Article pubs.acs.org/Langmuir
Control over Coffee-Ring Formation in Evaporating Liquid Drops Containing Ellipsoids Venkateshwar Rao Dugyala and Madivala G. Basavaraj* Polymer Engineering and Colloid Science Lab (PECS Lab), Department of Chemical Engineering, Indian Institute of TechnologyMadras, Chennai 600 036, India S Supporting Information *
ABSTRACT: A control over the nature of deposit pattern obtained after the evaporation of solvent from a sessile drop containing dispersed materials has been demonstrated to have applications in materials engineering, separation technology, printing technology, manufacture of printed circuit boards, biology, and agriculture. In this article, we report an experimental investigation of the effect of particle shape and DLVO (Derjaguin−Landau−Verwey−Overbeek) interactions on evaporation-driven pattern formation in sessile drops. The use of a model system containing monodisperse particles where particle aspect ratio and surface charge can be adjusted reveals that a control over the nature of deposit pattern can be achieved by tuning the particle−particle and particle−substrate interactions. A clear coffee-ring formation is observed when the strength of particle−particle repulsion is higher than the particle−substrate attraction. However, complete suppression of ringlike deposits leading to a uniform film is achieved when particle−substrate and particle−particle interactions are attractive. Results illustrate that for the system of submicron ellipsoids that are hydrophilic, the nature of deposit patterns obtained after evaporation depends on the nature of interactions and not on particle shape.
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DLVO interaction between particle−substrate, Marangoni flow, and the outward radial flow determines the drying pattern after evaporation of sessile drop.10 Recent studies also show coffeering formation when a sessile drop of suspensions of shaped anisotropic particles is subjected to evaporation.11,12 Recently, the effect of ellipsoidal shaped particles on the formation of coffee rings was studied with polystyrene ellipsoids.13 When the particle aspect ratio is greater than 1.5, a complete suppression of coffee-ring formation is observed. The adsorption of ellipsoidal particles to the drop surface forms an aggregated viscoelastic monolayer of a particle network that stops the migration of particles to the edge. Coffee-ring formation has been studied widely with spherical particles and more recently with different types of rodlike particles. Barring the work of Yunker et al., there are no reports on a systematic study of evaporation of sessile drops containing colloidal ellipsoids. In the present work, we study the evaporation of sessile drops containing submicron hematite ellipsoids. Because monodisperse particles of varying aspect ratio can be synthesized, use of hematite particles is a convenient model system to study shape and aspect ratio effects. Moreover, the charge on hematite particles can be tuned by adjusting the pH of the dispersion. Therefore, evaporation of suspension drops containing hematite ellipsoids is studied to understand the combined effect of particle shape and DLVO interactions. We show that a control over the nature of solid deposits, and formation of the coffee ring as well as its suppression, can be
INTRODUCTION The evaporation of solvent from a composite drop containing a variety of materials is scientifically relevant to several engineering disciplines, medicine, and agriculture and equally significant to numerous technological applications. The evaporation-driven self-assembly techniques have received considerable attention, especially for the controlled assembly of particles. The particle assemblies prepared through these routes find potential applications in photonic materials, sensors, catalysis, and other novel functional materials.1 Evaporation of sessile drops containing dispersed or dissolved species is a complex and nonequilibrium process. In general, when sessile drops containing insoluble or soluble solutes are dried on solid substrates, a ringlike deposit at the periphery of the drop is formed. These ringlike deposits are called “coffee-ring deposits or coffee stains”. However, the drying pattern depends on different parameters such as capillary flow, Marangoni flow, contact line pinning, substrate type, particle shape, and particle−particle and particle−substrate interactions. The combined or specific effects of these parameters determine the final deposit pattern. The outward capillary flow generated in the pinned contact line drop evaporation is the physical process responsible for formation of the coffee ring.2−4 The surface tension gradient at the drop surface creates a Marangoni flow, which acts to suppress or eliminate the coffee-ring formation.5 These surface tension gradients can develop either due to temperature fluctuations5,6 (evaporation of organic liquids) or concentration gradients7,8 (evaporation of surfactant−water or polymer−water solutions) at the drop surface. The direction of Marangoni flow depends on the ratio of thermal conductivity of substrate to the liquid.9 When a suspension containing spherical particles is evaporated, the © 2014 American Chemical Society
Received: February 28, 2014 Revised: July 8, 2014 Published: July 8, 2014 8680
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rinsed multiple times with Milli-Q water, and the residual water on the slides is removed with high pressure N2 gas. Glass slides are used immediately after this treatment to avoid contamination. The contact angle of suspension drop is measured with a goniometer (Digidrop, France). Hydrophobic glass slides are prepared by using a silanization treatment.17 Cleaned glass slides are dipped in a hexadecyltrimethoxysilane (Sigma-Aldrich) solution for 1 h. Then the glass slides are cleaned several times with Millipore water and finally cleaned with acetone. The contact angle of water on hydrophobic glass is measured to be 85 ± 5°. All evaporation experiments are carried under constant temperature of 28 ± 2 °C and relative humidity 65 ± 5%. All experiments reported here are repeated at least three times. Particle volume fraction and number density are given in Table 2. A glass slide
achieved by tuning colloidal interactions irrespective of particle aspect ratio.
