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Understanding the Formation of Anisometric Supraparticles – A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface Marcel Sperling, Periklis Papadopoulos, and Michael Gradzielski Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01236 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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Understanding the Formation of Anisometric Supraparticles – A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface Marcel Sperling,† Periklis Papadopoulos,∗,‡ and Michael Gradzielski∗,† †Technische Universität Berlin (TU Berlin), Institut für Chemie - Stranski Laboratorium für Physikalische und Theoretische Chemie, Straße des 17. Juni 124, D-10623 Berlin, Germany ‡University of Ioannina, Department of Physics, P.O. Box 1186, GR-45110 Ioannina, Greece E-mail: [email protected]; [email protected] Abstract Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here, we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop-air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation.

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This leads to a build-up of a stiff fumed silica mantle of about 20 µm thickness, that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles.

Introduction Supraparticles represent a class of nano-structured materials prepared by self-assembly of micro- or nanoparticles. 1,2 Such particles have drawn much interest in recent research, as via application of various types of ingredients, they can be used for applications like catalysis, 3–5 photonics 6 or sensing. 7 However, the formation of such particles with controlled symmetry or more likely, asymmetry, remains a challenge. In colloidal suspensions the interplay between drop surface energy and nanoparticle interactions leads to nanoparticle arrangements that differ from bulk suspensions. 8 Confinement can also lead to highly ordered structures. 9 Recently, the synthesis of supraparticles with highly reproducible properties, such as shape anisometry, by evaporation of dispersions of fumed silica (FS) or TiO2 nanoparticles on superamphi- and superhydrophobic surfaces was demonstrated. 10–13 Similarly, but by using monodispersed silica microspheres of different sizes, the concurring mechanism of particle sedimentation and evaporative flux led to the controlled formation of a ring-like structure resulting in doughnut-like shaped supraprticles that can also be rendered patchy using additional Au- or Fe3 O4 -nanoparticles. 14 Thanks to low liquid-solid adhesion on these surfaces it is straightforward to form spherical particles that can easily be removed. By varying dispersion and surface properties it is also possible to form asymmetric particles, that can be used as self-propelled swimmers 15,16 or to form more complex liquid-crystal-like structures. 17,18 The formation of anisometric, mm-sized supraparticles has been recently discussed by Sperling et al.. 10–12 This was achieved by deposition and dyring of aqueous suspension droplets onto a superhydrophobic surface containing fumed silica (FS). It was shown that by careful adjustment of ionic strength via addition of NaCl to the initial suspensions, the degree 2


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of observed anisometry values was proportionaly correlating. 10,11 A mechanism hypothesis was discussed, which is briefly illustrated in Figure 1.

(a) 0%

(b) 50 %

(c) 79 % High [NaCl]

Low [NaCl]

Dried 0.001 mM

25 mM

Figure 1: Scheme of anisometric supraparticle formation reported by Sperling et al., 10,11 with corresponding drying time in percentage, with 100 % equaling dried state: (a) Deposited droplet on a superhydrophobic surface, containing 3.5 % w/vol of FS and 25 mM NaCl. (b) Onset of droplet deformation due to rigid shell, formed by FS particle accumulation near the water-air interface. (c) Deformed droplet shape is kept during further drying until total drying. (d) Examples of dried supraparticles at low (0.001 mM) and high ionic strength (25 mM). FS particles show an evaporation induced shell formation at the droplet interface, promoting droplet deformation upon further shrinkage. The observed anisometry extent, thereby strongly depends on the applied ionic strength. Scale bars within the micrographs correspond to 500 µm.

When the droplets are initially applied onto the superhydrophobic surface, we can assume an overall homogeneous distribution of ingredients (Figure 1(a)). Upon drying the FS particles get collected near the water-air interface forming a dense shell due to their diffusion being too slow to compensate for the droplet surface propagation promoted by evaporation (Figure 1(b)). Depending on the strength of particle interaction, hence ionic strength, this shell is tuned for its rigidity, i.e. viscosity. Accordingly, if high enough, deformation of the droplet occurs, as it is no longer able to isometrically adapt to the shrinking droplet causing a break of symmetry. It was also shown that when observing several droplets at different FS concentration while keeping the NaCl concentration at 25 mM, proven to be high enough 3


