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Suppression of Coffee-Ring Effect: Evaporation Driven Disorder to Order Transition in Colloidal Droplets Shyamashis Das, Atreya Dey, Govardhan Reddy, and D. D. Sarma J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01814 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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The Journal of Physical Chemistry Letters

Suppression of Coffee-Ring Effect: Evaporation Driven Disorder to Order Transition in Colloidal Droplets Shyamashis Das, Atreya Dey, Govardhan Reddy and D. D. Sarma* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru 560012, India Corresponding Author *[email protected]

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ABSTRACT: The formation of a ring-like deposit at the periphery of a drying colloidal droplet is a vexing problem in many applications. We show a complete suppression of such deposits when a droplet of aqueous colloidal suspension, deposited on a glass substrate coated with a thin layer of silicone oil, is evaporated. This coating prevents the periphery of the aqueous droplet from getting pinned to the substrate and helps in suppressing the ring formation. It also decreases the surface area of the droplet, thereby decreasing the evaporation rate. These two factors together, driving the colloidal particles slowly to the center of the droplet, contribute to form an ordered crystallite at the end of the evaporation process. Brownian dynamics simulations performed to study ordering in the aggregate show that the spherical colloidal particles form face centered cubic structures. Experiments and simulations show that slow rates of droplet evaporation and smaller sized colloidal particles further lead to high quality ordered colloidal crystallites.

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Ordered crystallites of colloidal particles are used as building blocks in fabrication of optoelectronic devices,1 DNA/RNA microarrays,2-4 biosensors5 and photonic crystals.6-8 One of the ways of growing colloidal crystals is by controlled evaporation of a colloidal suspension deposited as a droplet on a substrate. Crystals of sizes nearly a micrometer in length scale have been realized in many cases, suggesting exciting possibilities of achieving highly ordered mesoscopic structures by simple evaporation.9-11 A major hindrance in obtaining ordered crystallites by evaporation is the accumulation of colloidal particles at the periphery of the droplet forming a ring, which prevents ordering from taking place on a large scale.12-13 This phenomenon of ring like deposition of colloidal particles is known as coffee-ring effect. Deegan et al.14-15 attributes the coffee-ring formation to the capillary flow of particles initiated by a diverging evaporating flux from the center to the periphery of a pinned droplet on a solid surface. Repeated pinning-depinning process, leading to successive coffee-ring like structures were also shown to form.16-17 There are reports of extensive efforts to mitigate the coffee-ring formation on substrates when droplets of colloidal suspensions are evaporated.18-27 Suppression of ring-deposition is highly desired particularly in applications like ink-jet printing, DNA microstructure assembly etc.28-29 where uniform deposition of particles is required. However, the ubiquitous nature of ringlike formation causes a major problem in all these applications. Studies show that coffee-ring can be suppressed by using specific particle shapes (e.g. non-spherical)19 or sizes,20 patterning the substrate to make it superhydrophobic,21-23 application of external electric/magnetic fields,24-25 making use of surface tension gradient driven recirculatory Marangoni flow of solvent,18, 26 tuning the concentration of colloidal suspension, medium of suspension, and evaporation rates.27 However, these methods are complicated to adopt for practical purposes. 3 ACS Paragon Plus Environment


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In the present work, we describe a simple technique of coating a flat glass substrate with silicone oil, which completely suppresses the coffee-ring formation when a droplet of aqueous colloidal suspension deposited on this substrate is evaporated. To the best of our knowledge such a complete suppression of coffee-ring effect with spherical colloidal particles on a smooth substrate like glass has not been reported before. The suppression of the coffee-ring leads to the formation of a compact aggregate at the center of the droplet. Furthermore, the structure of the aggregate formed does not depend on the morphology of the substrate unlike what is observed in the case of superhydrophobic surfaces.21-22 We have characterized the crystallinity in the final aggregate structure, and identified experimental parameters, which can be easily tuned that affect the degree of ordering in the aggregate. Aqueous colloidal droplets with the same initial concentration (0.2% (w/v)) of silica microspheres (0.57 m) and volume (~2 l) are left for evaporation on bare glass substrates and on silicone oil coated glass substrates (see Supporting Information (SI) for details of experiment). A distinct coffee-ring forms when the droplet is dried on a bare glass substrate (Figure 1a). However, on the glass substrate coated with silicone oil, coffee-ring formation is completely suppressed and the particles aggregate at the center of droplet (Figure 1b). The aggregate formed at the center of the droplet is further ordered when the droplet is dried at a slower rate of evaporation by placing it inside a highly humid closed container (inset of Figure 1b). To understand the differences in the drying processes on the two types of substrates, we recorded the receding contact line during the course of droplet evaporation using video microscopy. A few snapshots of the drying droplet are shown in Figure S1. Videos of the drying process on both bare glass substrate


