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Dec 18, 2014 - Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along th...
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Evaporation of Sessile Drops Containing Colloidal Rods: Coffee-Ring and Order−Disorder Transition Venkateshwar Rao Dugyala and Madivala G. Basavaraj* Polymer Engineering and Colloid Science (PECS), Laboratory Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India S Supporting Information *

ABSTRACT: Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along the three-phase contact line−a phenomena popularly known as the coffee-ring or the coffee stain effect. The formation of such rings as well as their suppression is shown to have applications in particle separation and disease diagnostics. We present an experimental study of the evaporation of sessile drops containing silica rods to elucidate the structural arrangement of particles in the ring, an effect of the addition of surfactant and salt. To this end, the evaporation of aqueous sessile drops containing model rod-like silica particles of aspect ratio ranging from ∼4 to 15 on a glass slide is studied. We first show that when the conditions such as (1) solvent evaporation, (2) nonzero contact angle, (3) contact line pinning, (4) no surface tension gradient driven flow, and (5) repulsive particle−particle/particle−substrate interactions, that are necessary for the formation of the coffee-ring are met, the suspension drops containing silica rods upon evaporation leave a ring-like deposit. A closer examination of the ring deposits reveals that several layers of silica rods close to the edge of the drop are ordered such that the major axis of the rods are oriented parallel to the contact line. After the first few layers of ordered arrangement of particles, a random arrangement of particles in the drop interior is observed indicating an order−disorder transition in the ring. We monitor the evolution of the ring width and particle velocity during evaporation to elucidate the mechanism of the order−disorder transition. Moreover, when the evaporation rate is lowered, the ordering of silica rods is observed to extend over large areas. We demonstrate that the nature of the deposit can be tuned by the addition of a small quantity of surfactant or salt.

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INTRODUCTION The drying of solutions containing insoluble solutes such as nano- and microparticles, polymers, proteins, and other biological molecules, if controlled, is a powerful method to create diverse assemblies for applications in biological science, electronics, chemistry, and materials engineering.1 There is enormous interest in evaporation driven assembly because it is simple, cost-effective, and a range of ordered structures of nanometer or micrometer sized matter can be created. The controlled evaporative self-assembly of materials is generally carried out in several different configurations: (1) sessile or pendant drop evaporation; (2) evaporation under confinement, for example, wedge, parallel plate, or cylindrical confinement; (3) vertical deposition.2 The evaporation of sessile drops containing dispersed material in particular is relevant to conventional and circuit board printing. It has recently been shown to have applications in particle separation and disease diagnostics as well.3,4 A sessile drop containing colloidal particles dried on a substrate under certain conditions leaves a ring like structure, generally called “coffee-ring”. With the help of optical microscopy, Deegan and co-workers directly visualized the © XXXX American Chemical Society

migration of suspended particles to the drop edge and elucidated “the capillary flow mechanism” as the physical process responsible for the formation of such ring-like deposits.5 When the three-phase contact line is pinned, the solvent that evaporates from the edge is replenished by the flow of solvent and this solvent flowing radially toward the edge carries the suspended particles from the interior to the drop edge. Hu et al. showed that the Marangoni flow, if present, can eliminate the coffee-ring formation.6 During evaporation either temperature fluctuations6,7 or concentration gradient of the surfactant8,9 at the drop interface creates the Marangoni flow which leads to uniform deposition of the solutes on solid substrates. In general the distribution of solids in deposits formed after the evaporation of the sessile drop can be affected by−capillary flow,5 Marangoni flows,6 particle−particle interactions, particle−substrate interactions,10−12 and particle shape.13,14 Received: November 20, 2014 Revised: December 12, 2014

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DOI: 10.1021/jp511611v J. Phys. Chem. B XXXX, XXX, XXX−XXX

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transition. We show that the width of ring can be controlled by the addition of surfactant and the ring can be completely eliminated when a sufficient amount of surfactant or salt is added.