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to form anisometric particles (≥ 0.5 mM), the deformation always takes place at a constant surface excess concentration of FS. This was calculated taking into account evaporated liquid volume and redistributing the corresponding amount of contained particles onto the droplet surface at the point in time right before its anisometric deformation. 12 After deformation the resulting shape is kept, while shrinking further until reaching dried state (Figure 1(c)). The so formed supraparticles, as indicated before, show various kinds of shapes, from isometric spherical at low towards highly anisometric ellipsoidal at high ionic strength, which is indpendent from FS content. Moreover, this process allows for addition of other components, which depending on type can amend several functionalities, such as magnetic response or fluorescence. 10–12 So far the details of the mechanism leading to asymmetric supraparticles formed from FS dispersions are not completely clear. Particle formation was mostly studied by optical microscopy. Even though this is sufficient to quantify particle size, symmetry and evaporation rate, it does not allow a direct observation of the interior and contact line of the droplets, i.e., to follow the mechanism in detail. Thus it was not possible to conclude whether the contained nanoparticles aggregate at the drop surface, or within the interior. Heading towards a more detailed view, confocal laser scanning microscopy (CLSM) was successfully used to study droplets on superhydro- and superamphiphobic surfaces. 19 In contrast to conventional optical microscopy, CLSM can resolve details at the three-phase contact line, at the drop-surface contact area and in the interior of the drop. For instance, in a recent work this technique showed that even micrometer-sized drops form high contact angles with a superamphiphobic surface throughout the evaporation process, leading to defect-free spherical particles. 13 In the present work, we use confocal laser scanning microscopy to investigate the formation of anisotropic supraparticles from colloidally dispersed fumed silica (FS) inside water droplets on a transparent superhydophobic surface. This is done in order to verify and quantify the mechanism of formation of these structures that was suggested in previous studies, 4


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namely shell formation by FS particles due to diffusion-controlled aggregation throughout droplet evaporation. 10–12 All measurements were done in a specially designed chamber with controlled temperature and humidity. In contrast to previous studies we used fluorescence labelling for both solid and liquid phases of the dispersion. Thus, by using one reflection and two separate fluorescence detector channels, we correct for the variation of the solid volume fraction that increases in course of evaporation. By careful image analysis that corrects possible artifacts, we compare changes in fumed silica concentration near the water-air interface and in the interior.

Experimental Superhydrophobic Surface The exact procedure was published previously. 20 Briefly, cover slips with a thickness of 170 ± 5 µm (Carl Roth) were covered with a layer of soot by gently moving inside the flame of a commercially available candle right above the wick for about 20 s. The soot-coated cover slips were then covered with a thin silica shell by chemical vapor deposition (CVD). For this, vials with tetraethyl orthosilicate and ammonia (each 200 µl, Sigma-Aldrich) were placed into a sealed chamber made of perfluoroalkoxy alkane (PAF, volume 120 ml) with the cover slips for about 2 h at 80-90 ◦ C. After combustion at 550 ◦ C for 3-4 h the surface became translucent. It was finally hydrophobized via CVD using 100 µl of n-octyldimethylchlorosilane (Sigma-Aldrich) inside a PFA chamber at 80-90 ◦ C for 3 h. The contact angle of water was 167 ± 2◦ .

Labelled Fumed Silica Briefly, 0.07 g of fumed silica (FS, Sigma Aldrich, particle aggregate size r = 120-200 nm, which are built from primary particles of 7 nm diameter, 395 m2/g surface area) was dispersed in 17.5 mL of water under vigorous stirring. FS was then labelled with Rhodamine 6G 5


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em (absorption λabs max = 535 nm, emission λmax = 560 nm) by simple adsorption onto the FS

particles. For that a stock solution of Rhodamine 6G in water was prepared at 1.2 mg/mL, which was added in order to set the overall ratio of Rhodamine:FS to 0.28 · 10−3 within the suspension. After further stirring for 3 h, the suspension containing coloured particles was subjected to purification by centrifugation. The centrifugation was done each time for 20 min at 3160 g, then the supernatant was discarded and replaced by deionized water. It is noted here that the supernatant was water-clear and did not show any visible colour. After repeating this process twice, the particles were concentrated by a last centrifugation and their concentration was measured by gravimetry after drying. The final FS concentration was set to 7 % w/v, similar to previous experiments. After having achieved the desired concentration, a small R em amount of < 1 mg AlexaFluor 488 (Invitrogen, λabs max = 490 nm, λmax = 525 nm) was added

to the suspension and dispersed. The droplets for the CLSM experiments were adjusted to a concentration of 3.5 % w/v FS (i.e. 9.6 µg/ml Rhodamine 6G) and 0 or 25 mM of NaCl, respectively.

