Towards the self-assembly of anisotropic colloids: Monodisperse oblate ellipsoids

Journal of Colloid and Interface Science 416 (2014) 30–37

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Towards the self-assembly of anisotropic colloids: Monodisperse oblate ellipsoids Daniel Florea, Hans M. Wyss ⇑ Eindhoven University of Technology, Materials Technology, Eindhoven, Netherlands Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, Netherlands

a r t i c l e

i n f o

Article history: Received 28 June 2013 Accepted 17 October 2013 Available online 28 October 2013 Keywords: Anisotropic colloids Oblate ellipsoids Thermo/mechanical stretching Biaxial

a b s t r a c t We present a robust and straightforward method for producing colloidal particles of oblate ellipsoidal shape via thermo/mechanical stretching of elastomeric films with embedded spherical particles. Our method produces uniformly sized and shaped colloidal particles. The method can be used for producing biaxially stretched particles of different aspect ratios and volumes; moreover, the method has a higher yield and batch size than previously reported methods for producing non-spherical particles via film stretching. These particles are ideal model systems for studying the self-assembly and gel formation for systems with anisotropic shapes and interactions. We illustrate this by adding of a non-adsorbing polymer to the solvent, thereby inducing directional depletion interactions between the particles. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Most studies of colloidal self-assembly and phase behavior employ spherical, uniformly sized colloids. The interactions between these colloids are generally isotropic, with no preferred direction for the interaction forces between the particles. Even for these simple colloidal materials, a wide range of structural arrangements can be realized, ranging from colloidal liquids to glasses, fractal gels, as well as gels formed by arrested phase separation in weakly attractive colloidal systems [1,2]. Colloidal particles with nonspherical shapes and directional instead of isotropic interactions offer great potential for extending on the controlled self-assembly and the structure formation of colloidal materials [3]. Anisotropically shaped colloids are predicted to self-assemble in crystalline arrays that can generate photonic band gap materials, negative index materials and metamaterials [4]. In out-of-equilibrium structure formation through colloidal gelation, qualitatively different structures are expected than those seen for spherical colloids. These anisotropically shaped building blocks also offer the opportunity to induce highly directional interactions, which greatly extends the possibilities for colloidal self-assembly and thus the creation of novel bottom up structures [5]. Other interesting applications of such particles are found in drug delivery and encapsulation systems [6], or in the efficient formation of Pickering emulsions [7]. Such anisotropic colloidal particles could be used as models to study and mimic systems that are governed by directional interac⇑ Corresponding author. E-mail address: [email protected] (H.M. Wyss). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.027

tions. Examples for systems and processes of interest are the blood rouleaux formation of red blood cells [8], and the self-assembly of supramolecular polymers [9] which form as a result of directional, non-covalent interactions. In order to study the dynamics of such systems, including any self-assembly behavior, direct visualization methods are needed. Using fluorescent colloids enables their tracking in either 2D or 3D with a confocal microscope [10]. Such fluorescently labeled colloids have enabled studies where colloids are used to mimic the behavior of atoms and molecules in conventional materials [11]. We would like to extend the power of these colloidal model systems to systems that exhibit directional interactions. To do so, an anisotropic interaction between colloidal particles is desired, preferably an attractive interaction that is strongest along a single axis of the particles, which should lead to the formation of columnar structures. For this reason, it would be interesting to produce and study particles of oblate ellipsoidal shape. When inducing a depletion interaction between such oblate ellipsoidal particles in a colloid–polymer mixture, these interactions should be strongest when they are in contact along the minor axis of the ellipsoids, which provides a closer contact of the surfaces and higher overlap volume. As a consequence, such particles should preferentially form columnar stacks; contacts between particles should preferably occur along the flatter side as opposed to the more curved edges of the ellipsoid. Uniaxially deformed colloids (prolate ellipsoids) are already produced and investigated [12–14]. Recent studies have investigated the behavior of such particles in quasi-2D environments, either trapped at a liquid–liquid interface [7], or between two glass walls [15]. These studies have revealed that such prolate particles

