Morphological deformation during evaporation ...

Morphological deformation during evaporation induced assembly of mixed colloidal suspension D. Sen1*, J. S. Melo2, J. Bahadur1, S. Mazumder1, S. Bhattacharya3 and S.F. D’ Souza2 1

Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India Nuclear Agriculture and Biotechnology Division, Bhabha Atomic Research Centre, Mumbai-400085, India 3 Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai-400085, India 2

Abstract. Sphere to deformed doughnut type transformation of colloidal droplets during evaporation induced assembly of colloidal silica and E. coli was observed. Distortion modulations get amplified with increase in volume fraction of anisotropic soft colloidal component. Reduction in elastic constants of formed shell, at the boundary of a drying droplet, and the anisotropic nature of bacterial component facilitate the deformation process. The charge modification of E. coli surface by Poly cationic Polytheleneimine ceases the morphological transformation and results spherical assembled grains. Hierarchical structures of these assembled colloidal grains have been probed using electron microscopy and small- angle neutron scattering techniques. Keywords: Morphological transformation, Mixed colloids, Spray drying, Small-angle Scattering. PACS: 68.03.Fg, 68.08.Bc, 68.37.Hk, 61.05.fg, 61.05.fg

morphological transformation occurs.1,6 It has been proposed that morphology of assembled grains may be tuned1-3 by drying parameters, constituent particle morphology, inter-particle correlation and also by the physicochemical properties of the drying medium. Although such transformation has been observed experimentally for drying of droplets containing only single type of colloidal suspension1,6, the understanding of such transformation in droplets that contain mixed colloids is more recent.7 It is noteworthy to mention here that if initial droplets contain mixed colloids of both organic and inorganic components, then porous grains can be synthesized by removal of the organic template species from assembled grains. In this paper it has been shown that the morphological transformation during drying of droplets with mixed colloids can be altered by introducing soft bacterial component. Further such morphological transformation may be controlled by the surface modification of the soft component. Cylindrical macro porous grains could be achieved by such evaporation induced assembly process.

INTRODUCTION Evaporation induced self assembly (EISA) of nanopartcles has found an important place in science and technology.1-9 The reason for this is mainly twofold: (i) To understand the kinetics of the assembly process for various physicochemical and thermodynamical conditions, (ii) to synthesize various novel nanocomposites, colloidal crystals, template porous materials etc. Morphological transition during EISA remains an open question even today. It is observed that the assembled grains of colloidal particles, prepared by spray drying, a novel way to realize EISA, often lead to various non-spherical morphologies1,6,7 like doughnut, mushroom etc. In the drying process of a micrometric droplet of colloidal dispersion, evaporation drives the shrinkage of the droplets and the constituent colloidal particles get assembled because of the capillary forces that appear due to the existence of wetting liquid between two colloidal particles. It has been demonstrated5 that if the rate of drying is slow enough, the droplet shrinks in an isotropic manner and the dried grains remains spherical in shape apart from the overall shrinkage in size. However, if the speed of drying is fast enough, the deformation forces overcome the electrostatic forces that stabilize the particles and the

EXPERIMENTAL Initial silica colloidal dispersion [40% silica (Visa Chemicals, Mumbai, India)] was diluted in pure water

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for further applications. E. coli BL21DE3 bacteria were used in this study as the soft component of the mixed colloidal suspension. A mixed colloidal dispersion of fixed 2% silica and varying E. coli (0%, 2%, 4% and 6% in weight) was prepared using pure water (Milli-Q, Millipore) and was used for EISA using spray drying.

SEM micrographs were obtained using VEGA, TeScan instrument. SEM micrographs for E0, E2, E6 and Emodified samples are shown in Figure 1. SANS profile for the samples are depicted in the Figure 2.

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Spray drying of the mixed colloidal dispersions was carried out using a spray dryer LU222 (LABULTIMA, India). Removal of the imprinted materials was done by incineration in muffle furnace at 300 oC for 10 hrs. The powders, with 0, 2, 4 and 6% of E. coli are labeled as E0, E2, E4 and E6, respectively. From the measured weight of the collected powder, before and after calcination, it was observed that ~70 % of template was removed by calcination. To modify the surface of E. coli, poly-cationic Polytheleneimine was coated on the E. coli surface. Spray drying was also carried out with silica and modified (Emodified) E. coli (6%) suspension.