UV-VIS spectroscopy We verified that no Rhodamine 6G remains in the water after labeling of the FS particles, by measuring absorption spectra after particle sedimentation enforced by centrifugation. The final supernatant showed no absorption (Figure S1b). In contrast, Alexa488 did not adsorb to the particles and its absorption was stable with time (Figure S1a). Spectra were recorded with a Cary 50 UV/VIS spectrometer (Varian). FS particle and dye concentrations in the final solution were set to 3.5 w/v and 9.6 µg/ml, respectively, as also used within the experimental droplets.

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Rheology We performed rheological measurements of FS aqueous dispersions at concentrations of 5, 7 and 9 % w/v at NaCl ionic strength vaying from 1 to 100 mM. Before measurements the samples were pre-sheared at a rate γ˙ = 5 s−1 for 30 s followed by equilibration for 60 s. Concentrated NaCl solution was added by vigorous mixing for about 30 s and then the sample was measured immediately with a Gemini 200 HR rheometer (Malvern Instruments). The dispersions showed solid-like viscoelastic properties whose viscosity increased with the concentration of FS. The ionic strength showed similar influence, but on a narrower scale (Figure S2).

Humidity Chamber & Preparation In order to provide homogeneous conditions during the drying process, the experiments were conducted in a custom-built chamber allowing for control of both humidity and temperature (Figure 2a). Preparing the measurement, 3 µl-droplets of FS dispersion were dispensed onto a transparent superhydrophobic surface and the chamber was properly closed. Cotton balls soaked with saturated K2 CO3 aqueous solution were placed into the chamber, providing a constant humidity of about 43 % and the temperature was equilibrated at 25 ◦ C. Overall, the process of droplet application until start of the measurement took about 1 min, which was taken into account for the times reffering to the image sequences.

Confocal Laser Scanning Microscopy (CLSM) Setup The humidity chamber was attached to a confocal microscope (Leica TCS SP5 II). We used a dry objective from Leica (HCX PL APO 40×/0.85). Refractive index matching could be done in our case, as the SH-surface is mostly air and has refractive index close to 1, whereas water has nwater = 1.33 and FS particles ≈ 1.46. 21,22 FS has low bulk volume fraction, at f ≤ 3 % at t = 0, during the observed time span, so the refractive index of the drop is close

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(a)

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CoverGwithGslideGglassGopening HT-Sensor

GasGflux

HeatingGmodules

T-ControllerGBPT100k SampleGholderGBflexibleGviaGfoilk

(b)

FumedGsilicaGB570Gnmk BackgroundGB500Gnmk

SH-layer GlassGslideGB633Gnmk

Figure 2: (a) Humidity chamber. (b) Illustration of a typical experiment including the overlay of measured confocal images, where the presented colors correspond to the detected wavelengths for reflection (red, 633 nm) and the two fluorescence channels for the liquid phase background (green, ~500 nm) and the FS particles (yellow, ~570 nm).

to that of water, 1.33. Ideally, a water objective should be used to image the drop interior. However, the air cushion in the SH-surface prohibited the use of a water objective, because total reflection at the glass-SH-surface interface occurs. The unavoidable aberrations resulted in lower resolution and loss of intensity. This loss of intensity is not uniform, but depends on depth and shape of the drop-air interface, so it gives rise to “shadows” in the background images. To correct for intensity variations due to aberration, water was also labeled with fluorescence using Alexa488. As both, FS and Alexa488 are negatively charged in water, Alexa did not adsorb to FS, but was repelled by it. Without salt it was repelled at distances comparable to the Debye length revealing the presence of even loose aggregates, so some particles appear as “holes” in the Alexa fluorescence image (Figure 3(a)). At cNaCl = 25 mM, the Debye length decreases to 2 nm (Eq. 1). s