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Figure 1. (a) A droplet of silica particles form a ring like deposition, when it is evaporated on a bare glass substrate and (b) When a droplet from the same suspension is deposited and evaporated on a glass substrate coated with silicone oil, the ring formation is suppressed, and the particles accumulate at the center. Insets show magnified images of the disordered and ordered crystallites, respectively. (c) The droplet basal area on a glass substrate with (black circles) and without (red squares) silicone oil coating plotted as a function of the drying time. The black and red arrows indicate the fraction of time, the respective droplets spend in pinned state. (d) The variation of droplet contact angle (red squares) and contact radius (blue circles) with drying time when deposited on a glass substrate with silicone oil coating. and on silicone oil coated glass substrate can be found in the Movie S1 and S2, respectively. To quantify the time spent by a droplet in the pinned and depinned state, we plotted the droplet basal area as a function of the drying time (Figure 1c). In the case of the bare glass substrate, the droplet remains pinned till 80% of the total drying time. Whereas in the case of silicone oil coated substrate total pinning time is only 25%, thus providing a window of 75% of the drying time in the depinned state, which is critical to suppress the coffee-ring formation. Clearly the coffee-ring deposition is observed when the contact line remains pinned to the substrate for a large part of the droplet lifetime (Figure 1a). The pinned contact line maintains the diverging evaporative flux from the center to the edge resulting in a capillary flow that makes the particles move towards the periphery



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of the droplet. As reported earlier,30 velocity of the particles diverges towards the end of droplet lifetime. This prevents ordering of the particles from taking place at the inner periphery of the coffee ring where particles arrive during the later stages of drying with high velocity (inset of Figure 1a). The figure reveals a decreased local ordering of the particles at the inner periphery compared to the outer periphery of the coffee ring. We see small domains of close-packing of the particles near the outer periphery, which becomes disordered in the region of the inner part of the periphery, connected via intermediate region where non-close-packed structures are observed over short domains (inset of Figure 1a). This order to disorder transition on going from the outer periphery to inner periphery of the coffee-ring is in agreement with the results of Marin et al.30 When a droplet deposited on the glass substrate coated with silicone oil is evaporated, the contact line does not get pinned (Figure 1c) and the droplet contracts in size as evaporation progresses, forcing the particles to accumulate at the center (Figure 1b) and form an ordered crystallite (inset of Figure 1b). The extent of ordering varies from the center to the periphery of the crystallite surface. The region near the periphery is less ordered as those particles experience large force by the moving solvent front as seen in Movie S2 and also they have less time to settle into an ordered state before the solvent evaporates. The particles at the center remain relatively unaffected throughout the droplet lifetime and hence get more time to settle down as ordered crystallites. In the depinned state, the time evolution of the droplet contact angle and its radius on the substrate shows that there are three stages through which the droplet passes during the evaporation process (Figure 1d). In the first stage, called the constant-contact-radius (CCR) mode,31-32 the droplet remains pinned and only the contact angle decreases. In the second stage called the constant-contact-angle (CCA) stage, depinning of the contact line takes place and only contact radius decreases. In the final stage known as the mixed mode of drying, both the contact angle and 6 ACS Paragon Plus Environment

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radius decreases uniformly. The uniform decrease of contact angle and radius during the last stage helps in forming an ordered crystallite on the substrate. To identify the effect of evaporation rate on the ordering inside the final aggregate, we varied the drying time of the droplet. Figure 2a,b show two SEM images obtained using two vastly different drying times, namely 36 hours and 10 minutes, respectively, illustrating typical cases of ordered and disordered aggregates obtained at the end of the drying process. In both the cases, silica particles of size 0.57 m and initial concentration of 0.2% (w/v) are used. Ordered crystallites of silica particles are obtained, when droplets are dried with a slow evaporation rate

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Figure 2. SEM images of the aggregates obtained when suspensions of silica microspheres of size 0.57 m are dried in (a) 36 hrs and (b) 10 min. Structure factor of the SEM images in (a) and (b) are shown in (c) and (d), respectively. Distribution of the Steinhardt bond order parameters computed for the aggregates obtained from Brownian dynamics simulations by decreasing the basal area of the droplet at the rate (e) 330 m2/s and (f) 1332 m2/s. The number of LJ particles in the simulation droplet is N = 2500, the diameter of the LJ particles is  = 1.0 m, and the attractive well-depth of the LJ potential is εLJ = 2.5 kBT, where kB is the Boltzmann constant and T is the temperature. 7 ACS Paragon Plus Environment