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EXPERIMENTAL SECTION
Synthesis of Hematite Ellipsoids. Different aspect ratio hematite ellipsoidal particles are synthesized by using the forced hydrolysis of Fe+3 in the presence of urea.14 The aspect ratio of the particles is tuned by varying the molar ratio of NaH2PO4 to Fe+3. In a typical experiment, 3.54 g of Fe(ClO4)3 (Sigma-Aldrich, India) and 0.6 g of urea (Merck, India) are added to 100 mL of Milli-Q water (18.2 MΩ· cm) in a clean 250 mL Pyrex bottle. Different amounts of NaH2PO4 (Merck, India) are added to the Fe+3 solution to control particle aspect ratio. The Pyrex bottle is kept in a preheated oven at 100 °C for 24 h. After 24 h, the reaction product is centrifuged at 4000g for 30 min and washed with deionized double distilled water multiple times. The final suspension is obtained by redispersing the sediment consisting of hematite ellipsoids in Milli-Q water. During the course of cleaning, the pH of the supernatant was measured after every washing. The washing was stopped when the supernatant pH was found to be approximately equal to pH of the deionized water from Milli Q. Scanning electron microscopy (Hitachi S-4800, Japan) is used to characterize the particles (Table 1). Hematite particles of different aspect ratios are shown in Figure 1.
Table 2. Concentration of Particles in Sessile Drops: Volume Fraction and Number of Particles per mL in Suspensions of Hematite Used in the Present Investigation serial number 1 2 3 4
Table 1. Characterization of Hematite Ellipsoids: Dimensions and Aspect Ratio Obtained from Scanning Electron Micrographs serial number 1 2 3 4
length (nm) 119 ± 10 196 ± 6 179 ± 21
diameter (nm)
aspect ratio
± ± ± ±
∼1 2.23 ± 0.23 4.01 ± 0.13 6.18 ± 0.63
105 53 49 29
12 3 2 4
aspect ratio ∼1 2.23 ± 0.23 4.01 ± 0.13 6.18 ± 0.63
volume fraction −3
1.04 × 10 2.048 × 10−3 1.190 × 10−3 1.784 × 10−3
number of particles/mL 1.72 1.17 4.82 2.20
× × × ×
1014 1013 1012 1013
coated with a mono- and multilayer of hematite ellipsoids was prepared by the dip-coating method. When a water drop was deposited on the hematite particle film, a stable drop having a contact angle of 40° ± 5 was observed in both cases, indicating that the particles are hydrophilic. A 1 μL suspension is placed on a cleaned glass substrate with a micropipette. The time sequence images of the sessile drop during evaporation are captured with an inverted microscope (Leica DMI3000B, Buffalo Grove, IL). The images are taken at 1 min intervals with a 20× objective at 6.9 mm free working distance. Low magnification images of dried patterns are captured with a digital camera (Sony, DSC-750) which is connected to a stereoscopic microscope (Zoom stereoscopic microscope, India). The 3D surface profile measurements of dried structures are analyzed with an optical surface profile meter (Bruker, ContourGT-I, Germany) with a 20× objective. The 3D plots and radial height profiles are created from the surface profile date by using the Gwyddion (data visualization and analysis tool) software.