κ−1 D =

r 0 kB T = 2.0 nm 2NA e2 cNaCl

(1)

Thereby, r is the dielectric function of water at low frequencies, NA Avogadro’s constant,

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and e the elementary charge. Therefore, Alexa appears uniformly distributed (Figure 3), as the volume fraction of water is everywhere close to 1. All measurements analyzed herein were done at this salt concentration of 25 mM NaCl, which was previously found to favor the formation of highly anisotropic particles. 10–12 Axial and lateral resolution reached 1.0 and 0.3 µm, respectively, near the drop-air interface, allowing to resolve aggregates down to a few µm, but single particles could not be resolved. Deeper inside the drop the axial resolution decreases due to refractive index mismatch and may reach several µm. Time series of vertical xz-plane scans were measured in time intervals of 10 s. The scan frequency was set to 100 Hz at a pinhole size of 100 µm. Thereby, the xz-plane was carefully set to the droplet center, obtained from the circular droplet curvature from xy-plane scan. We focused at the microscopic contact line of the droplet (Figure 2(b)). During scanning three channels were used, one in reflection and two in fluorescence mode. In order to observe the surface and interfacial dropet contour line we used the reflection from the HeNe laser at 633 nm, where the PMT detector was set to a range of 630-636 nm. For the two fluorescence channels, two Argon laser lines were used at 488 and 514 nm. Detection ranges were set to 493-509 nm and 570-586 nm, respectively. This allowed to separate the emission of Rhodamine 6G originating from the FS particles from that of Alexa488 in the liquid phase, which did not adsorb onto the particles as already indicated by UV-VIS experiments (Figure S1). As both FS and Alexa were negatively charged in water, Alexa was strongly repelled by FS, especially when no salt was added (Figure 3(a)).

Results and discussion The distribution of FS particles in sessile drops in three dimensions was measured with CLSM. Thanks to the transparency of the SH-surface and the use of an inverted microscope, it was possible to monitor the part of the drop close to the macroscopic three-phase contact line (TPCL) (Figure 2(b)). Vertical slicing provided by the xz-scan mode proved that the

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(a)

t = 2 min FS dispersion 0 mM NaCl

t = 8 min

(b)

t = 18 min

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t = 2 min

t = 8 min

t = 18 min

FS dispersion 25 mM NaCl

Air

Air SH layer

SH-layer

Background

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FS fluorescence

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20 µm

20 µm

Figure 3: Sessile drops of FS dispersion evaporating on a superhydrophobic surface. Vertical (xz) confocal images are shown. FS particles were labelled fluorescently with Rhodamine B (yellow), and the aqueous phase (“background”, NaCl solution, concentration of (a) 0 or (b) 25 mM) was labelled with Alexa488 (green). The two channels were measured simultaneously. Reflection (white) shows the drop-air interface and the nanoparticles in the SH-layer. Some large aggregates are marked with a red circle in both channels. Whereas repellency of the dye by FS in (a) gives rise to “holes” in the background channel, in (b) high salt concentration allows for uniform dye distribution.

drop is in the Cassie wetting state, 23 as the SH-surface is not penetrated. In addition, excellent water repellency is confirmed by the high static macroscopic contact angle of 167 ◦ . In course of evaporation drop volume decreased and FS concentration increased (Figure 3, Movie S1, Figure S3). For about t = 13 min the TPCL remained pinned and the contact angle decreased. When the contact angle reached 159 ± 1 ◦ the TPCL depinned and started to recede. At t = 18 min, well before complete dryness, fluorescence from FS particles near the surface increased much more than the drop interior, implying that a dense shell of FS particles started to form (Figure 3). Verification of the formation of a dense shell requires quantitative interpretation of confocal images through further detailed analysis. First, optical artifacts must be corrected. Ideally, CLSM requires a system with uniform refractive index that is measured with an objective designed for this refractive index. This cannot be done in our case, as explained above. To correct for intensity variations due to aberration, water was also labeled with fluorescence using Alexa488. Both fluorescence channels are affected by abberation the same way. Therefore, to correct for intensity variations that are not due to real variation of FS particle concentration, we performed a normalization of the fluorescence intensity with the Alexa background.