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FIGURE 2. SANS data of assembled E. coli template macro porous silica grains.

The dried grains were probed using scanning electron microscopy (SEM) and small-angle neutron scattering (SANS). SANS experiments on spray dried grains were performed using two instruments: (i) a double crystal based SANS facility9 and (ii) a 5 meter long slit geometry instrument10 at the Guide Tube Laboratory of Dhruva reactor at Trombay, Mumbai, India, in order to access a wide range of wave vector transfer. (a)

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RESULTS AND DISCUSSIONS It is obvious from the electron micrographs that the deformation modulation of the assembled grains enhances with increase in fraction of unmodified E. coli and the grains are buckled doughnut type. However, when the bacteria surface is modified with Polytheleneimine, the deformation gets hindered and spherical grains are realized without any buckling effect. It has been conjectured that the deformation during the spray drying process may occur due to various factors like thermo-dynamical instabilities, hydro-dynamical instabilities or particle-particle interactions in a drying droplet. The quantitative measure of the strength of drying is represented by Peclet number (Pe=R2/Dτdry, where R is the radius of the droplets, D is the diffusion coefficient of the colloidal particles in the droplet and τdry is the drying time). If Pe 1, the drying is fast enough and there is a possibility of formation of hollow grains during drying depending on the other physical parameters. The grains possess density fluctuations at three different levels, (i) individual colloids, (ii) individual bacteria/bacterial template pores and (iii) overall assembled grains. This three level hierarchy is verified from the three distinct zones of the scattering

FIGURE 1. SEM micrograph of assembled grains of (a) E0 (b) E2 (c) E6 and (d) Emodified.


profiles (Figure 2). It is seen from the profiles in the intermediate and higher wave vector transfer (q) range that the correlation of the macro-pores gets modified with modification of bacterial surface. From zeta potential measurements it was found that the silica and the unmodified E. coli possess negative surface charge. However, the surface charge of the bacterial surface gradually becomes positive with increase in the amount of Polytheleneimine. During loss of water, evaporation brings the particles onto the air-water interface, where their concentration gets enhanced. Eventually this gives rise to the formation of a viscoelastic shell of densely packed particles at its surface. The thickness and nature of the shell are important parameters and determine the required stress to drive the deformation. Initially, such shell is produced and gets thicker as droplet shrinks. At this stage the main mechanism of water transfer is governed by the pressure gradient resulting from the capillary pressure. However, at certain instant, the capillary forces driving the deformation of the shell overcome the electrostatic forces stabilizing the colloidal particles. Then the shell becomes elastic and undergoes a sol-gel type transition and buckles.1 Once buckling occurs, it becomes energetically favourable to force plastic deformation of the shell through the tip of the invagination. This gives rise to doughnut like particles with a central hole.

ACKNOWLEDGMENTS Authors are thankful to Dr. V.K. Aswal of SSPD, BARC, Mumbai, India for his help in scattering measurements with slit geometry SANS instrument. Authors are grateful to M/S. Visa Chemicals, Mumbai, India for providing silica colloids. DS would like to thank Dr. Antoine Thill and Dr. Olivier Spalla of LIONS, SCM, DRECAM, CEA Saclay, France for many fruitful discussions on evaporation driven self assembly during his visit at CEA Saclay, France.

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When the charges on silica surface and the surface of the bacterial component are of same sign (ve), i.e, when unmodified E. coli is used, the surface of E. coli can be exposed directly at the air water interface. It is worthy to mention that E. coli is much softer (Longitudinal Young’s modulus ~25 Mpa and circumferential Young’s modulus ~75 MPa density ~1.1 gm/cm3)11 than silica (~75000 MPa, density ~2.5 gm/cm3). This enhances the buckling probability of the shell formed at the air water interface and the grains become deformed doughnuts. The deformation increases with increase in the volume fraction of the unmodified E. coli.

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When the E. coli surface is modified with 2% Polytheleneimine, charge reversal occurs at the E. coli. surface. This leads to a situation where the surface of the +ve E. coli gets partially covered by –ve silica. Hence during drying of the micrometric droplets, the soft surface of E. coli does not get directly exposed to the air-water interface. In turn, this hinders the buckling and the spherical grains are realized. The column of these bacterial templated macroporous powder grains has been found12-13 to be a potential candidate for filtration of E. coli bacteria from water.

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