Bull. Mater. Sci.
(2020) 43:173
https://doi.org/10.1007/s12034-020-02096-5© Indian Academy of Sciences

Evaporation-driven self-assembly in the mixtures of microand nanoparticles
RAVI KUMAR PUJALA1 , DEVIKA VENKUZHY SUDHAKARAN2and SURAJIT DHARA2,∗

1 Soft and Active Matter Group, Department of Physics, Indian Institute of Science Education and Research (IISER),Tirupati, Andhra Pradesh 517507, India
2 School of Physics, University of Hyderabad, Hyderabad 500046, India∗ Author for correspondence (sdsp@uohyd.ernet.in)MS received 17 August 2019; accepted 24 November 2019Abstract. We report experimental studies on the self-assembly of silica microspheres and Laponite nanoplatelets (NPs)in evaporating sessile droplets and in thin films, respectively. A ring-like stain of the silica microspheres with positional
order is observed after the evaporation of sessile droplets due to the coffee-ring effect. This effect is suppressed in the binary
mixtures of silica microspheres and Laponite NPs. A depletion zone has been observed in the mixtures during the sessile
droplet evaporation, the width of which can be tuned by varying the compositions. We demonstrate a simple method for
preparing core–shell particles by evaporating thin films of binary mixtures in which the Laponite NPs self-assemble to forma crystalline shell on the amorphous silica microspheres. We present a possible orientation of the Laponite NPs in the shell.
Keywords.

Self-assembly; micro and nanoparticles; depletion zone; evaporation; core–shell particles.

1. Introduction
The self-assembly of micro- and nano-sized objects into larger
structures is a subject of immense interest in the field ofmaterial science [1–3]. In case of anisotropic nanoparticles
such as the nanoplatelets (NPs), the self-assembly and phasebehaviour are very different than that of the spherical nanoparticles [4–8]. The self-assembly of colloidal particles, driven
by evaporation, exhibits several complex phenomena. One
such intricate phenomenon is the coffee-ring effect in which apattern of dispersed particles is left out by the droplets when
they are evaporated from a solid surface [9–12]. For a given
suspension, the wetting properties of the substrate dictate thepattern and the final structure of the deposits.
The evaporation of sessile droplets and films have beenstudied due to its relevance in industrial applications suchas printing, painting, coatings and films [13–17]. Suspended
particles in the droplets are transported towards the periphery, while drying, due to the radial capillary flow, the so-called
Marangoni flow; as a consequence stains are formed which
are undesirable in many applications. There have been various efforts to suppress the stain formation. For example
Shi et al [18] used hydrophobic fumed silica in an evaporating ethanol/water droplet that suppresses the capillary
flow and leads to a uniform deposits. Talbot et al [19] used
Laponite NPs to control the radial flow inside the picolitre
droplets to produce uniform deposit. They argued that thiseffect is due to the sol–gel transition of Laponite during
the evaporation. Liu et al [20] prepared supraparticles using
0123456789().: V,-volbi-disperse colloidal aqueous suspensions, where the smaller
colloids mostly self-assemble into ordered structures at theperiphery. Recently, we reported artificial nacre-like structures produced by evaporation of Laponite NP suspensions[21]. So far, drying experiments are mostly focused on spherical nano or microparticles in different suspension geometries.
Such studies in the binary mixture consisting of microspheres
and NPs are largely unexplored. In this paper, we present somepreliminary and new results on drying experiments, of sessile
droplets and thin films of binary suspensions consisting ofsilica microspheres and Laponite NPs. Our results include
a novel method for preparing core–shell particles where theshell is a crystalline film on the amorphous silica core.

2. Methods and materials
Laponite is a plate-like charged silicate of average diameter 25 nm and thickness 1 nm [5]. They are available in thepowder form which is used in this study. They were used without further purification. The powder is dissolved in a desired
amount in milliQ water and mixed thoroughly by using magnetic stirrer for 2 h, followed by sonication for about 10 minfor breaking any clusters present in the dispersion [22]. The
binary mixtures of Laponite with monodispersed silica microspheres of diameters ranging from 1.2 to 8 µm, are preparedby mixing the stock solutions of individual suspension. Concentration of the microspheres varied from 0.1 to 2 wt% and173

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the platelet concentration varied from 0.1 to 3 wt%. Zeta
potentials of the Laponite and microspheres are −40 and−75 mV, respectively. Sessile droplets or thin films of the(2020) 43:173

mixture are prepared on hydrophilic glass plates. We used
10 µl for the sessile droplet and 50 µl for the thin film
experiments. The droplets or films are dried at an ambient

Figure 1. Schematic overview of suspension geometries for evaporation. Evaporation of (a) sessile dropletand (b) thin film (5 × 5 mm2 ). Silica microparticles (sphere) and Laponite NPs (platelet) are uniformly
suspended in milliQ water. Arrows indicate the direction of evaporation.