A large number of articles on the coffee stain effect published to-date deal with particles that are spherical in shape. There are several studies recently in which authors observed the selfassembly anisotropic particles that include carbon nanotubes,15,16 gold nanorods, 13,17 CdS/CdSe,18 fd virus, 19 DNA20,21 and different shape gold nanoparticles by using sessile drop evaporation. The observation of deposits left from the evaporation of suspensions of gold nanoparticles shows the long-range ordering of particles. The sessile drop evaporation technique has been also used to obtain several types of close packed structures with different shaped particles as well as their mixtures. The structural arrangement of particles in the deposit strongly depends on the particle shape. Several types of particle arrangements such as hexagonal packing, nematic/3D, tetragonal packing, and nematic/smectic with polyhedral, bypiramid, nanocubes, and nanorods, respectively, have been observed.13 The shape induced separation of particles is also obtained when suspensions containing a mixture of rods and spheres is evaporated.17 In these systems, the interaction between particles of different shape leads to the phase separation. Noble et al. observed an increase in the ordering of nanorods in the ring regions when a suspension of CdS/ CdSe rods is evaporated in the presence of an external electric field.18 By evaporating a suspension containing polystyrene ellipsoids, Yunker and co-workers demonstrated that the coffeering formation can be completely suppressed.14 However, the authors did not consider the effect of interaction forces in their study. Recently, we have investigated the coffee-ring formation with hydrophilic hematite ellipsoidal particles at different pH on a glass slide. We show that the particles shape alone cannot suppress the coffee-ring formation. The particle−particle (PP) and particle−substrate (PS) interactions determine the ring structure.10,11 A clear ring is observed when the PP and the PS interactions are repulsive, and a uniform film is observed when PP interactions are attractive. In the various investigations on the evaporation of sessile drops containing colloidal rods, the effect of electrostatic interactions and more importantly, a detailed investigation of the structural arrangement of particles in the deposits obtained has not been carried out. It must also be noted that in several investigations involving gold particles surfactants are invariably used to stabilize the nanoparticles, which may alter the flow patterns during evaporation, and it is also difficult to tune interactions in such systems. In the present work, we study the evaporation of sessile water drops containing well-characterized model silica rods of varying aspect ratio. The rodlike silica particles are synthesized by a recently developed emulsionbased synthesis technique.22 Bright field optical video microscopy and scanning electron microscopy are used to visualize particle migration and to characterize the deposits obtained upon evaporation. We show that when the conditions necessary for the formation of ring stains are met, the evaporation of drops containing anisotropic particles (AR ≈ 4 to 15) dried on glass substrate at various particle volume fractions leave a coffee ring deposit. Moreover, we also observe an order-to-disorder transition in the ring deposit. The particles in the outer region of the ring, that is close to the three-phase contact line, were arranged with their major axis parallel to the drop boundary. After a few layers of ordered particles, the interior of the ring mostly contained particles deposited in random configuration. The measurement of local particle velocities and time evolution of ring width are used to understand the mechanism responsible for the order−disorder

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MATERIALS AND METHODS Synthesis of Silica Rods. Rod-shaped silica particles were synthesized by a recently developed emulsion-based synthesis method where the aspect ratio of silica rods can be tuned by adjusting the concentration of reagents, temperature, and reaction time.22 The dependence of these parameters on the aspect ratio of the particles can be found in Kuijk et al. (2011). The following procedure was followed to synthesize silica rods of length 2.426 ± 0.531 μm and diameter of 0.248 ± 0.086 μm. All the glassware used in the synthesis and experiments were first sonicated (Bandelin Sonorex, Germany) with soap solution for about an hour, rinsed several times with triple distilled water, rinsed with piranha solution (3:1 mixture of concentrated H2SO4 and H2O2), and rinsed several times with triple distilled water to adjust the pH to neutral. A 300 mL aliquot of pentanol (Spectrochem, India) was taken in a 500 mL pyrex bottle. A 30 g sample of polyvinylpyrrolidine (MW ≈ 40 000 Sigma-Aldrich, USA) was dissolved in pentanol by sonication until a transparent solution was obtained. Then, 30 mL of absolute ethanol (analytical grade, Hong Yang Chemical, China), 4.2 mL of Milli-Q water (Millipore system, 18.2 MΩcm), and 1 mL of 0.18 M sodium citrate (Sigma-Aldrich, USA) were added to the reaction mixture. The bottle was manually shaken vigorously for approximately 5 min so as to mix the contents. Then, 6.75 mL of 25% aqueous ammonia (Merck, India) was added. The bottle was shaken again followed by the slow addition of 3 mL of tetra-ethyl orthosilicate (TEOS 99% Merck, India). The flask was shaken again to mix the contents. The reaction mixture was kept in a circulating water bath at 20 °C for 24 h. At the end of the reaction, the particles were separated from the reaction mixture by centrifugation at 1500g for 1 h. The supernatant was discarded, replaced with ethanol, and sonicated to obtain a homogeneous dispersion. The dispersion was then centrifuged at 1500g for 15 min; the supernatant was replaced with alcohol and then sonicated. This washing procedure was repeated two times. Particles were then dispersed in deionized water, centrifuged for 15 min at 1500g and sonicated; this procedure was repeated three times. The particle suspension was kept undisturbed for overnight to sediment large and nearly uniform sized particles. Sedimented particles were dispersed in fresh deionized water, centrifuged at 700g for 15 min to remove smaller particles. This final washing was done at least two times. Particle Characterization. Synthesized rod-like particles were characterized by scanning electron microscopy (SEM) (Hitachi S-4800, Japan) (Figure 1). A dilute drop of silica suspension was taken and spread on a clean silicon wafer. The suspension drop was dried at room temperature and subsequently coated with a layer of gold by sputtering (Hitachi E-1010, Japan) for 40 s. Particle size was estimated by using ImageJ software. Electrophoretic light scattering (Horiba, Japan) was used to measure the mobility of the particles at different salt concentration. The zeta potential of the particles was calculated from the mobility data by using the Smoluchowski equation. Evaporation Experiments. Evaporation experiments were performed on a glass slide. All glass slides were cleaned with piranha solution (70% concentrated H2SO4 and 30% H2O2 by B