D. Florea, H.M. Wyss / Journal of Colloid and Interface Science 416 (2014) 30–37

are able to stabilize Pickering emulsions much more efficiently than conventional spherical colloids [7], and their behavior at an interface has been exploited in a new mechanism for circumventing the so-called coffee-ring effect that occurs for liquid drops dried on a surface [16]. It has also been shown that the colloidal glass transition of ellipsoids is fundamentally different from that of spheres, as rotational degrees of freedom become important, thus leading to two separate glass transitions, for translational and rotational dynamics, respectively [15]. Also in a 3-dimensional environment, recent studies indicate that the effects of particle shape on the local structure of fluids of hard ellipsoids are nontrivial and depend sensitively on the interplay between the translational and the rotational diffusion [17,18]. However, studies on well-defined, oblate particles are still lacking; this is mainly due to limitations in the currently available fabrication methods. Different chemical [19–21] and physical methods [22] are used to produce a variety of anisotropic shapes. However, the methods currently used for creating oblate particles have significant drawbacks, including relatively low yields or insufficient control over the shape distribution of the particles. Existing methods are based on embedding particles in an elastic film and subsequently deforming this film by film blowing [23] or using a 2D stretcher, consisting of two pairs of orthogonal blocks that move simultaneously [24] or by compression of the elastomer under load [18]. In all these reported methods the stress field applied on the film is not uniform, and, as a consequence, the shapes of the particles produced by this method are not uniform. Other methods are able to produce highly uniform particles, but in turn suffer from limitations to the batch sizes that can be achieved [22]. As a consequence, to our knowledge there is currently no adequate method for producing monodisperse, fluorescent oblate particles with controlled aspect ratios in reasonable quantities, a key requirement for any systematic study of the phase behavior and dynamics of such systems. In this article, we present a new procedure for producing such particles with high accuracy and repeatability, thus extending the range of available colloidal model systems to biaxially stretched, ellipsoidal particles. We prepare our particles using a method based on the principles developed by Keville et al. [12] who embedded spherical latex particles in a polymer film which was then heated above the glass transition temperature (Tg) of the latex and subsequently stretched. We have adapted and extended the method by introducing a biaxial stretching protocol that is able to produce large amounts of uniformly deformed particles in a highly repeatable fashion. We also optimized the method for the processing of fluorescently labeled particles, minimizing the loss of fluorescence that occurs as particles are exposed to elevated temperatures for extended periods of time.

2. Experimental 2.1. Particle deformation: materials and methods Poly(methyl methacrylate) (PMMA) latex particles have been used as model colloidal materials [25–28], as both their refractive index and their buoyancy can be matched with a mixture of organic solvents [26,29,30]. Moreover, when suspended in this solvent mixture, they behave as almost perfect hard spheres. For this reason, particles produced by the same method have been used in a wide range of experimental studies on spherical colloids, including studies on colloidal crystallization [28] , glass formation [31] and gelation of colloid–polymer mixtures [26]. Because the same advantages apply to non-spherical particles we also use PMMA particles in our current study. The particles are purchased from Andrew Schofield at the University of Edinburgh; they are