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RESULTS AND DISCUSSION Effect of DLVO Interactions at Fixed Aspect Ratio. To elucidate the combined effect of various interactions and particle shape, the deposits obtained after the evaporation of sessile aqueous drops containing ellipsoids on a clean glass substrate are studied. In these experiments, ellipsoids of fixed aspect ratio of 4 are used. The PP (particle−particle) and PS (particle−substrate) interactions are controlled by systematically changing the pH of ellipsoid suspensions. The zeta potential of hematite ellipsoidal particles and the glass slide18 at different pH values is shown in Figure 2A. From Figure 2A, the isoelectric point (IEP) of hematite particles is around pH 9. Particles carry positive and negative charges in acid and base medium, respectively, due to the adsorption of H+ and OH‑ ions on the particle surface. Similarly, the glass surface exhibits IEP around pH 2 and is negatively charged above pH 2. A 1 μL drop is placed on a precleaned glass slide, and evaporation experiments are conducted at 28 ± 2 °C and 65 ± 5% relative humidity. The volume fraction of particles in the drop is maintained constant (φ ∼ 0.00119). The experiments are designed such that the conditions necessary for the formation of coffee ring are satisfied:2 (1) Solvent evaporation: the suspending medium for all the particles used in this study is
Figure 1. Scanning electron microscopy (SEM) images of hematite particles with different aspect ratios. (A) Aspect ratio 1; (B) aspect ratio 2; (C) aspect ratio 4; (D) aspect ratio 4. Zeta Potential. The suspension pH is adjusted by the addition of either aqueous HNO3 or aqueous NaOH solutions of known concentration. To obtain hematite dispersions at a particular pH, the particles which are originally dispersed in water are centrifuged multiple times with solutions of different pH values. Suspensions containing aspect ratio 4 particles are used to measure the zeta potential (Horiba Nano partica SZ-100, Japan). The zeta potential values of all samples were measured at a known salt concentration of 0.01 M NaCl. Because high salt concentration conditions were maintained during the measurement of electrophoretic mobility of ellipsoidal particles, the Smoluchowski equation was used to calculate the zeta potential from measured electrophoretic mobility data.15,16 Sessile Drop Evaporation. In all experiments, glass slides are used as substrate. Initially, glass slides are dipped in Piranha solution (70% H2SO4 and 30% H2O2 by volume) for 1 h. Then the slides are 8681
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DLVO interactions are evaluated at 5 nm surface-to-surface separation and at a Debye length of 30 nm. The PP and PS DLVO interaction under experimental conditions is given in Table 3. Table 3. Particle−Particle (PP) and Particle−Substrate (PS) DLVO Interactionsa pH
particle charge (mV)
substrate charge (mV)
Up-p/ KBT
Up-s/KBT
2 3 6.5 8 11 12
40 50 16.5 8.4 −75.8 −75
−0.5 −6 −26 −30 −30 −30
61.5 141.2 −56.3 −74.2 429.24 418.5
−84.77 −133.7 −156.6 −125.7 323 318.7
a
PP interactions are calculated for particle major axes parallel to each other and PS interactions are calculated for the particle major axis parallel to the substrate. Interactions are calculated at surface-tosurface separation of 5 nm between particle−particle and particle− substrate.
Figure 2. (A) The zeta potential measurements of hematite ellipsoidal particles (aspect ratio 4) and the glass slide as a function of pH. The zeta potential values of the glass slide are taken from Somasundaran et al.18 (B) Microscopy images of dried patterns obtained with aspect ratio 4 particles on a glass slide at different pH values. The volume fraction of the particles is 1.19 × 10−3. The scale bar is 500 μm.