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In the calculations, all images were converted to 8-bit grayscale l×c-matrices of float-type (pixel size, l,c = 512, l ∼ = row, c ∼ = column) using the python PIL library in combination with NumPy and SciPy modules. 24–27 The general evaluation prcoedure is illustrated in Figure 4. This procedure was repeated for all images of the time sequence of the drying process. The FS fluorescence matrix, Fl×c = (fi,j )i=1...l,j=1...c , was converted using the background matrix, Bl×c = (bi,j )i=1......l,j=1...c (both Eq. 2a) by means of incremental division of brightness values, BG fi,j and bi,j , yielding Fl×c , shown in Figure 4(b) (yellow: FS and green: background). In

case of empty pixels, i.e. bi,j = 0, the corresponding value for

f b i,j

is set to 0 in order to

avoid division artifacts (Eq. 2b).

 f  b 1,1    f  b 2,1 BG Fl×c =  .  .  .   f b l,1

f b 1,2 f b 2,2

.. .

f b l,2



···

··· .. .



···

f b 1,c   f  b 2,c  

.. .

    

(2a)

f b l,c

with,

f b

!

:= i,j

   fi,j    bi,j             

0

| bi,j 6= 0 (2b) | bi,j = 0

In addition to aberration, there is another distortion of confocal images, when the refractive index of the medium does not match the one of the objective. Measured axial distances are longer or shorter than real ones, if the medium has a lower or higher refractive index, respectively, than the objective. As the microscope has a dry objective and is inverted, all vertical distances under the drop up to the drop surface are measured correctly. Inside the drop all distances appear shorter by a factor approximately equal to the ratio of refractive indices, i.e. 1.33. Before further analyis we also corrected vertical distances after normal11


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ization, as shown in Figure 4(c). Each location above the interface, represented by position lmax , resembling the droplet area is reset according to Eq. 3a & 3b, by means of stretching BG,Idx by the factor of nwater , yielding Fl×c .





BG×Idx = Fl,c

a1,1   a  2,1   .  ..   

al,1

a1,2 · · · a1,c  a2,2 · · · .. . . . .

  a2,c    ..  .    

(3a)

al,2 · · · al,c

with,

ai,j :=

   f    b lmax +(i−lmax )/nwater ,j              f

| i > lmax (3b) | i ≤ lmax

b i,j

The final image is free from artifacts caused by refractive index mismatch and scattering and shows the distribution of FS particles. The intensity of each pixel is proportional to FS particle concentration. To quantify the formation of an dense shell of FS particles at the drop surface, we analyzed the time series of corrected images. We extracted average fluorescence intensity as a function of depth z, where z is measured in orthogonal direction to the drop surface (Figure 4(d)). All points at given depth were averaged. Accordingly, rotation of the corresponding fluorescence image by the appropriate angle α, aligns the depth vector along the vertical direction. Therefore α was determined via a tangential fit along the droplet contour line obtained from the reflection image as shown in Figure 4(a). We extracted each individual slope, s, thereby obtaining the respective α-value using Eq. 4.

α = arctan (s)

(4)

Mathematically, the contour line defining z = 0 was extracted determining the position 12


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(lmax , cmax ) of the brightest pixel in the reflection channel along each vertical line, i.e., within each column vector. Then we extracted intensity profiles starting from each point in the contour line, orthogonal to the local tangential. Following the droplet’s interface contour BG,Idx line in incrementally constant steps k, typically 10 pixels, the matrix Fl,c , obtained