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synthesized by dispersion polymerization, following the protocols reported in [32]. The fluorescence of the particles results from the incorporation of a (7-nitrobenzo-2-oxa-1,3-diazol (NBD)) dye which is covalently bound to the PMMA. In order to deform these particles into oblate ellipsoids, we use the method developed by Keville et al. [12] as a starting point and extend it to enable the production of bi-axially stretched colloids; this requires several experimental steps, schematically shown in Fig. 1. In the first step, fluorescent PMMA spheres are dispersed in pentane. In a Teflon mold, a PDMS mixture is cast and then mixed with the dispersed PMMA particles. All the components are homogenously stirred until all the pentane has evaporated and then the film is cross-linked inside the circular mold. Previous studies have used hexane in this dispersion step; our experiments indicate that the use of pentane in the processing of these particles has significant advantages over hexane; in particular, we observe a reduced formation of bubbles/inclusions during drying, which improves the homogeneity of the films. This is likely related to the lower boiling point and smaller molecular size of pentane, which are expected to accommodate evaporation of solvent and its transport through the matrix. In a next step, the elastic, crosslinked PDMS film is clamped to a custom-designed stretching device, which applies a uniform strain deformation to the composite. Subsequently, the entire device is transferred to a heating bath set to a temperature above the glass transition of PMMA. As a result, the PMMA particles assume a non-spherical shape that minimizes the elastic stresses that occur in the composite as a result of the applied macroscopic strain deformation. Upon cooling, the particles remain in their new oblate shape and can be recovered by etching the cross-linked PDMS using sodium methoxide [33]. The particles are then washed in multiple steps and finally collected by centrifugation. As reported previously for unidirectionally stretched colloids, the etching process affects the stabilizing layer grafted on the PMMA surface, which can lead to an uneven sterical stabilization depending on the local amount of stretching on the surface of the particles. In fact, it has been previously shown in studies of prolate ellipsoids that this effect can be exploited for inducing anisotropic interactions [34]. If undesired, the effect can also be circumvented by reabsorbing a stabilizing layer on the deformed particles; this can be achieved in a one-step process, as described by Pathmamanoharan et al. [35]. A more complete description of the materials and the procedures used can be found in Supporting Information. 2.2. Characterization methods The particles were investigated using a range of experimental techniques in order to get a better description of their size distribution, shape and fluorescence. Scanning electron microscopy (SEM) was used to measure the lengths of the long axis and, knowing the particle volume, the corresponding aspect ratios between the short and long axes of the particles. To obtain the length of the long axis of the particles, we make use of the fact that in a 2D projection, an oblate ellipsoid will appear as an ellipse, the long axis of which corresponds exactly to the long axis of the oblate ellipsoid. The corresponding property of prolate ellipsoids was recently exploited for characterizing the aspect ratio of such particles [17]; in this case the short axis of the 2D projection is exactly equal to the short axis of the prolate ellipsoid. By performing these measurements on a large number of particles and analyzing the distribution of the observed aspect ratios we quantify the uniformity of the stretching. Further, to study the surface properties of the particles we used Energy-dispersive X-ray spectroscopy (EDAX), which provides information on the chemical composition at the particle surface. We

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Fig. 1. Schematic of the fabrication process for creating oblate ellipsoids (1) particles are embedded in a PDMS film, (2) the film is attached to a stretching device, (3) the film is stretched and heated above the Tg, (4) the PDMS film is etched and the particles are recovered.

used this technique to check whether the PDMS matrix was completely removed from the particle surface after the etching and washing steps. Confocal microscopy was used to image the oblate ellipsoids and also to monitor the fluorescence intensity after each step of the process in order to minimize the loss of fluorescence caused by the different processing steps. By optimizing the processing conditions we find conditions where only a minimal reduction in the fluorescence intensity is observed. Besides the shape of individual deformed particles, another important aspect is the uniformity of this deformation for the full batch of particles. The polydispersity of the particle shapes can be estimated by analyzing a large number of SEM images, to obtain a representative distribution of particles aspect ratios. We have further used Dynamic light scattering (DLS) measurements, which provide a quantification of the polydispersity before and after stretching and also a measure of the effective hydrodynamic radius of the particles. 3. Results and discussion 3.1. Fluorescence intensity As we would like to image the final particles in a confocal microscope, we aim at producing fluorescent oblate PMMA ellipsoids. However, following the typical protocols reported for uniaxial stretching [12,34], we observe a clear change in the color of the PDMS composite film, indicating a significant change in the fluorescence of the particles. The fluorescent activity that was initially present is almost entirely lost in the process. This change is observable even by eye: just after film preparation a bright yellow color is present, but this color is no longer observed after the curing and heating steps are performed; instead it changes to brown with the film becoming mostly transparent to visible light. The fluorescent NBD dye present in the PMMA particles is chemically linked to the PMMA polymer. It has been previously shown that such doped fluorescently dyed polymers exhibit a thermal bleaching that is linked to a higher mobility of the polymer chains, which occurs as the temperature is raised above the glass transition temperature of the material. This changes the photochemical or photophysical behavior of the dye [36–38], leading to bleaching. The loss of fluorescence activity should thus be directly dependent on the glass transition temperature Tg of the PMMA material. Indeed, the change in the fluorescence intensity of dyes has previously even been used to study the glass transition and quantify the Tg of polymers [36,37]. To investigate this temperature-induced effect in more detail, we directly investigate the changes in fluorescence that occur in the material, using confocal microscopy. Following the different fabrication protocols, we suspend PMMA particles in uncured elastomer and place drops of this material on glass cover slips. These drops are then observed at a range of different temperatures to directly follow changes in fluorescence as a function of temperature and time. To enable a direct quantitative comparison, all experiments are performed at constant exposure time, pinhole size, laser