At pH 2, the PP and PS interactions are repulsive (highly) and attractive, respectively. Because of electrostatic attraction between particles and glass surface, a mono- or multilayer of hematite particles is observed all across the dried pattern. However, most of the individual particles are observed to migrate to the drop edge because of the outward radial flow that is created to replace the fluid evaporating from the drop edge due to pinned contact line. The time sequence images acquired during the evaporation of a suspension drop at pH 2 is shown in Figure 3A. These images clearly demonstrate the accumulation of particles at the drop edge and a monotonic increase in ring width. When suspension drops at pH 6.5 are placed on the glass slides, reduced PP repulsion and the van der Waals interactions due to high Hamaker constant19 (H131 = 20 × 10‑20 J and H132 = 10 × 10‑20 J, 1, hematite; 2, glass; 3, water) lead to the formation of small aggregation (see supporting information for more details). However, as the glass substrate is more negatively charged, increase in PS attraction pulls the particles and particle aggregates to the glass substrate due to enhanced electrostatic attraction. Therefore, a uniform film is obtained at the end of the evaporation. The time sequence images of an evaporating drop at an initial pH of 6.5 in Figure 3B show that the particles are uniformly deposited during evaporation. Above pH 12, particles and substrate have negative charge; therefore, both PP and PS interactions are repulsive. During evaporation, the particles are carried to the edge due to radial flow of the solvent. In the case of pH 12 drop, both height and width of the deposit increase during initial stages of evaporation. However, as shown in Figure 3C, deposition of more particles inside the drop is observed despite repulsive interactions. A possible reason for this phenomenon is the recrystallization of NaOH from the pH 12 drop and modification of the flow field in the drop interior due to deposition of these crystals as a result of supersaturation. The presence of NaOH crystals is confirmed by studying the evaporation of the pristine sessile aqueous NaOH drop. However, the formation of ring deposits is only observed in suspensions of drops containing ellipsoids but not in drops without particles (in both of these drops, NaOH is used to adjust pH to 12; see Supporting Information). Further study is
water at different pH values, which evaporates under experimental conditions. (2) Nonzero contact angle: on the substrate used in this study, that is, glass slides, the suspension drops containing hematite particles form a drop having nonzero contact angle. An average contact angle of ∼10° is recorded. (3) Pinning of contact line: the contact line pinning is confirmed by the measurement of time evolution of contact line radius. During the evaporation of droplets containing particles, the strength of the Marangoni and outward radial flow depends on the contact angle of droplet (other parameters such as temperature and humidity being constant). As the contact angle decreases during the evaporation of pinned contact line drops, the Marngoni flow decreases and radial flow velocity increases. In all experiments, due to low contact angle (∼10°), the radial flow dominates the Maranogni flow.5 Therefore, under experimental conditions reported in this paper, Marangoni flow effects were negligible and hydrodynamic forces that carry particles to the drop edge were of similar magnitude for all the aspect ratio particles and pH values. Therefore, the current investigation was aimed at elucidating the effect of DLVO interactions. The effect of pH of the drop on the nature of deposit pattern at particle aspect ratio of 4 is illustrated in Figure 2B. A clear coffee ring is observed under highly acidic and basic conditions, that is, below pH 4 and when the pH is greater than 10. However, when the pH of the drop is close to the IEP, the particles are deposited homogeneously across the drop surface, leaving a uniform film. The deposit pattern in the three different regions (acidic, basic, and intermediate pH) is determined by PP and PS interactions. The PP interactions are calculated for side−side configuration, and PS interactions are calculated for the major axis parallel to substrate configuration (see Supporting Information). For DLVO calculations, zeta potential values measured at low salt concentration (0.0001 M NaCl) are used, as they closely correspond to experimental conditions (Smoluchowski theory predicts zeta potential values lower than actual values for ellipsoid particles at low electrolyte concentration).16 The 8682
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drop surface−air interface (which is due to shape-induced capillary attraction) prevents the transport of particles to the periphery. It must be noted that the particles used in the work of Yunker et al. are partially hydrophobic. The particles used in this investigation are hydrophilic and, moreover, highly charged at low and high pH. To further investigate the role of surface charge of the particle on the adsorption of particles to drop surface, pendant drop experiments were designed. Adsorption of Particles to Drop Surface: Role of Particle Charge. The adsorption of particles to interfaces during any nonequilibrium processes such as evaporation of drops containing particles depends on several factors such as the rate at which the interface recedes due to evaporation, density of the particles (and hence settling of the particles in an evaporating drop), and particle charge.20,21 In the case of charged particles, image charge effect is an important phenomenon to understand the