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RESULTS AND DISCUSSION Evaporation of Aqueous Drops Containing Silica Rods. All the suspensions studied were stable during the course of evaporation experiments. The first set of experiments were carried out to verify whether the conditions necessary for the formation of a coffee ring were satisfied: (1) The suspending medium for all the particles used in this study was water, which evaporates under ambient laboratory conditions, and therefore the solvent evaporation criteria is satisfied. (2) On the glass substrate used in this study, the suspension drops containing silica rods form a drop having nonzero contact angle. An average contact angle obtained with suspension drops on a glass slide was 20 ± 3°. The contact angle was measured by placing drops containing silica rods of AR ≈ 15 at a volume fraction of Φ = 1.469 × 10−4. (3) From the microscopy images it was evident that the contact line was pinned for most of the time during evaporation (see Figure S1 in Supporting Information). (4) In the absence of any additives, silica particles and glass substrate were negatively charged in deionized water. The repulsive interactions between the PP and PS were favorable conditions for the ring formation. The addition of salt does alter the interactions, and hence the nature of the deposit in the presence of electrolyte will be discussed later in the manuscript. Aqueous suspension drops containing silica rods of three different aspect ratios were dried on a glass slide. For each aspect ratio, suspension drops containing three different volume fractions ranging from 10−4 to 10−6 were studied to investigate the effect of particle loading. The SEM images of the deposits obtained on a glass slide after the evaporation of water are shown in Figure 2. As evident in Figure 2, a clear ring-like

Figure 1. Scanning electron microscopy (SEM) images of anisotropic particles−silica particles used for coffee-ring experiments and their corresponding size characterization.

volume) and rinsed with Milli-Q water multiple times and kept in an oven at 70 °C for 24 h. Aqueous suspensions of silica particles containing a known number of particles (or volume fraction) were prepared and sonicated for 15 min prior to the experiments. Experiments were done at a temperature of 30 ± 2 °C and 80 ± 5% relative humidity. Evaporation experiments were conducted by placing a 2 μL sessile drop of aqueous suspension on cleaned glass slide with the help of a micropipette. The scanning electron microscope (SEM) was used to visualize the spatial arrangement of particles in the deposits. All samples were sputter-coated with gold for 40 s before SEM imaging. To investigate the effect of particle−particle interactions, sessile drops containing silica rods and a known concentration of sodium chloride (NaCl) were evaporated. The NaCl was used to screen the surface charge of silica rods; that is, the particle−particle repulsion decreases with an increase in salt concentration. The volume fraction of silica rods in the aqueous NaCl sessile drops was fixed at Φ = 2.032 × 10−4 and the concentration of NaCl (RFCL, India) was varied from 0.1 mM to 10 mM. All solutions were sonicated for 15 min prior to the evaporation experiments. The experiments were also performed to study the effect of the addition of surfactant using sessile drops containing silica rods of AR ≈ 10. The volume fraction of particles in the water−sodium dodecyl sulfate (SDS)−silica rods mixture was Φ = 1.058 × 10−4 but the weight fraction of sodium dodecyl sulfate (Mw ≈ 288.38, RFCL, India) was varied from 0 to 0.5% by weight. Video Microscopy. Bright field optical video microscopy was used to capture the migration of particles during evaporation of sessile drop suspension. A 2 μL suspension (silica particles with AR ≈ 15, Φ = 1.469 × 10−4) was placed on a clean glass slide and images were captured with a 63× objective. The images were recorded with a CCD camera (Leica, Germany) that was fixed to an inverted microscope (Leica, Germany). To visualize particle migration, the microscope objective was focused on a region close to the contact line, a few micrometers above the surface of the glass substrate over which the sessile drop was placed. The change in the ring width during evaporation was captured with a 20× objective by acquiring images at every 15 s interval during the evaporation. A 2 μL suspension containing particles with an aspect ratio ≈ 4 was used for the ring width measurements. The ImageJ software was used to calculate the ring width as a function of time.