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inside a humid container (Figure 2a). The same suspension forms a disordered aggregate when the droplet is evaporated fast inside an oven kept at 60º C (Figure 2b). Structure factor33 𝑆(𝑞⃗) computed using Fourier transformation of the image (see SI for details of structure factor computation) in Figure 2a shows sharp distinct spots with hexagonal features, quantifying a high degree of order over macroscopic length scale in the aggregate (Figure 2c). In contrast, the diffuse spot in the 𝑆(𝑞⃗) map in Figure 2d indicates co-existence of randomly oriented very small domains of ordering and disordered patches in the structure shown in Figure 2b. We have expanded a small part of the ordered region in Figure 2a for clarity. The area of ordered crystallite is much larger as shown in Figure S2a. We further note that the extent of ordering did not depend on the concentration of silica particles, as demonstrated in Figure S3. To gain insight into the structure of the aggregate and factors that affect the ordering in the aggregate, we performed Brownian dynamics simulations using Lennard-Jones particles mimicking the droplet evaporation (see SI for model details and simulation methods). A simulation snapshot of the final aggregate shows three kinds of particles: crystalline, disordered and solvated particles, which are shown in red, dark blue and light blue colors, respectively (Figure S4). To identify the structure of the crystalline particles we calculated the Steinhardt bond order parameters,34 q4, and q6 (see SI for details), for each of the crystalline particles and plotted a distribution of these values to identify the crystal structure in the aggregate. The depinning of the contact line, and the decrease of droplet size with evaporation in the experiments is mimicked in the simulation by decreasing the basal area of the droplet at a constant rate, ν. This corresponds to the CCA stage of droplet drying, where the basal area of the droplet decreases linearly. Figure 2e,f show distributions of the parameters q4 and q6 for the aggregate obtained using two different values of ν. For a perfectly crystalline face-centered cubic (FCC) structure the values of the parameters 8 ACS Paragon Plus Environment

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q4 and q6 are 0.19 and 0.57, respectively and for a hexagonal close pack (HCP) structure, the values are 0.09 and 0.48, respectively.34 The peaks in the distribution of q4 and q6 values indicate that both the FCC and HCP structures are present in the aggregate obtained from simulations (Figure 2e,f). However, the HCP peaks are smaller in intensity compared to the FCC peaks indicating that FCC structure is more favored over HCP. These simulations also show that increasing the value of ν, reduces ordering in the final aggregate as broadening in the q4 and q6 peaks is observed and extraneous peaks also appear indicating increased disorder in the aggregate (Figure 2f). Increasing the evaporation rate reduces the time available for the particles to explore the configurational space and settle down into their ordered free energy minima as they are pushed inwards faster. Overall this suggests that a slow evaporation rate of the droplet favors the formation of more ordered structures in the final aggregate. Although both the FCC and HCP like regions are present in the aggregates, we believe that the aggregates in our experiments probably have dominantly the FCC structure for two reasons: (i) the silica particles in the late stages of droplet evaporation cannot be approximated as Brownian particles as most of the solvent has evaporated. So, when we simulate the late stages of droplet evaporation in a microcanonical ensemble, the distribution of q4 and q6 values for the aggregate obtained are dominated by the FCC peaks (Figure S5a). (ii) The Brownian dynamics simulations with a larger system size also show that the FCC peaks dominate over other peaks in the distribution (Figure S5b). We have also investigated the effect of particle size on the extent of ordering in the final aggregates. Figure 3a-d show the final aggregates of particles with sizes 0.57, 0.9, 1.49 and 2.34 m, respectively formed by the evaporation of droplets deposited on silicone oil coated glass substrates. In all these aggregates, colloidal suspensions with the same initial concentration (0.2% 9 ACS Paragon Plus Environment


(w/v)) are used and the droplets are subjected to a long drying time (~36 hour) inside a humid chamber to promote ordering. We obtained similar results from independent experiments for same sized particles as shown in Figure S6. It is evident that the extent of ordering decreases with the increase in particle size spanning the entire range from nearly perfectly ordered assembly for the smallest sized (0.57 m) particles to a nearly completely disordered assembly for the largest size particles (2.34 m) (Figure 3a-d). We quantified the extent of hexagonal ordering on the surface of the aggregates shown in Figure 3a- d using the bond-orientational order parameter (BOP),33 m6 (see SI for details). The m6 value, which is ~0.8 for particles with the smaller diameter (0.57 m) decreases, with the increase in particle size reaching a value ~0.17 for particles with diameter 2.34 m (Figure 3e). This supports the conclusion that hexagonal ordering on the surface of the aggregates decreases with an increase in the particle size.