power, and gain ratio. To obtain statistically relevant data, we record and analyze the fluorescence intensity in z-stacks of the samples, consisting of 25 frames. The interslice separation for these confocal measurements was 300 nm and was kept constant in all experiments. From each of these frames, we extract a maximal and a minimal fluorescence intensity from digital image analysis (ImageJ) and use these values to compute the average maximum and minimum intensities and their standard deviations for each of the different experimental conditions. Doing so, we quantify how the different heating procedures affect the fluorescent properties of the particles. There are two distinct steps in the protocol that involve heating of the material: the curing of the film and the heating during the elongation process. To study the influence of the curing temperature on the particle fluorescence we measured samples preheated to three different temperatures (room temperature, 90 °C and 110 °C, the optimal curing temperature for undyed PMMA [12]) and we kept the samples in the oven for 2 h to fully cure. We then measured the fluorescence of the film in the confocal microscope. The results are shown in Fig. 2a, where we plot the measured intensity after the curing procedure, rescaled with the value measured at room temperature before the treatment, as a function of the curing temperature. We observe a clear decrease in the fluorescence intensity between 90 °C and 110 °C, where the intensity drops by almost 70% compared to the untreated samples. It thus appears that the maximum temperature reached in the heating protocol, compared to the glass transition temperature of PMMA clearly is of key importance in determining the final fluorescence of the particles. If the curing temperature is kept below Tg, the fluorescence intensity remains nearly unchanged; it appears that at a curing temperature of 90 °C the fluorescence intensity is essentially not affected, as shown in Fig. 2a. We have measured the DSC of the particles, and show the results in Supplementary Information. From these measurements we identify the glass transition temperature of our PMMA at 105 °C, but the rearrangement of the polymeric chains starts already at around 86 °C. Therefore, to ensure that all the fluorescent sites remain active during the curing procedure, we generally used a curing temperature of 70 °C, well below the temperature range where we expect bleaching to occur. In order to produce deformed particles, the films should be treated at temperatures above their glass transition temperature, which for PMMA is around 105 °C. Ideally, we would want to perform the deformation at temperatures significantly above the glass transition temperature, where the particles are liquid-like, thus enabling a quick and pronounced deformation of the particles, with stretching ratios comparable to the stretch applied on the matrix material. However, at high temperatures significant thermal bleaching is observed for the fluorescent dye embedded in the material. The rate of this bleaching process is temperature-dependent, but significant for all temperatures comparable or higher than the glass transition temperature [36]. When treating the particles at high temperature, the time of exposure should therefore be as short as possible. We found in our experiments that a treatment time of 10 min is sufficient for ensuring a uniform

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Fig. 2. Influence of curing and deformation temperatures on the final fluorescence intensity, rescaled with the initial intensity measured at room temperature. (maximum intensity – solid line, minimum intensity – solid dashed line) (a) rescaled final intensity as a function of the curing temperature for samples cured for 2 h (b) rescaled final intensity for particles treated for 10 min as a function of the deformation temperature, after curing for 2 h at 70 °C.