adsorption of particles at interface.21 If a charged particle approaches a low dielectric medium (air or oil) from a high dielectric medium (water), the particle experiences repulsion from an image charge present in the low dielectric medium. Because of the electrostatic repulsion between the original and the image particle, there exists an energy barrier for the particles to reach the interface. The magnitude of image charge depends on the charge on the particle (q) and the dielectric constants of the two mediums: ε − ε2 qimage = q 1 ε1 + ε2 (1) In eq 1, ε1 is the dilectric constant of the medium which contains the charged particle and ε2 is the dielectric constant of the particle free medium (in the present case−air). The image charge effect depends on the particle surface charge density and the dielectric constant of two mediums. For hematite particles used in present study, the zeta potentials values vary from +40 mV to −70 mV depending on the pH values of suspension. Because recent simulations and experiments show that particle adsorption to interfaces can significantly influence the type of deposit obtained after evaporation, we used a pendant drop method to qualitatively observe the possible role of particle charge on the adsorption of ellipsoids to interfaces. Due to the submicron size of ellipsoids, we could not use optical microscopy for the direct visualization of particle adsorption to drop surface. To understand the role of particle charge, initially, a 10 μL droplet of a pH 2 suspension containing hematite ellipsids (AR ∼ 4) is suspended in a decane medium with the help of a syringe. Decane medium (dielectric constant ∼2) instead of air (dielectric constant ∼1) is used to avoid drop evaporation during experiments. The drop is allowed to stand for 1 h to provide sufficient time for particles to reach the interface. After 1 h, the drop is subjected to compression by withdrawing the suspension from the pendant drop at a slow rate and simultaneously images of the drop are recorded during compression. Even after 1 h, a uniform decrease in the drop size is observed, indicating that the particles remain in the drop and no particles are adsorbed to the interface (see supporting video 1, Supporting Information). If the particles are adsorbed to the interface, buckling phenomena must be expected because of the compression of the monolayer of ellipsoids. In a pH 6.5 drop, where the particles are weakly charged, a clear buckling is observed, indicating adsorption of particles to the interface (see supporting video 2, Supporting Information). A snapshot of a pendant drop maintained at pH = 2 and pH = 6.5 undergoing a
Figure 3. Temporal images of a sessile drop during evaporation of suspensions at different pH values. Microscopy images are taken at 1 min intervals. The sessile drop contains aspect ratio 4 particles (φ = 1.190 × 10−3). (A) pH 2 suspension. (B) pH 6.5 suspension. (C) pH 12 suspension. The scale bar is 100 μm.
required to understand the pH change inside the drop during evaporation. Surface profile measurements are performed to further investigate the particle deposition close to the ring edge. The 3D images of ring structures are shown in Figure 4A. From Figure 4A, it is evident that a ring where the particle density is high is formed for high zeta potential or highly charged suspensions and a uniform film is observed near the IEP. The radial ring height profile calculated from the 3D images is shown in Figure 4B. The height of the ring formed due to deposition from a pH 2 drop is higher compared with the other pH drops. The height profile in the interior of the drop at pH 11 and 12 is more because of deposition of NaOH crystals and because of particles that fail to reach the edge. Effect of Particle Aspect Ratio. To investigate the effect of particle aspect ratio, we study evaporation of sessile suspension drops containing particles of aspect ratio 1, 2, 4, and 6 at pH of 2, 6.5, and 12. The microscopy images of dried patterns are shown in Figure 5. At all aspect ratios, ring formation is observed when pH is 2 and 12, that is, when the particles are highly charged (PP interactions are repulsive), and a uniform film is observed at pH 6.5, that is, when particles are weakly charged and PS interactions are attractive. Recently, Yunker et al. observed the suppression of coffee-ring formation when the suspension drops contain ellipsoidal particles of aspect ratio greater than 1.5.13 The authors attribute this coffeering suppression to the adsorption of particles to the interface. The formation of the viscoelastic particulate network at the 8683
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Figure 4. (A) The 3D surface profile images of dried structures on glass slides with suspensions of different pH values. The suspension drop contains aspect ratio 4 particles at a volume fraction of 1.19 × 10−3. (B) The ring height and width are calculated as a function of pH from the surface profile images.
Figure 6. Adsorption of hematite particles to the water−decane interface: (A) suspension containing hematite ellipsoids (AR ∼ 4) at pH = 2 (highly charge); the drop is isotropic and shows no buckling under compression; (B) suspension containing hematite ellipsoids (AR ∼ 4) at pH = 6.5 suspension (weakly charged); buckling is observed as evident from the ripples formed on the drop surface.
Figure 5. Effect of particle aspect ratio: Microscopy images of dried droplet observed with pH 2, 6.5, and 12. (A) Particles of aspect ratio ∼1 (φ = 1.04 × 10−3). (B) Particles of aspect ratio 2 (φ = 2.048 × 10−3). (C) Particles of aspect ratio 4 (φ = 1.190 × 10−3). (D) Particles of aspect ratio 6 (φ = 1.784 × 10−3). The scale bar is 500 μm.