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Figure 3. SEM images of the aggregates formed from silica particles with sizes (a) 0.57 m, (b) 0.9 m, (c) 1.49 m and (d) 2.34 m. (e) Bond order parameter, m6, corresponding to all the four aggregates (a-d) plotted as a function of the size of the particles. Steinhardt bond order parameter distributions for the aggregates obtained from Brownian dynamics simulations using particle diameters (f) 2.0 m, (g) 1.0 m and (h) 0.5 m. The values of the parameters N = 2500, εLJ = 2.5 kBT, and ν = 661 m2/s are equal in the simulations of all three particle sizes. 10 ACS Paragon Plus Environment

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Brownian dynamics simulations also show that reducing the particle size improves ordering in the aggregate. The FCC peaks in the distribution become sharper and extraneous peaks reduce in size as the particle size decreases (Figure 3f-h). The movie of the drying droplet from the experiment with silica particle diameter 2.34 m (Movie S2) shows that the particles present at the center of the droplet are relatively unaffected until the end of the drying process. Whereas, the particles at the edges of the droplet are pushed towards the center by the moving solvent front. Smaller diameters allow for faster diffusion, which in turn should allow for delayed aggregation, as the particles should be able to move away from the solvent edge. Thus, diffusivity should play an important role in the crystallization observed here. Diffusivity (D0) of a Brownian particle is related to its diameter () as 𝐷0 ∝ 1⁄𝜎 , from the Stokes-Einstein relationship.35 In the simulation of a drying droplet it can be seen that particles with diameter 2.0 m pile up at the edge of the droplet. This pile-up is less pronounced for 1.0 m particles and even lesser for 0.5 m particles. Smaller particles with higher D0 values also perceive the dynamics of their environment to be slower than larger particles with lower D0 values because 𝑡 ∝ 𝜎 2 ⁄𝐷0 . Faster diffusion leads to better ordering, because the particles can efficiently explore their configurational space and reach the free-energy minima before the solvent has completely evaporated. The particles with smaller diffusivity, comparatively are more likely to get trapped in a disordered state as the solvent evaporates before they can settle down into their free energy minima. It is to be noted that the free energy surface dynamically evolves as the evaporation proceeds. In fact, for most part of the evaporation process, the corresponding free energy is more or less irrelevant, since at large volumes, the contributions from particle-particle interactions, responsible for ordering is marginal. It is only the last stage of evaporation that is relevant for the ordering process and this constitutes a very small fraction of the total drying time. Since it is impossible to define precisely the relevant



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time-scale, we have labelled the two drying processes with the corresponding total drying time. This is reasonable as an indicator of the rapidity of the evaporation process, since conditions that lead to a very long total drying time also allows the system to sample the free energy minima for a correspondingly longer time compared to the case with a much shorter overall drying time. In summary, we show that a simple coating of a thin layer of silicone oil on a flat glass substrate can influence the dynamics of colloidal assembly to overcome the problem of coffeering formation and grow colloidal crystals. By tuning the evaporation rate and particle size, highly ordered colloidal crystals can be grown. Brownian dynamics simulations show that the ordered crystallite formed by an evaporating droplet with spherical particles most likely forms an FCC structure. Both the simulations and experiments show that ordering in the condensate improves with slow evaporation rates and smaller particle sizes. The simple technique of coating the glass substrate with silicone oil to suppress the coffee-ring formation has several advantages over other self-assembly routes including its simplicity, the ability to control particle deposition without permanent modification of the nature of substrate, and to enable crystallization to take place at length scales, making this approach extremely appealing for practical purposes.


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ASSOCIATED CONTENT AUTHOR INFORMATION The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank IISc for financial support. S.D. and A.D. acknowledge research fellowships from CSIR, India and IISc, Bengaluru respectively. Authors acknowledge support from the Science and Engineering Research Board (EMR/2016/001356) and Nano mission, Department of Science and Technology, India. We thank Dr. S. Mukherjee, Prof. S. V. Kailas and Ms. S. Hemanataraj for helpful discussion and help with contact angle measurements. Supporting Information The following files are available free of charge. Details of experimental set-up, description of simulation model, and methods of data analysis. Figures S1-S7 relevant to the discussion in this article. Movies S1- S2 showing coffee-ring formation and suppression in the AVI format.



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Suppression of the Coffee-Ring Effect and ... - ACS Publications

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