deformation of the particles, while the loss of fluorescence activity is still not very pronounced. To study the effect of the treatment temperature on fluorescence activity in detail, we perform measurements where the time of exposure is kept constant at 10 min, while the deformation temperature is varied. We perform deformations of the original spherical particles at various temperatures and measure the fluorescence activity in the sample before and after this treatment. In Fig. 2b we plot these intensities, normalized with the initial intensity of the undeformed material, as a function of the deformation temperature. We observe a strong influence on the fluorescent response of the particles. As the temperature is increased above the Tg of PMMA, we observe a pronounced drop in the signal to noise ratio starting from the room temperature samples to the ones treated in the range of 140–220 °C. Keville et al. [12] showed that the PMMA particles can be stretched uniaxially at temperatures between 160° and 220 °C, yielding the same final results. The optimum temperature for deforming the particles used in their study was 180 °C, which is also close to the degradation temperature of the PMMA at 190 °C. Therefore, this temperature should also be a good choice for our experiments for two main reasons: the fact that there is not such a large effect on the final fluorescence intensity between 140 °C and 180 °C and the big drop for the signal to noise ratio is between 180 °C and 200 °C.

To test this approach we perform measurements where the stretching of the film is performed at room temperature and the film is subsequently heated to 175 °C in this stretched state. The time of exposure to high temperatures is limited to 10 min. To study both the optimal etching time and the adhesion of PDMS to PMMA, we use SEM combined with EDAX analysis. The two complementary techniques have been performed to study particles after the stretching, deformation and recovery of particles. A typical image is shown in Fig. 3, where three particles are completely clean, while one is still mostly covered with a film of PDMS, even after the entire cleaning process. We confirm that the material sticking to the particle indeed is PDMS by using EDAX, thus mapping the elements present in the material (see Supplementary Information). The same analysis is also used to identify the most adequate concentration and etching time for the etching process, where the PDMS is completely removed while minimizing the exposure of the PMMA particles that prevents the grafted stabilizing layer from being removed by etching (for more detailed information see Supplementary Information). 3.3. Uniformity of the film stretching process To test the degree and the uniformity of the film deformation that occurs during the stretching procedure, a pattern is drawn

3.2. Cold stretching approach We have shown that, by modifying the working temperature and reducing the time during which samples are exposed to this temperature, the temperature-induced bleaching effects of the fluorescent particles can be dramatically reduced. However, in practice it is not straightforward to significantly reduce the time needed for the entire process including the mechanical stretching of the film. A possible approach towards further reducing this exposure time could be to perform the mechanical stretching of the films at a lower temperature and subsequently heat the films to a higher temperature, where particle deformation can take place. Keville [39] showed that the cold stretching approach yields prolate ellipsoidal particles with aspect ratios that are comparable to those obtained when the entire stretching procedure is performed at elevated temperatures. It thus appears that for the range of stretching ratios employed, delamination is not an important issue and uniformly shaped oblate ellipsoids can be formed.

Fig. 3. Typical scanning electron microscopy image of PMMA particles after the etching procedure. Three PMMA particles appear completely etched and one still has the PDMS attached. This indicates that PDMS remains sticky even after the cold stretching.