favored and therefore during drop evaporation, the concentration of the particles at the drop surface should increase with time. However, as the timescale of the drop evaporation experiments is about 8 minutes, it is highly unlikely that the drop surface is completely covered with particles. To demonstrate the role of PS interactions, we use a hydrophobic substrate instead of negatively charged glass substrate so that PS attraction is reduced. Evaporation Experiments on a Hydrophobic Surface. Drying patterns were obtained by evaporating aqueous sessile drops containing hematite particles (aspect ratio ∼4) at pH = 2 and pH = 6.5 evaporated at a constant temperature of 28 ± 2 °C and a relative humidity of 65 ± 5% on a silanized
compression cycle due to slow withdrawal of respective suspensions is shown in Figure 6A,B. Moreover, when the surface tension of these drops were measured over the equilibration time, a steeper drop in surface tension was observed for pH = 6.5 (data not shown), indicating particle adsorption when particles are weakly charged. It must be noted that the adsorbed monolayer of ellipsoidal particles are known to be highly viscoelastic.22 We conclude from these experiments that − particle adsorption to the interface at intermediate pH is 8684
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drop edge due to radial flow. Therefore, the nature of deposit can be controlled by changing the pH of the suspension. The possibility of tuning the PS interactions provides an alternate method to control the nature of drying pattern at intermediate pH. Therefore by an appropriate choice of substrate, a uniform deposit as well as coffee-ring can be obtained. The interplay of interactions and shape presented in this article can be extended to other configurations commonly used for evaporation-driven self-assembly of particles on substrates and for the controlled deposition of molecules that are inherently shape anisotropic for application in biological sciences.
hydrophobic glass slide. The microscopy images of dried patterns are showed in Figure 7. Figure 7 clearly demonstrates
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ASSOCIATED CONTENT
S Supporting Information *
DLVO interaction calculations, surface tension data for different pH solutions, NaOH crystal formation image, side view image of drop on glass slide, calculation of hydrodynamic and Marangoni force and images charge effect video. This material is available free of charge via the Internet at http:// pubs.acs.org
Figure 7. Evaporation of a sessile drop containing hematite particles (AR ∼ 4) at pH = 2 and pH = 6.5 on the hydrophobic glass surface. (A) Nature of the deposit at pH = 2. (B) A higher magnification image of the dried pattern shown in panel A. (C) Nature of the deposit at pH = 6.5. (D) A higher magnification image of the dried pattern shown in panel C. The presence of particle aggregates inside the ring is evident in panel D.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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the formation of coffee-ring deposits at pH = 2 as well as at pH = 6.5. Figure 7A and 7C, respectively, show ringlike deposits upon evaporation of the pH = 2 and pH = 6.5 suspension. Note that at pH = 6.5, when negatively charged substrate is used, uniform deposits are observed. Because the particle−substrate attraction strength decreases, the radial flow carries the particles toward the edge and leads to the formation of a ring at the drop periphery even at intermediate pH. When the pH = 6.5 suspension is evaporated on a hydrophobic surface, few particle aggregates are observed in the interior of ring due because of particle−particle attraction. From the evaporation experiments on the hydrophilic and hydrophobic surface at intermediate pH, we conclude the following: (1) a coffee ring forms when particles can adsorb to the interface, and the particle−substrate interaction is negligible; (2) the coffee ring is suppressed when particles can adsorb to the interface, and the particle−substrate interaction is attractive. It must be noted that the adsorption of particles to the interface during the course of evaporation (∼8 min) may not be sufficient to completely cover the drop surface − that is − the surface coverage is much smaller than those required to form a visco-elastic interface. Because high surface coverage is required for the formation of a highly viscoelastic monolayer to suppress coffee-ring formation, we conclude that the particle− substrate interactions are dominant and determine the nature of the dried pattern under the experimental conditions used.
ACKNOWLEDGMENTS We thank reviewers for careful reading of the manuscript and insightful comments which helped us improve the quality of the manuscript. V.R.D. acknowledges the research fellowship from IIT-Madras (awarded through MHRD), and M.G.B. is grateful for the research grant from the Department of Science and Technology, India. We acknowledge the scanning electron microscopy facility at the department of chemical engineering, IIT-Madras (procured through DST-FIST grant).
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REFERENCES
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CONCLUSION We have demonstrated the effect of DLVO interactions on the nature of drying patterns obtained upon the evaporation of suspension drops containing colloidal ellipsoids. The coffeering formation is observed upon evaporation of drops maintained at low (acidic) and at high (basic) pH conditions, and a uniform deposit is obtained at intermediate pH. When suspension pH is low (acidic) and high (basic), the repulsive PP interactions are dominant and the particles are carried to the 8685
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