Figure 2. Deposition patterns obtained from a sessile dried droplet. The droplet is preparedfrom the solution state in which 0.5 wt% of silica microsphere (diameter 1.2 µm) is dispersed.
(a) Scanning electron microscope (SEM) and (b) confocal images of ring-shaped stain of
the sessile droplet left on the substrate after evaporation. (c, d) A close-up view of the stains.
Square and hexagonal ordering are indicated in yellow and white, respectively.

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temperature of 25°C and relative humidity of 50%. In caseof film drying, the glass plates are partially covered toprevent contamination due to airborne debris and to slow
down the evaporation. The dried samples are studied using
a high-resolution FESEM (Carl Zeiss Ultra 55) and confocal microscope (Leica SP8) with an oil immersion objective.
Optical and birefringence imaging is done using a polarizing
optical microscope (Olympus BX51) and a first-order retardation plate, also known as λ-plate (retardation of 530 nm).
3. Results and discussion
In this study, we used two types of suspension geometries
namely, the sessile droplets and the thin films in a confinedarea (5 × 5 mm2 ). Figure 1 shows the schematic overview
of the two suspension geometries. Evaporation in sessile

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droplets takes place in all directions, whereas in thin films
it occurs in the vertical direction.

3.1 Ring-shaped colloidal stains from sessile droplets
Our study begins with the investigation on sessile dropletsin which spherical particles of diameter 1.2 µm are dispersedin milliQ water. When the water from the droplet is completely evaporated, the particles are deposited on the substrate.
Figure 2a and b shows scanning electron microscope (SEM)
and confocal images of deposition near the edge. The deposition is not uniform throughout the area. A close-up view
by SEM (figure 2c) and confocal microscope (figure 2d)shows that the density of the deposited particles is larger ina ring-shaped stain at the edge due to the so-called coffeering effect. A structural change from ordered to disordered

Figure 3. (a, c, d) Scanning electron microscopy and (b) confocal images of a dried droplet consistingof silica microspheres (diameter 1.2 µm) and Laponite NPs. Concentration of microspheres is 1 wt% andLaponite concentration is 1 wt% in (a, b) and 0.5 wt% in (c, d). (c) An annular-ring-like depletion zone(~30 µm) is observed near the periphery of the stain. (d) A close-up view of the depletion zone.

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Figure 4. Core–shell microparticles prepared by evaporating thin films. (a) Light microscope image of silica particles(diameter 8 µm) after drying. (b) FESEM image of the core–shell silica particles (diameter 1.2 µm) with shell thickness~200 nm. The inset shows microparticles with broken shell. (c) Light microscope image taken between a crossed
polarizer (P) and analyser (A). (d) Image after inserting a λ-plate, fast axis oriented at 45° with respect to the polarizer.
(e) Refractive index ellipsoids corresponding to blue and yellow regions. Extraordinary (n e ) and ordinary (n o ) refractive
indices are labelled. (f) Schematic orientation of the Laponite NPs in the shell.

Figure 5. Effect of coating thickness on the silica microspheres. (a, b) SEM image of a few microspheresshowing rough shell surface after third coating. (c) Crack formation on the shell. (d) A close-up view of acrack showing a nacre-like structure of the shell.