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on the original film so that we can track the amount of stretching as well as its uniformity over the film (Supplementary Information Fig. 1). The uniaxial stretching of these types of composites is quite a straightforward process, which is also why most of the articles regarding ellipsoids are about prolate ellipsoids [12–14,34]. However, in order to biaxially deform fluorescent particles, many aspects of the process need to be refined. For this type of elastomer, any fault in the fabrication of the film (trapped bubbles, heterogeneity, small tears) will lead to either rupture of the films or to significant inhomogenities in the material’s deformation. 3.4. Uniformity of the stretched particles To obtain information on the average aspect ratio and the uniformity of the stretching procedure, we use both dynamic light scattering measurements and scanning electron microscopy (SEM). For the light scattering measurements we used non-fluorescent particles stretched by the same protocol, because the excitation and the emission spectrums give a signal at 532 nm, which is the same as the DLS laser wavelength. Because this technique relies on detecting only scattered light, we use samples that do not absorb or emit light at 532 nm. We first investigate how the diffusion coefficient changes as we stretch the particles. Compared to spherical particles, we observe a slowing-down of the dynamics that can be followed at all angles, with the data for a scattering angle of 60° shown in Fig. 4a. For these oblate ellipsoids we can detect both a translational and a rotational diffusion coefficient. We can distinguish between these two modes of diffusion by using depolarized dynamic light scattering (DDLS), a method able to quantify the rotational diffusion coefficient Dr for optically anisotropic particles. In this method, only the light is detected that has changed its polarization during scattering, which makes the signal sensitive to rotational diffusion for optically anisotropic scatterers. A vertical polarizer is placed on the incident beam and a horizontal polarizer on the detector side (VH geometry). The dynamic structure factor  f(q, t) then still exhibits a single exponential decay f ðq; tÞ ¼ e Ct , but in contrast to conventional (VV-geometry) measurements, the decay rate now depends on both the translational diffusion coefficient Dt as well the rotational diffusion coefficient Dr as

 ¼ q2 Dt þ 6Dr C

ð1Þ

where the translational diffusion coefficient can be separately determined from conventional VV measurements, where the decay rate is given as

 ¼ q2 Dt C

ð2Þ

After fitting the correlation curves using cumulant analysis [40], we obtain an average relaxation rate, which scales with q2 for both the spheres and the oblate ellipsoids. Further, the polydispersity index (PDI) of the distribution of relaxation rates is computed as the ratio of the second cumulant to the squared averaged relaxation time, with data presented in Table 1. The effective hydrodynamic radius is 481 nm for the undeformed spheres, while in the case of oblate ellipsoids it is 642 nm, a significant increase of the effective radius. For the standard deviation of this distribution we estimate a average PDI of 0.05 for undeformed spheres, while in the case of the stretched ellipsoid it is 0.21. As we are using the same particles, the additional polydispersity has to come from the shape polydispersity. By plotting the average decay rates (for the spherical and the ellipsoidal particles in both VV and VH) in Fig. 4b, we directly extract the average diffusion coefficients using Eqs. (1) and (2); the translational diffusion for the oblate ellipsoids is determined from the slope of the VV measurement (obtained using a linear fit with the offset set to 0, Dt ¼ 1:5683  109 cm2/s, R2 = 0.9969) and the rotational diffusion for the oblate ellipsoids is obtained from the intercept of a linear fit to the VH data (Dr = 0.3754 s1, R2 = 0.9931). This analysis of the average decay rate also indicates that the oblate ellipsoids diffuse slower, as evident by the higher slope observed in the case of spheres compared to ellipsoids. There is a very small difference between the VV and VH signal, thus indicating that the rotational diffusion is very slow, close to the resolution of our technique. However, using the Perrin equation for the diffusion of spheroids [41,42] as a model prediction (see Eqs. (1)– (4) in Supplementary Information), we find that the DLS data is consistent with spheroids (long axis: 0.608 lm, short axis: 0.310 lm) with a final shape ratio of L=D  1:96. For such oblate spheroids the Perrin equations yield the following values for the diffusion coefficients: Dt = 1.5680  109 cm2/s and Dr = 0.1709 s1. This ratio corresponds to a stretching ratio of 25.1%, a reasonable value given that it was achieved by stretching the composite film by 26% during processing of the particles. The average shapes of our particles as obtained by light scattering and the Perrin equations are thus con-

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Fig. 4. Dynamic structure factors f ðq; tÞ measured in DLS measurements for spheres (solid line) and for oblate ellipsoids (VV – dash-dot line and VH – dotted line) that were stretched with 26% at an measuring angle of 60° corresponding to a q2 value of 5:01  1014 m2 . (b) Decay rate C as a function of q2 from polarized dynamic light scattering measurements for the original PMMA spheres (circle) and for oblate ellipsoids stretched at 26%, for which SEM measurements indicate a deformation ratio of 27%. Polarized light scattering (diamonds) and depolarized light scattering (squares) measurements of the oblate particles show comparable behavior.