above (Eq. 3b), was rotated by the corresponding αk -value derived from step in Figure 4(a). Thereby, the rotation axis was set to the respective interfacial tangential point from which αk was determined. This resulted in the alignment of the tangential of the contour along the horizotal axis for each incremental location at the interfacial boundary, as shown in Figure 4(d). The values of the rotated matrix starting from the selected contour point give the local intensity profile with depth z (Figure 4(e)). Finally, all intensity profiles were averaged over all incremental steps. In principle it is possible to calculate the absolute FS concentration as a function of depth from the respective intensity profile. However, the exact initial concentration of FS particles near the contact line was unknown, as sedimentation occured within a few seconds. Note that for agglomerates of > 5 µm size, which are clearly visible in Figure 3, the estimated sedimentation time according to Stoke’s law for a travel distance equal to the typical initial droplet height of less than 2 mm, is less than ~1 min. As the amount of time between droplet deposition and start of measurement already exceeds that time limit, we conclude that this early sedimentation occurred already before the first image of the sequence. This is also proven by the image series in Figure 3. Therefore, in a final analysis step, we divided all curves by the one at t = 1 min to get the normalized average intensity as a function of depth I(z). Changes in I(z) are proportional to the spatially resolved change in concentration. Moreover this procedure eliminates fluctuations due to sedimentation at short times. The final result is shown in Figure 5 as function of time and depth z. Within a few minutes, we indeed observe a relative increase in intensity close to the interface until depths of about z ≈ 25 µm. In addition to this relative increase there is, of course, an overall 13


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Figure 4: Process chart for the evaluation of image sequence measurement channels (white: reflection, yellow-FS: FS particle fluorescence Rhodamine 6G, green-BG: background fluorescence Alexa-488) obtained from confocal microscopy. Top to bottom: (a) incremental tangential fit of the droplet contour line in the reflection image to obtain the angle α from the slope (Eq. 4), (b) division of FS particle fluorescence intensity by the background, (c) correction of the refractive index for the droplet area (Eq. 3a & 3b), (d) rotation of the obtained image from (c) by α with rotation axis located at the interfacial tangential point, (e) vertical walk from contour point and extraction of corresponding brightness values resembling the effective fluorescence intensity. All calculations were performed on 8-bit grayscale images.

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(a)

1.5

Normalized Intensity, I/I0

1.5

1.4

1.4

1.3 1.2

1.3

1.1

1.2

1 0.9

1.1 1 0

10

20

30

Distance / μm

(b) 1.5 1.4 1.3 1.2 1.1 1

Normalized Intensity, I/I0

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0

10

40

50

20

18 1416 1012 8 6 2 4 Drying Time / min 60 0 30

40

50

60

18 min

1.5 1.4 1.3 1.2 1.1 1

10 min

1.5 1.4 1.3 1.2 1.1 1

4 min

1.5 1.4 1.3 1.2 1.1 1

2 min

0

10

20

30

Distance / μm

40

50

60

Figure 5: (a) Measured normalized average intensity over vertical distance from the droplet contour towards the interior at different drying times for a droplet initial concentration of 3.5 % w/v FS and 25 mM NaCl. The intensity was normalized with respect to the first image of the time sequence to enhance visibility of the relative FS concentration increase. The color chart on the right represents the z-axis volume fraction. (b) Excerpt for 2, 4, 8, and 18 min, respectively from (a). A clear increase in fluorescence intensity up to a distance of 22 µm can be observed reaching about 1.5 times the initial values.

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increase of concentration everywhere in the drop that is reflected at the increased I(z) at large distances from the surface. We fitted the normalized average intensity curves with the following smooth step function:

I(z) = I0 +

∆I 1 + exp((z − z0 )/w)

(5)

where I0 is the intensity at the drop interior, ∆I intensity increase at the shell, z0 the shell thickness and w the shell boundary width (Figure 5(b)). These values are summarized as a function of time in Figure 6.

Figure 6: Variation of FS shell at the surface of the drop with time. Parameters of Eq. 5 extracted by fits of the normalized intensity I(z) in Figure 5. (a) Position refers to the step in intensity and corresponds to the thickness of the shell. (b) The intensity step is the difference in normalized intensity between the shell and the bulk and is proportional to the concentration difference. (c) The width of the shell boundary.

Already at short times of 3 − 4 min a shell with low density contrast compared to the rest of the drop forms. In course of evaporation this shell initially becomes thinner, but reaches a constant thickness of about 25 µm after 5 min. In contrast, ∆I increases with time, showing that the shell becomes increasingly dense compared to the bulk. The apparent decrease after about t = 18 min is an artifact caused by the saturation of intensity at the shell. Surprisingly, this shell has a rather sharp transitional region with a thickness of only 2 µm throughout the 16


