the LCST.6,21 Figure 3 shows the change in crystallinity in a 6.89 wt % microgel dispersion following laser irradiation as a function of φeff,b. In all images the sample was irradiated for 5 min at 19 mW/µm2. The bath temperature was controlled to produce φeff,b values of 0.762 (T ) 24.5 °C), 0.746 (T ) 25.0 °C) and 0.730 (T ) 25.5 °C). Due to the size of the microgels and the polycrystalline nature of these locally annealed regions, it can be difficult to distinguish ordered areas at this magnification. Areas of crystallization, where well-defined lattice fringes are present, have therefore been highlighted in red to help guide the eye. Note that as the value of φeff,b is decreased, a greater degree of crystallization is observed. The high degree of polycrystallinity observed in these samples is expected as there is no surface or seed crystal off of which to template further crystal growth. Furthermore, the growth process is being quenched at a rapid rate once the laser is turned off, which also contributes to the polycrystallinity. Each region of order is presumably produced from a homogeneous nucleation event,
(1)
Freezing transitions were determined experimentally for each sample using previously established methods.6 Samples were heated in increments of 0.2 °C, allowing for 15 min of thermal equilibration at each temperature point with constant microscopic examination. Another important factor to take into account when increasing the microgel concentration is the differences in heat capacity (and hence heat dissipation) associated with differences in polymer concentration. Specifically, higher polymer concentrations should result in samples with a higher specific heat, which will require longer irradiation times to produce similarly sized melted regions. The irradiation time was thus normalized to the wt % polymer in each sample in an attempt to equalize the overall heat dissipation times across different microgel concentrations. Theoretically, this should produce experimental conditions wherein samples of different polymer concentrations have identical diameters of the melted regions. Table 1 lists experimental variables for the samples observed in this study. The laser power at the sample and QD was held constant at 11 mW/µm2 and 0.84, respectively, with only the irradiation time being varied, as described above. In general, we find that higher microgel concentrations frustrate the crystallization process during cooling and therefore, samples with increasing concentrations produce decreasing degrees of crystallization. Quantifying the degree of crystallization in a colloidal system can be done using rigorous statistical analysis methods such as Ψ6 bond order calculations.22,23 This method measures the bond orientation between particles and their nearest neighbors, and assigns a value of |Ψ6|2 ) 1 for perfect hexagonal lattice packing,23 where:
Phase Transitions in Microgel Assemblies
Ψ6(j) ≡ (1/Z)Σkexp[6iθjk]
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(2)
In this definition, j represents a given particle, Z is the number of bonds between that particle and its nearest neighbors, k is a bonded neighbor, and θjk is the angle between the j-k bond and the x-axis. Attempts to quantify the crystallinity of our samples in this manner failed due to the polycrystalline nature of the observed regions. In those polycrystalline areas, many of the particles lie slightly out of focus and cannot be unambiguously identified, and therefore will not be accounted for in the data analysis. Additionally, the Ψ6 calculations assign value to particles that have bond angles with nearest neighbors that result in a hexagonal lattice structure. Although the microgels do assemble into an FCC lattice, it is not always the case that we are looking at the 111 face of the crystal as these crystallites have been randomly nucleated. Thus, many ordered regions are misrepresented in the analysis for having only four nearest neighbors with bond angles that are not commensurate with hexagonal packing. Averages of Ψ6 calculations that were performed on particles as a function of distance from the irradiation site were performed for samples with a large degree of ordering, such as the image in Figure 4(a). They were not found to be statistically different from averages of calculations made on the same sample in a completely glassy or disordered region. For these reasons, we have taken a less rigorous approach to quantifying the degree of crystallization for each sample. To quantify crystallization for each image, a circle with an origin located at the coordinates of the laser irradiation spot (Vide supra) was drawn. The circle was expanded out from the origin to encapsulate all regions of crystalline order visible in the image. We have defined the radius of this circle as the crystallization radius for each sample. Images a-c of Figure 4 are representative images from each sample concentration that was investigated. The green dot in the middle is centered on the point of laser irradiation. The large circle denotes the crystallization radius. No crystallization was observed at 8.02 wt %, and so the crystallization radius is defined as zero (0) for Figure 4c. For each microgel concentration, 6 different regions of the sample were irradiated and imaged after cooling. The average crystallization radius at each microgel concentration is plotted as a function of polymer wt % in Figure 4d. The ratio r/σ is a measure of microgel compression, where r is the center-to-center distance between particles at 20 °C and σ is the fully swollen microgel diameter at 20 °C, as determined by dynamic light scattering in a dilute dispersion (Rh ) 337 ( 10 nm). The center-to-center distance between particles is determined from calculations of the radial pair distribution function, g(r), following a modified form of particle-tracking routines originally developed by Crocker and Grier.24 Images are analyzed to establish particle position maps from which the radial pair distributions are calculated. The distance, r, at the first nearest neighbor peak maximum is the center-to-center distance used in quantifying the particle compression. Values of r/σ for the microgel assemblies are listed in Table 1. At high microgel concentrations, interpreting data as a function of compression, rather than calculated values of φeff, provides a better physical representation of the assembly. This is due to the manner in which the shift factor is calculated and the highly deformable and compressible nature of the microgels. The shift factor is calculated from the dilute solution regime where there are virtually no particle-particle interactions and therefore no perturbations to the particle. In more concentrated assemblies, a packing maximum is reached. This maximum is 74.0% for an FCC crystal and ∼64% for a disorded glass.25 To
Figure 4. Images showing the crystallization radius for (a) 6.20 wt %, (b) 6.89 wt %, and (c) 8.02 wt % samples. The green dot near the center of each image indicates the location of the laser spot and the black circle highlights the approximate perimeter of the crystallization radius. Scale bar ) 10 µm. Average crystallization radius is plotted in part d as a function of polymer wt %.