Figure 2. SEM images of ring deposits obtained by evaporating suspensions of silica rods on a glass slide. (A) AR ≈ 4 suspension (Φ = 6.692 × 10−5), (B) AR ≈ 10 suspension (Φ = 1.058 × 10−4), (C) AR ≈ 15 suspension (Φ = 9.796 × 10−5).

deposit was observed for all the three aspect ratio particles, and the edge of the deposit contains most of the particles. Similar ring deposits were observed at other volume fractions considered in this work. The radial flow that carries silica rods of AR ≈ 10, which eventually leaves a ring-deposit at the edge was directly visualized with optical video microscopy. The video microscopy results clearly showed that the particles move toward the drop edge with their major axis aligned in the flow direction due to the outward capillary flow as a result of pinned C

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rods will adsorb to the interface. The silica rods once adsorbed interact via the shape-induced long ranged capillary attractions forming larger clusters at the interface. In general, the anisotropic particles show higher capillary attraction than the spherical particles because of the nonuniform deformation of the interface due to particle shape effects.25−28 The clusters of silica rods at the interface were found to settle inside the ring eventually at the end of drying. The drying patterns were consistent with the simulation results of Crivoi et al.29 The salt effect was also studied for lower aspect ratio particles. At the same salt concentration, smaller clusters were observed for the lower aspect ratio particles. (Supporting Information for more details Figure S2). The capillary interaction between the anisotropic particles depends on the amplitude of the elliptical quadrupolar deformation (He). For anisotropic particles that are elliptical and rodlike in shape, the capillary interaction has been shown to be proportional to the square of the deformation amplitude (Ec α He2).28 The deformation amplitude is also a function of particle aspect ratio−that is− deformation amplitude is lower for smaller aspect ratio particles than for the larger aspect ratio particles. Therefore, at the same salt concentration, lower aspect ratio particles are expected to form smaller aggregates than the higher aspect ratio particles when adsorbed at the droplet surface. However, the effect of salt concentration was the same in particles of all aspect ratio. But the required salt concentration to suppress the ring formation was much higher for the lower aspect ratio particle. Order-to-Disorder Transition in the Ring Deposits. A closer look at the ring-deposits obtained from the evaporation of silica rods in deionized water revealed a local ordering of particles at the edge of the ring as shown in Figure 4. Most of

contact line evaporation (see Supporting Information, video S1). Recently we had shown that the PP and the PS DLVO (Derjaguin and Landau, Verwey and Overbeek) interactions control the nature of the final deposit pattern in the evaporation of suspensions containing anisotropic particles.11 A clear ring was observed when PP interactions were highly repulsive, and a uniform film was observed when the PP interactions were weakly repulsive and PS interactions were attractive.11 In the present investigation, the silica rods of −80 mV zeta potential (in pure water) were used, and the zeta potential of these particles in 5 mM aqueous NaCl solution was −15.5 mV. Generally glass slides carry a negative charge in water.23 So the overall PP and PS DLVO interactions were repulsive, and this favors the formation of ring like patterns. To manipulate electrostatic interactions, the evaporation of droplets containing silica rods (AR ≈ 10) with a known amount of monovalent salt were carried out. The formation of deposits at three different NaCl concentrations has been shown in Figure 3. From Figure 3, as the salt concentration increases, the