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evaporation process. Our CLSM experiments show the shell formation that could not be observed directly in previous works. The high concentration of FS particles at the surface builds up due to evaporation at the droplet surface, and the slow diffusion of the FS particles in the shell. Assuming an average hydrodynamic radius of RH = 150 nm and viscosity in the shell equal to that of water η = 1 mPas the diffusion coefficient for FS aggregates is

D=

kB T 6πηRH

(6)

In ∆t = 20 min after drop deposition the shell formation is complete (Figure 6b). Within this time the mean displacement of FS particles along one dimension is

x2

D

E1/2

√ = 2D∆t =

s

kB T ∆t = 0.06 mm 3πηRH

(7)

This distance is much smaller than the decrease of drop radius ∆r = 0.25 mm (Figure S3) after ∆t. The slow diffusion that does not allow FS particles near the surface to return to the bulk explains the formation of the shell. Water diffusion through this shell is fast enough to follow this rate of retracting, thus no slowing down of evaporation is observed, in agreement to previous studies. 13 In fact, viscosity increases with FS particle concentration (Figure S2), thus diffusion of FS particles becomes even slower with time. Thus we conclude that diffusion is not sufficient to balance the increased concentration due to the evaporation through the shell. The enrichment of the shell is due to the inward motion of the drop-air interface. The shell has a nearly constant thickness, but increasing concentration difference from the rest of the drop. Thus, the contrast of stiffness between the shell and the interior increases, according to rheological measurements (Fig. S2). In earlier experiments we already showed that droplet deformation occurred at a FS surface excess concentration of csurf,ex = 8.1 ± 1 µg/mm2 . 12 Here we measured the shell thickness z0 , so the FS surface excess concentration

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translates with good approximation into an excess volume fraction according to Eq. 8, where ρF S = 2.2 g/ml.

θv (F S) =

csurf,ex (F S) ≈ 0.17 z0 ρF S

(8)

As a consequence, the viscosity within the interfacial region is higher than what we assumed above, resembling a robust FS mantle that bends like a solid with further droplet shrinking, hence promoting droplet deformation. Viscosity measurements as a function of the shear rate (Figure S2) indeed show a strong increase of viscosity with volume fraction, whereas salt addition has only a small influence on viscosity. However, all the samples exhibit pronounced shear thinning. These findings support the proposed mechanism originally reported on the formation of anisometric supraparticles via drying aqueous FS suspension droplets at high ionic strength. 10,11 This proposed mechanism is now supported by CLSM experiments that clearly show constantly thick, but increasingly stiff, surface layer of 22 µm of FS forming quickly. The increase of density occurs nearly linearly in time, until the shell becomes so stiff that it is no longer able to follow the contraction due to the continuing evaporation of the droplets. At this point anisometric deformation of the droplets sets in, which controls the finally formed anisometric supraparticles.

Conclusion We developed a setup and a comprehensive evaluation routine for the investigation of sessile drops of colloidal dispersions via Confocal Laser Scanning Microscopy. In order to quantify the shell formation mechanism associated with the formation of anisometric FS supraparticles, 10–12 we investigated evaporating aqueous suspension droplets deposited on a superhydrophobic surface. We showed that fluorescent labeling of both solid and liquid phases in combination with careful image processing can eliminate the artifacts caused by optical aberrations in systems with non-uniform refractive index. Thus, information similar 18


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to that of X-ray tomography is obtained. As a result, we could show that fast shrinking of the drop surface due to evaporation and slow diffusion lead to the formation of a thin, dense shell of particles on the surface. To the best of our knowledge, this process was observed directly for the first time and elucidates the formation of the anisometric supraparticles. The understanding of this process can facilitate the synthesis of multicomponent and complex supraparticles via an evaporation induced self-assembly.

Acknowledgement We gratefully thank the workshop team for fine mechanics from Technische Universität Berlin for their support for building the customized chamber for the experiments. These thanks include Rolf Kuhnert for providing the technical drawing and Dr. René Straßnick for the electronics setting up the heating system.

Supporting Information Available Movie S1: Video of evaporating drop. Figure S1: UV-Vis spectra of Alexa488 and Rhodamine 6G in FS dispersions. Figure S2: Rheological measurements of FS dispersions at different volume fractions and NaCl ionic strength. Figure S3: evaporation rate at varying relative humidity. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Understanding the Formation of Anisometric ... - ACS Publications

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