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state is observed as we move from the edge of the ring to thecentre. In particular, closer to the edge of the stain a hexagonal order is found, followed by square and again hexagonalorder, respectively. The positional order is destroyed as wemove towards the centre of the ring. This kind of structuraltransition was also observed by Marín et al [23]. They showedthat this transition originates from a temporal singularity of
the flow velocity inside the evaporating droplets. When thedeposition speed is low, that happens at the beginning of theevaporation, the particles have enough time to get arranged
due to the Brownian motion, whereas towards the end, thespeed of the particles is high enough that they get jammed
into a disordered state.
3.2 Suppression of coffee-ring effect and formation ofdepletion zone
As a next step, we have studied the effect of Laponite NPs on
the coffee ring. It is observed that the addition of Laponite NPs
suppresses the formation of a coffee ring. For concentrations> 1 wt% of both silica microparticles and Laponite, it forms
nearly a uniform deposition of the silica particles as shown infigure 3a and b. At lower concentrations (< 1 wt%), formation of an annular ring-like region near the edge is observed
where the silica microspheres are mostly depleted (figure 3c).
A close-up view of the depletion zone is shown in figure 3d.
The width of the depletion zone grows with time and reaches amaximum when the evaporation is completed. Similar depletion zones near the periphery have been observed in systemsof colloidal suspension containing surfactant molecules andin mixtures of polystyrene and poly(N-isopropylacrylamide)(pNIPAM) [20,24–27]. The formation of depletion zone isdue to the inhomogeneous adsorption of the colloidal particles at the water−vapour interface which sets up the solutal
Marangoni flow along the interface during the evaporation.
3.3 Formation of core–shell colloidal particles
In what follows, we present a method for the preparation ofcore–shell particles in which the Laponite NPs are depositeduniformly, making a thin shell on the surface of the silica
microparticles. The core–shell particles are prepared in twoways. In the first approach, the micron-sized silica particles
are spread and dried on the hydrophilic glass substrates andthen the suspension of Laponite NPs is poured on to it anddried. In the second approach, the silica microspheres andLaponite NPs are mixed thoroughly and then drop casted on
the hydrophilic substrates. In both cases the evaporation takes
place vertically as shown in figure 1b, consequently there is
no formation of the coffee ring. In the latter approach, themicrospheres sediment under the gravity and then the NPs
self-assemble on the surface of the silica microparticles tominimize the surface energy. The second approach is foundto be more efficient as the coating is more uniform than thefirst one (figure 4a). Figure 4b shows the SEM image of a few
core–shell particles of diameter 8 µm. The thickness of thePage 5 of 6

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shell is uniform (see broken shell in inset of figure 4b) andthe surface is highly smooth. The shell thickness depends onthe concentration of the NPs and for 1 wt% it is about 200 nm
on the silica particles of diameter 1.2 µm.
To explore the microstructure and properties of the shell,
we looked at the core–shell particles between two crossed
polarizers under the light microscope. Figure 4c shows theconcentric and colourful interference rings surrounding thespherical microparticles with four crossed extinction directions (better seen in the inset of figure 4c). This clearly
indicates that the shell is optically birefringent, possessing
ordinary and extraordinary refractive indices. It means theshell has crystalline order. To get a qualitative idea about
the retardation, we inserted a first-order waveplate (λ-plate
retardation of 530 nm) with fast axis oriented at 45◦ withrespect to the polarizer. According to the Michel−Levy chart,different colours represent different magnitudes of retardation
across the sample [27]. For example, the background magenta
colour indicates the region of minimum or zero retardation.
The retardation of second-order blue is higher than the firstorder yellow region. These two colours exchange places when
the fast axis is rotated by 90◦ . This means that the long axis of
the ellipsoid of index of the shell is parallel to the surface ofthe microsphere, as shown schematically in figure 4e. Basedon the orientation of index of ellipsoid it is easy to comprehend that during the drying process, the Laponite NPs aredeposited on to the surface of the microspheres as shown infigure 4f. In the shell, the NPs possess an orientational orderwith an average direction of orientation of the short axes of theNPs being perpendicular to the surface of the microspheres.
The thickness of the shell can be enhanced by adding multiple
coatings. For this purpose, the successive coatings are madeeach time after drying the sample completely. However, after
second coating the deposition is not uniform as a result ofwhich the surface becomes rough (figure 5a and b). In addition, cracks are formed as shown in figure 5c. A close-up viewof the cracks reveals an artificial nacre-like structure of thefilm (figure 5d), as reported previously [21].

4. Conclusion
After the evaporation of sessile droplets, the silica microspheres form a narrow ring-like zone where the particles areself-assembled due to the coffee-ring effect. The addition of
Laponite NPs suppresses the coffee-ring effect and forms a
uniform deposit at higher concentrations and a narrow depletion zone at lower concentrations. A valuable bonus of ourstudies is a simple method for preparing core–shell particles
in which the shell is crystalline. The Laponite NPs are spontaneously self-assembled, forming a shell upon drying andexhibit crystalline order. We have proposed an orientation of
the Laponite NPs in the shell. Such core–shell particles could
be useful as markers or tracer particles for various applications.

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Acknowledgements
RKP acknowledges the support from the Department of
Science and Technology, India for an INSPIRE FacultyAward Grant (DST/INSPIRE/04/2016/002370). SD gratefully acknowledges the support from the DST (DST/SJF/PSA02/2014-2015) and SERB. DVS acknowledges DST for
INSPIRE fellowship.

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