D. Florea, H.M. Wyss / Journal of Colloid and Interface Science 416 (2014) 30–37 Table 1 Distribution of the effective hydrodynamic radius for spheres, and for oblate ellipsoids prepared using a film stretch of 26%, calculated using cumulant analysis [40]. Spheres

Ellipsoids

Angle

Radius (nm)

C (s1)

PDI

Radius (nm)

C (s1)

PDI

40 50 60 70 80 90 100 110 120 130 140 150

437.21 469.27 488.94 496.77 477.08 472.21 480.14 480.22 488.87 500.00 482.86 497.25

32.18 45.78 61.50 79.65 104.16 127.35 147.00 168.06 184.52 197.58 219.95 225.67

0.06 0.08 0.00 0.02 0.02 0.05 0.03 0.14 0.06 0.00 0.00 0.19

621.03 634.22 618.17 681.16 688.10 723.82 632.74 610.85 616.36 613.89 628.93 631.44

22.65 33.87 48.64 58.09 72.22 83.08 111.55 132.12 146.35 160.93 168.86 177.71

0.39 0.19 0.19 0.14 0.26 0.13 0.21 0.11 0.19 0.27 0.22 0.26

Mean STD

480.90 16.925

0.05 0.1

641.73 35.94

0.21 0.1

sistent with the deformation expected from the film stretching ratios. To further investigate the shape of the particles as well as the distribution of these shapes, we also use SEM imaging. This technique provides a direct visual representation of the particles, as shown in Fig. 5a and b, where we present a comparison between the initial, spherical particles and the final oblate ellipsoids, for an example where the film was stretched by 40%. The two images have the same magnification, which enables us to readily extract the stretching ratio of the particles. Starting from spheres of 3 lm diameter before stretching (Fig. 5a), oblate ellipsoids with a long axis of around 4 lm are obtained. Because SEM is a projection method, the apparent long axis of an oblate ellipsoid always corresponds to the true long axis, irrespective of the particle’s orientation. Thus to characterize the size distributions of the ellipsoids, we have measured the long axis, while for the spheres we have measured the radii. This is seen even more clearly in the zoom-in of particle with a short axis that appears almost perfectly aligned parallel to the imaging axis, shown in Fig. 6a. To illustrate this, a circle is drawn on top of the image, matching the shape of the particle’s projection with only minimal deviations. These images clearly demonstrate that the particles have been deformed, and, knowing the size of the initially spherical particles, they enable us to quantify the deformation ratio for each individual particle. To quantify the distribution of particle shapes, we analyzed SEM images for the same batches of particles that have been analyzed previously by DLS. The resulting size distributions are shown in

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Fig. 6b, where we plot two histograms representing the size distributions for both the undeformed particles and for those obtained by stretching the PDMS film by 26%. The mean radius of the spheres is 479 nm, while for the oblate ellipsoids the average value is 610 nm. This corresponds to a stretching ratio of 27.3%, a value that almost perfectly matches the amount of stretching applied in the fabrication. Combining the information from both dynamic light scattering and SEM, we thus obtain quantitative information on the degree and the uniformity of the particle’s deformation. Both methods indicate that the stretching is uniform, and the degree of particle deformation is consistent with the amount of stretch applied to the PDMS films during the stretching procedure.