accommodate higher microgel concentrations within these packing restrictions, the microgel particles must compress to a smaller size. Recent neutron scattering experiments corroborate this hypothesis of compression.7 A decrease in the particle radius as a function of concentration for φeff > 0.35 was observed for particle radii determined from fits of both the form and structure factors. Shifts in the peaks of the polymer density profiles were also observed at higher concentrations, suggesting compression of the particles as a function of concentration. For this reason, values of φeff over the packing maximum are not accurate representations of the actual volume fraction of the microgels, but rather provide a qualitative picture of how “overpacked” the sample is. In the case of a perfectly ordered crystal, the dilute solution φeff is related to the compression ratio in the following manner:
φeff ) 0.740/(r/σ)3
(3)
Even though the experimental conditions for each sample have been normalized to account for changes in phase behavior and heat transfer with increasing microgel concentration, the behavior of each sample during the melting process is not consistent. When the samples were irradiated at the same laser power, using the same QD, and with wt % normalized irradiation times, the resultant melted regions of samples composed of 6.20 and 6.89 wt % were still slightly different in size. The decrease in melt radius was even more pronounced for the most concentrated assemblies. The melted region for samples of 8.02 wt % was merely a few particle diameters in radius at 11 mW/ µm2, the irradiation power used in the determination of crystallization radius. Figure 5 shows representative melting images for samples of 6.20, 6.89 and 8.02 wt % irradiated at 19 mW/µm2 and QD ) 0.84 after 10, 11, and 13 min, respectively. Even at the higher flux, the 8.02 wt % sample is only mildly perturbed by the laser, and its melted region is smaller than the other concentration samples by at least half. To better compare the behavior of the extremely compressed 8.02 wt % sample to that of other samples, we increased the irradiation power of the laser for this sample to 22 mW/µm2 where a melt region of similar size to sample 6.20 wt % irradiated at 11 mW/µm2 was obtained. The irradiation times
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St. John and Lyon
Figure 6. Partial particle trajectories during the cooling process started immediately after laser irradiation was ceased for (a and c) a 6.20 wt % sample, and (b and d) a 8.02 wt % sample. Trajectories a and b are the first 15 s and c and d the last 15 s of each 75 s movie.
Figure 5. Images depicting melted regions of 6.20, 6.89, and 8.02 wt % samples irradiated at 19 mW/µm2 and QD ) 0.84 after 10, 11, and 13 min respectively. The green dot near the center of each image indicates the location of the laser spot. Scale bar ) 10 µm.
and QD were kept the same as in the previous experiments. It should be noted that whereas the QD has been kept constant, the two samples were irradiated at very different laser powers. Therefore, the actual temperature gradients across the melted regions cannot be assumed to be constant. A 75 s movie of both samples was obtained as they cooled using digital video microscopy (20 frames per second, 1500 frames). The recording was started as soon as the laser had been shuttered. Recorded image time series were then analyzed to obtain particle trajectory maps with the same particle-tracking routines used above to calculate g(r). Trajectories of the first and last 15 s of the 6.20 and 8.02 wt % samples are shown in Figure 6. There are two striking observations that can be made from these trajectories. By comparing parts c and d of Figure 6, it is apparent that the less concentrated sample equilibrates at a much slower rate, even though the laser flux was only half-that which was impingent on the 8.02 wt % sample. In Figure 6c, the 6.20 wt % sample has not cooled completely and still has a large fluid region in the center at the end of 75 s. This is in contrast to Figure 6d, where the particles in the heated region of the 8.02 wt % sample have equilibrated with the surrounding bath before the last 15 s
Figure 7. Enlargements of the bottom left-hand corner of (a) Figure 6a and (b) Figure 6b, depicting concentration dependent dynamic behavior of particles at the periphery of the laser melted region.