Figure 3. Deposit patterns resulting from the addition of salt to a sessile drop containing AR ≈ 10 particles with different salt (NaCl) concentration. (A) Microscopy images of the dried patterns at different salt concentrations. At high salt concentration more number of particles can be seen inside the ring with a loose packing of particles at the edge. The SEM images at 0.1 mM (B), 1 mM (C), and 10 mM (D) salt concentrations clearly show that the width of the ring as well the formation of ring effect diminish as the salt concentration increases. The scale bar in image A corresponds to 20 μm.

ring width decreases and at the highest salt concentration (10 mM), most of the particles were deposited in the drop interior. At 10 mM salt concentration, the particles were observed to adsorb to the interface and form aggregated clusters as the drying process continued (see Supporting Information, video S2 for more information). The adsorption of particles at the interface can be attributed to the screening of particle charge and the associated image charge effect. The silica rods dispersed in deionized water were highly charged, and therefore, as the particles approach a low dielectric medium (air) from a high dielectric medium (water), the image charge repulsion prevents the particle adsorption to the drop surface.24 The repulsion depends on the particles charge and interface charge. At a higher salt concentration the image charge repulsion was screened by salt addition, so silica

Figure 4. SEM images of the ring deposits observed with silica particles at higher magnification. The images clearly show an order-todisorder transition. The suspension drops used to obtain these deposits contained silica rods of (A) AR ≈ 4 suspension at a particle volume fraction of Φ = 6.692 × 10−5, (B) AR ≈ 10 suspension at a particle volume fraction of Φ = 7.933 × 10−5, (C) AR ≈ 15 suspension at a particle volume fraction of Φ = 7.347 × 10−5.

the particles were aligned side by side (attached along major axis) and parallel to the contact line. This arrangement was observed all along the circumference of the contact line. Generally, sessile drop evaporation is a nonequilibrium process. During the evaporation of sessile drops, the capillary flow carries the particles toward the edge, and during this flow a D

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Figure 5. Sequential images of particle near the drop edge during evaporation of an aqueous drop containing silica rods of AR ≈ 15. Images were captured at an interval of 1 s.

Figure 6. Normalized ring width and velocity as a function of normalized time. (A) The normalized width of ring deposits obtained from aspect ratio 4 particles of two different volume fractions. The variation of normalized widths at the two volume fractions is similar. (B) The evolution of normalized ring width (measured experimentally) and flow velocity (calculated theoretically) as a function time.

After few layers of ordered arrangement of particles right next to the three-phase contact line, a random arrangement of particles in the drop interior was observed indicating an orderto-disorder transition in the ring. The order-to-disorder transition was observed for silica rods and β-FeOOH rods (see Supporting Information Figure S3) of aspect ratios ranging from 5 to 25 investigated in this work. Similar order-to-disorder transition has been recently observed in the coffee ring deposits from a suspension of spheres.30 It was shown that the structural transition in the ring is related to the time-dependent flow velocities during the evaporation process. During evaporation, there will be spatiotemporal variation of capillary flow velocity inside the drop. From the sequence of images obtained from video-microscopy of the particle migration, we measured the velocity of the particles by locating particle position in each image. The experimental particle velocities have been compared with the theoretical analysis based on the thin-film approximation.30,32 The velocities were calculated using eq 1 which has been developed from an analysis based on the thin-film approximation and the measured time evolution of contact radius.

majority of the particles−especially those close to the drop edge−were aligned parallel to the flow direction. It can be seen that in Figure 2, most of the particles in the ring interior (those that fail to reach the edge after complete evaporation of the solvent) appear to have aligned with their major axis in the flow direction. Once the particles approach the contact line, the exerted torque on one end of the particle because of solvent flow will turn the particle parallel to the contact line.18 This acts as a nucleation point and other particles follow the same arrangement in the ordered region. This was confirmed, by tracking several particles in the vicinity of the drop edge during the drying process. Consider the silica rod marked within the red circle in Figure 5, each frame in the figure differs by 1 s. As evident, the particle has aligned in the radial flow direction, and once the particle long axis touches the drop edge the exerted torque on the other end of the particle turns the particles parallel to the edge. The ordering of particles within the ring seen in Figure 4 is similar to that reported with spherical particles.30 In the case of anisotropic particles the orientation effects become important. In the ordered region the alignment of the particles was different for different aspect ratios. For low aspect ratio particles (AR ≈ 4) a smectic order was observed and for high aspect ratio (AR > 10) particles nematic order was observed. As explained earlier, the radial flow and associated torque on the particles will ensure that the particles will be oriented parallel to the contact line. Due to continuous migration of particles to the drop edge, the particles will be confined and exhibit predominantly translational diffusion. The ordering observed in the ring was due to the combined effect of constrained diffusion as well as electrostatic and excluded volume interactions between charged rods similar to those observed in the suspensions of charged rods close to a planar wall.31 The ordering of the particles was reduced with the addition of salt (see Figure 3B−D).