3.5. Indications for directional interactions As mentioned at the beginning of this paper, one important motivation for creating oblate ellipsoidal particles is the possibility to induce anisotropic interactions between these particles and to study structure formation and self-assembly in such materials. To illustrate that this is possible, we have performed a number of test experiments in colloid–polymer mixtures, using our newly obtained oblate particles; the addition of polymer to a suspension of these particles should lead to an attractive depletion interaction, where the shape anisotropy directly translates to a directiondependent interaction, which is strongest along the minor axis of the ellipsoids. While PMMA spheres in colloid–polymer mixtures have been widely used as colloidal model systems [26,29,41–43], for oblate ellipsoids no such studies of structure formation have yet been reported. To illustrate these effects in preliminary test experiments, we have used polystyrene as a depletant, as is wellestablished from studies on colloidal PMMA spheres [26,29,41– 43]. The attractive interaction is controllable and it scales with the radius of gyration and the concentration of the polystyrene molecules [44]. To obtain a first indication on the structuring behavior for oblate ellipsoids, we prepare samples at different conditions and investigate the resulting structures using a confocal microscope. At a volume fraction of U = 0.1 and a depletion interaction of U  5.25 kT, which was estimated using the formula described by Dinsmore [41], we obtain clustering of the particles into structures which indeed indicate that the interactions are strongest along the short axis of the ellipsoids, as shown in Fig. 7. However, the formed columnar structures consist of typically only 5 or fewer ellipsoids. Also, non-columnar, but still directional structures like those presented in Fig. 7c are frequently observed in our experiments on these systems. Although the observation of these structures

Fig. 5. Scanning electron microscopy images of PMMA particles before and after stretching. (a) Original spherical particles (diameter 3 lm, as measured from SEM) (b) oblate ellipsoidal particles obtained for a 40% stretching. The scale bars are 5 lm.

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Fig. 6. (a) Close-up view of an oblate ellipsoid obtained for a 35% stretching. A solid black circle indicates the edge of the particle; it has a diameter of 3.95 lm, corresponding to a stretching ratio of 31%. The scale bar is 1 lm. (b) Particle size distributions (distribution of longest axis) obtained from SEM images for the original PMMA spheres (black bars) and for oblate ellipsoids stretched at 26% (grey bars).

Fig. 7. Typical clusters formed by oblate ellipsoids (U  0.1) in a polystyrene solution (at an estimated interaction energy of U  5.25 kT) in a density matched solvent mixture of decalin and tetralin. The typical formed structures (a–c) indicate an affinity towards aggregation along the short axis of the oblate ellipsoids.

indicates directional interactions, these preliminary data do not yet give us information on the phase behavior or out-of-equilibrium structure formation in these materials. More detailed, systematic studies will be performed in the future to investigate the behavior of these oblate ellipsoidal as a function of both their aspect ratio and the magnitude of the induced depletion interaction forces.

4. Conclusions Our modified stretching protocol is able to produce uniformly shaped fluorescent oblate ellipsoids of controlled size and shape. With our current device we are able to produce in one batch around 500 mg of particles, but upscaling to even larger amounts should be straightforward, as the batch size scales with the square radius of the composite film. The uniformity of the stretching is adequate even when small differences in the L/D ratio are needed. We modify the shapes of particles with various L/D ratios directly by modifying the stretching of the original composite films. The availability of such well-defined fluorescent oblate spheroids in large enough volumes can be the basis for a range of interesting experimental studies on the effects of particle shape on the behavior of colloidal suspensions. As a result, phase diagrams, dynamics and structure formation can be studied directly in experiments, and predictions from simulations and theory can be tested. Moreover, based on these well-defined oblate particles, several convenient mechanisms can be exploited to induce and tune direc-

tional interactions between colloidal particles. The most straightforward mechanism is the introduction of depletion interactions by the addition of non-adsorbing polymers to the solvent; here the anisotropic shape of the particles directly translates to an anisotropic, directional interaction. Other options include the surface treatment of one side of the ellipsoids in order to create Janus particles or the direction-dependent modification of surface properties by modifying the etching time. In analogy to the work of Zhang et al. [34] for prolate ellipsoids, in our case this effect should lead to weakly attractive interactions at the edges of the oblate spheroids. The presented procedure for producing oblate spheroids thus opens up a wide range of new possibilities in the study of colloidal self-assembly by extending the available model systems to oblate spheroids. These systems offer a convenient way of inducing highly directional interactions between particles. The self-assembly and phase behavior of these materials should be qualitatively different from that of spherical colloids and offer insight into structure formation in systems where directional interactions play an important role, ranging from supramolecular polymers to biological systems.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.027.

D. Florea, H.M. Wyss / Journal of Colloid and Interface Science 416 (2014) 30–37

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