of the movie, as evidenced by the confined trajectories shown in Figure 6d. These data suggest that thermal equilibrium of the perturbed region with the surrounding bath is not the main driving force in the freezing process for the 8.02 wt % sample. The strength of this overriding force is even more apparent when considering that the 8.02 wt % sample received a much higher input of energy from the laser than the 6.20 wt % sample and still managed to equilibrate at a much faster rate. In Figure 6b, trajectories for particles that are outside of the melt region of the 8.02 wt % sample show the particles moving in a directed manner toward the center of the melt region, or
Phase Transitions in Microgel Assemblies point of irradiation. This phenomenon is not observed in Figure 6a for the 6.20 wt % sample. Instead, this sample has a thick ordered (crystallized) layer around the perimeter of the melted region. This can be seen in greater detail in Figure 7 where the bottom left-hand corner of Figure 6a and Figure 6b have been enlarged and depicted in Figure 7a and Figure 7b, respectively. The directed motion of particles toward the point of irradiation in the 8.02 wt % sample can be clearly differentiated from the caged behavior of the crystallized particles at the periphery of the melted region in the 6.20 wt % sample. The presence of crystallization at the periphery of the melted region in Figure 6, parts a and c, as well as the crystallization observable in Figure 2 while the laser is still on, suggest that the rate of crystal nucleation at the appropriate volume fraction can be quite fast for pNIPAm microgel particles. This crystallization also provides these samples with a template off of which to grow when the irradiation has stopped, producing less polycrystallinity in the region after cooling. In contrast, the rate of nucleation is slowed in samples with higher microgel concentrations, such as Figure 6, parts b and d, depicts, and nucleation is not observed until after the irradiation has stopped and the sample has partially cooled. This adversely effects the quality of the crystalline region after cooling, as any crystallization has most likely originated from individual nucleation events. It can be deduced from these observations that the high osmotic pressure due to the higher particle concentration in the 8.02 wt % sample is frustrating the mobility of the particles faster than the thermal equilibration of the sample can take place. The compression is so large in this sample that the fluid phase near the irradiation site exerts a force on the surrounding particles and essentially pushes particles away from the irradiated region as the assembly expands from a compact solid to a fluid. As observed in Figures 6b and 7b, when the laser is turned off, the now even more compressed surrounding particles quickly try to regain their lost free volume and constrict the fluid back into the solid phase. This essentially increases the rate of particle arrest, thereby producing a rapidly quenched sample. Conclusions In conclusion, we have demonstrated local control over phase behavior in microgel assemblies over length scales on the order tens of microns, and have applied this technique to investigate the effects of concentration on crystallization dynamics. We have found that the mobility of particles in locally heated regions of samples containing high concentrations of microgels is frustrated very quickly once energy input has stopped. This frustration is attributed to compression from the surrounding bath due to the increased osmotic pressure associated with high concentrations
J. Phys. Chem. B, Vol. 112, No. 36, 2008 11263 of microgels. This result is particularly interesting when compared with our previous equilibrium heating experiments.6 When heated uniformly across the entire sample, the osmotic pressure in highly concentrated microgel assemblies was shown to increase mobility of the particles, as observed by the shift of freezing transition to higher volume fractions. It is extremely important to the development of a complete understanding of soft materials to recognize that local changes or perturbations to a sample can have a drastically different effect than the same perturbation implemented on a global scale. Acknowledgment. The authors would like to thank Dr. Satish Nayak for synthesis of the Au nanoparticles used in this study. L.A.L. acknowledges financial support from the donors of the Petroleum Research Fund, administered by the American Chemical Society. References and Notes (1) Kratz, K.; Eimer, W. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 848–854. (2) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1–33. (3) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (4) Alsayed, A. M.; Islam, M. F.; Zhang, J.; Collings, P. J.; Yodh, A. G. Science 2005, 309, 1207–1210. (5) Hellweg, T.; Dewhurst, C. D.; Bruckner, E.; Kratz, K.; Eimer, W. Colloid Polym. Sci. 2000, 278, 972–978. (6) St. John, A. N.; Breedveld, V.; Lyon, L. A. J. Phys. Chem. B 2007, 111, 7796–7801. (7) Stieger, M.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2004, 20, 7283–7292. (8) Tang, S. J.; Hu, Z. B.; Cheng, Z. D.; Wu, J. Z. Langmuir 2004, 20, 8858–8864. (9) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959–960. (10) Jones, C. D.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 460–465. (11) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292–5293. (12) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409– 453. (13) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247–256. (14) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735–743. (15) Debord, S. B.; Lyon, L. A. J. Phys. Chem. B 2003, 107, 2927– 2932. (16) Alder, B. J.; Wainwright, T. E. J. Chem. Phys. 1957, 27, 1208. (17) Hoover, W. G.; Ree, F. H. J. Chem. Phys. 1968, 49, 3609–3617. (18) Pusey, P. N.; Van Megen, W. Nature 1986, 320, 340–342. (19) Batchelor, G. K. J. Fluid Mech. 1976, 74 (1), 1–29. (20) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705–1711. (21) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B 2004, 108, 19099–19108. (22) Halperin, B. I.; Nelson, D. R. Phys. ReV. Lett. 1978, 41, 121–124. (23) Savage, J. R.; Blair, D. W.; Levine, A. J.; Guyer, R. A.; Dinsmore, A. D. Science 2006, 314, 795–798. (24) Crocker, J. C.; Grier, D. G. J. Colloid Interface Sci. 1996, 179, 298–310. (25) Scott, G. D. Nature 1960, 188, 908–909.
JP803848M