u ̅ (r , t ) =

4RDva ΔC 1 ⎡ (R2 − r 2) ⎤ 1 ⎢ ⎥ − ⎥⎦ πρr θ(t ) ⎢⎣ R2 − r 2 R3

(1)

where Dva is the diffusion constant of vapor in air at 30 °C, ΔC is the vapor concentration difference between the drop surface and the surroundings in the experiments. In eq 1, θ(t) is the contact angle at time t. To find the contact angle the following equation was used. h(r , t ) =

(R2 − r 2) θ (t ) 2R

(2)

where h(r, t) is the local drop height at radius r and time t. The particle velocities obtained from eq 1 were in the range of ca. 0.1 to 15 μm/sec. A plot comparing the experimental data E

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The Journal of Physical Chemistry B and theoretical predictions from eq 1 and 2 was shown in the Supporting Information (Figure S5). During the initial stage of evaporation, the particles migrate toward the edge at low velocities and the velocity increases significantly during the final stages of drying (Supporting Information, video S3). Therefore, the particles that reach the edge at first have sufficient time to arrange. At a later time, close to the end of the drying period, the velocities increase abruptly and therefore the particles rush to the edge leading to a sudden increase in the concentration of the particles. The slower flow velocities during the initial stages of evaporation and a sudden increase in the velocities toward the final stages of drying and the evolution of ring width with time were both consistent with each other as one would expect. The measured ring width for an evaporating drop containing silica rods of AR ≈ 4 at two different particle loadings is shown in Figure 6A. The normalized velocity and ring width as a function of normalized time is shown in Figure 6B. The ring width was normalized with the maximum ring width, that is, the ring width obtained after evaporation; the velocity was normalized with the maximum velocity and the drying time was normalized with the total drying time, tfinal. The overlapping of the two curves in Figure 6A shows that the rate of increment of the ring width is independent of the particle concentration at least at low volume fraction. From Figure 6B, both the rate of increase of ring width and the flow velocities were lower at initial drying times. When drying time reaches 0.8tfinal, the velocities and the ring width increased suddenly. From Figure 6B, approximately 40% of the width increased in the 0.8tfina to 1.0tfina time interval. This indicated that in the last 20% of the total drying time, more number of particles reached the edge. The sudden increase in the particle number density near the drop can be viewed as an “edge effect” that hinders particle mobility and arrests the orientation and positional translation of the silica rods. Therefore, the particles that reach the edge in the final stage of evaporation were arrested in the disordered arrangement. The results presented in Figures 4 and 6 suggest that the evaporation rate can be controlled to alter the structural transition in the ring, which can be exploited to create an ordered arrangement of silica rods over large areas. To increase the ordering of the particles, either particle size should be small or the flow velocity inside drop should be low. With smaller sized gold and CdS/CdSe nano particles, a longrange smectic order has been observed near the edge.13,17,18 In the current investigation, the effect of slow rate of evaporation on ordering of particles in the ring region was studied. To this end, a sessile drop containing silica rods of AR ≈12 was dried in a high relative humidity environment such that a 2 μL drop takes approximately 12 h for complete drying (compared to normal drying time of 12 min). The polarization optical microscopy and SEM images of the ring patterns due to slow evaporation has been shown in Figure 7. These images clearly reveal that the slow evaporation increases particle ordering. In this case, the flow velocities inside the drop is low and due to prolonged drying time, the particles have sufficient time to rearrange in an ordered pattern. Therefore, the controlled evaporation technique can be used to obtain a long-range ordering of particles over a large area. Effect of Surfactant Addition on the Ring-Formation. To study the effect of the presence of surfactant on the formation of the coffee ring, the evaporation of aqueous suspensions of silica rods containing sodium dodecyl sulfate (SDS) were considered. Under the experimental conditions,

Figure 7. Slow evaporation of aspect ratio ≈ 12 particles on a glass slide. (A) Polarization optical microscopy image of the ring shows an increase in the ordering of silica rods in the ring region. Panels B and C show the SEM images of particles alignment inside the ring at different regions.

SDS in water has a critical micelle concentration (CMC) of approximately 0.23% by weight.33 The concentration of SDS in the drop was varied up to a maximum of about 1% by weight. The volume fraction of particles of AR ≈ 10 in each drop was kept constant at Φ = 1.058 × 10−4. Figure 8 shows the SEM

Figure 8. SEM images of silica particles with AR ≈ 10 particles on a glass slide with different SDS concentrations. A ring to uniform deposition was observed with increasing SDS concentration (A) SDS = 0%, (B) SDS = 0.125%, (C) SDS = 0.5%, (by weight).

images of the deposits consisting of silica particles with and without SDS. As shown in Figure 8A, the suspension without SDS exhibits a clear ring formation, as discussed in earlier sections. The addition of a small amount of SDS, 0.125% by weight, much less than the CMC, significantly reduced the number of particles in the ring deposits and the ring width was appreciably reduced (Figure 8B). At higher SDS concentrations, 0.5% by weight, larger than the CMC, the coffee-ring completely disappears (Figure 8C). Suppression of coffee ring formation in the presence of surfactant molecules is consistent F

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with recent results.8 When a sessile drop containing particles and surfactant is dried, the final drying pattern in principle depends on the adsorption of surfactant on the particles and also their adsorption to the drop surface. As a consequence both particle−particle interactions and surface tension changes may occur. Because of similar charge on SDS molecules and anionic silica rods, SDS−silica interactions will be repulsive and SDS would not adsorb to the silica surface as shown in recent studies.34,35 Therefore, both SDS and silica rods will migrate to drop edge due to the radial capillary flow. The surfactant molecules that reach the edge will adsorb to the interface thereby reducing the surface tension of the water at the edge. Thus, the drop spreads initially reaching a contact angle much less than that observed for pure water. The higher concentration of the surfactant at the edge than that in the middle of the drop leads to a surface tension gradient-driven Marangoni flow within the drop.6 Because of this Marangoni flow, the particles were carried away from the edge to the drop interior, thus eliminating the coffee-ring formation. The Marangoni flow strength depends on the surfactant concentration. At low concentration, the Marangonic flow is not sufficient to bring all the particles to the drop interior (Figure 8B). However, at higher concentration all the particles from the edge were carried to the drop interior.

ACKNOWLEDGMENTS We would like to thank Mr. Santosh V. Daware for his help in the synthesis of particles. We acknowledge the scanning electron microscopy facility at the Department of Chemical Engineering, IIT-Madras (procured through a DST-FIST grant).

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CONCLUSIONS We considered the evaporation of aqueous sessile drops containing rodlike colloids and studied the nature of the deposit and arrangement of particles in the deposit using silica rods of different aspect ratios. When both PP and PS interactions were repulsive, the ring-deposits were obtained. The capillary flow responsible for the migration of particles to the edge of the ring was visualized via bright field optical video microscopy. In the coffee-ring deposits, a local ordering of particles where the particles adjacent to the three-phase contact line are oriented parallel to the contact line was observed. As one move away from the contact line, an order-to-disorder transition was seen, and the particles were deposited in a random manner at the ring interior. The structural transition in the ring deposit was evident in particles of various aspect ratios studied. We have demonstrated that the evaporation rate can be exploited to produce deposits with ordered assemblies of colloidal rods over large areas. The coffee ring stains can be completely eliminated by the addition of sufficient amount of surfactant or salt. ASSOCIATED CONTENT

S Supporting Information *

Details about contact line pinning, coffee-ring with β-FeOOH particles, salt effect on ring formation with lower aspect ratio particles, particles diffusivity calculation and fluid velocity during drying. Videos containing drying of sessile drop (AR ≈ 10 and 15) with salt and without salt. This material is available free of charge via the Internet at http://pubs.acs.org.

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DOI: 10.1021/jp511611v J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/jp511611v J. Phys. Chem. B XXXX, XXX, XXX−XXX