Evaporation of Nanoparticle Droplets on Smooth Hydrophobic

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Evaporation of Nanoparticle Droplets on Smooth Hydrophobic Surfaces: The Inner Coffee Ring Deposits Tuan A. H. Nguyen, Marc A. Hampton, and Anh V. Nguyen* School of Chemical Engineering, The University of Queensland, QLD 4072, Australia S Supporting Information *

ABSTRACT: The solid surfaces used in evaporation studies of nanoparticle sessile droplets usually exhibit significant surface roughness, causing significant pinning of the three-phase contact lines and producing different types of nanoparticle deposits, from single and multiple coffee rings (formed at the initial pining of triple contact lines) to central bumps. Here we used nanometer-scale smooth hydrophobic surfaces to investigate the evaporation of sessile water droplets containing silica nanoparticles and organic pigment nanoparticles. We observed a new type of coffee ring deposits which were not formed at the initial pinning but at the later pinning. We referred them to as the inner coffee ring deposits (ICRDs). The radius of ICRDs was smaller than the radius of the initially pinned contact area and increased with increasing concentration of added salts and nanoparticles and with increasing contact angle hysteresis of hydrophobic surfaces. We also observed different dendrite deposit patterns inside ICRDs. We argue that all the deposit patterns are due to the second pinning of the three-phase contact lines, which occur when the forces on particles are balanced. The hypothesis is further supported by the transient changes of the dynamic contact angles and contact base area radius. The contact angle hysteresis, the particle concentration, and the colloidal interaction forces such as the electrical double-layer forces play a vital role in determining the size and patterns of ICRDs and the evaporation kinetics of nanoparticle sessile droplets.

1. INTRODUCTION Assemblies of micro- and nanoparticles within evaporating liquid droplets have potential applications in many areas such as photonics, electronics, sensing devices, etc. These applications are very much influenced by the morphology of particle deposit patterns and their position on the solid surfaces. Nanoparticle suspension droplets evaporating on the surfaces can produce many types of particle deposit structures, including single rings (or “coffee rings”)1−3 and multiple rings,4,5 central bumps, fingering deposits at the initial wetting lines, hexagonal cells and uniformly distributed patterns,6 etc. The coffee ring deposits are usually formed at the initial pinning of triple contact lines. Despite many experimental and theoretical research efforts, the mechanisms controlling the formation of the deposit patterns have not yet been clarified successfully. The majority of the studies agree that the pinning and receding of the three-phase (triple) contact lines (TPCLs) is one of the main reasons.1,4,7,8 However, a wide range of pinned and receding features of the TPCLs has been observed and reported for a number of experimental systems with different advancing, receding, and initial contact angles; different solute and particle compositions and concentrations; different surface roughness scales; and different environment (humidity and temperature) conditions. The coupling between the solid surface roughness and the freshly formed colloidal patterns provides an intrinsic force barrier that can pin the TPCLs. Because of that coupling, the role of colloid itself in pinning the contact line has been poorly understood. © 2013 American Chemical Society

Dynamics of the TPCL is mainly driven by the unbalance between the interfacial forces. Both physical and chemical properties of nanoparticle suspensions and solid surfaces, therefore, play important roles in the evolution of a nanoparticle suspension droplet. The unbalanced Young force and the anchoring forces induced by the solid surface roughness were used to interpret the pinned-receding behaviors of the TPCLs.7 The scenario becomes even more complicated with the presence of other additives, i.e., salts,9 surfactants,5 and polymers.9 These components not only change the physical and chemical properties of the working system but also make them time-dependent.10 The pinning of the TPCL was believed to commence when the local concentration of solute components close to the TPCL reaches a critical value. For instance, Fukai et al.11 observed the pinning of xylene−polystyrene droplets on a hydrophilic surface commence as soon as the polymer concentration is in the range of 20−40 wt %. A self-pinned mode of anisole−polystyrene droplets was also reported by Kajiya et al.12 The proposed reason for the pinned mode was that the polymer solution concentration near the edge became so high that the TPCL mobility was lost near the end of the droplet lifetime. Evaporation of aqueous drops containing sodium polystyrenesulfonate and sodium chloride was studied by Kaya et al.9 They suggested that the role of polymer was to Received: December 23, 2012 Revised: February 6, 2013 Published: February 7, 2013 4707

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wafers (Silicon Valley Microelectronics, Santa Clara, CA). The wafers were made hydrophobic by esterification using 1-octanol (Sigma-Aldrich, Australia) and by silanation using n-octadecyltrichlorosilane (Sigma-Aldrich, Australia). The details about the hydrophobization techniques were previously reported.14 In this paper, the hydrophobized silicon wafer surfaces will be referred to as the Oct-silicon and OTS-silicon, respectively. Prior to experimentation, the hydrophobized surfaces were cleaned with acetone (AR grade, Sigma-Aldrich, Australia) and then left in DI water overnight to hydroxylase. Surface roughness of the Oct-silicon and OTS-silicon surfaces was determined by atomic force microscopy (AFM) using a MFP3D AFM (Asylum Research, Santa Barbara, CA) in contact mode in air with triangular silicon nitride cantilevers (Veeco, Santa Barbara, CA). The surface potential of the silicon wafers surfaces was measured by streaming potential method using a streaming potential device with an asymmetric clamping cell (EKA Electro Kinetic Analyzer, Anton Paar GmbH, Graz, Austria). Contact angle hysteresis (CAH) loops were measured by pendant drop tensiometry using a PAT1 tensiometer (SInterface Technologies, Berlin, Germany). 2.2. Experimental Setup and Data Analysis. Evaporation of single sessile droplets on the hydrophobic silicon wafer surfaces was investigated under natural diffusion condition in a closed glass box (30 cm × 30 cm × 30 cm) to minimize possible external perturbations. The experiments were conducted at room temperature (22 ± 1 °C) and controlled relative humidity of 53 ± 1% (using the saturated MgNO3 salt solution). Small nanoparticle sessile droplets of 0.5 μL volume were gently deposited onto the flat solid surfaces using a pipet (Eppendorf Research, Germany) with precisely controllable volume. A progressive scan CCD camera (model XCD-SX910, Sony, Japan) and a 10× objective (Nikon, Japan) were used to capture the transient pictures of evaporating droplets at the rate of 15 frames/s. A fiber light with diffuser was used to illuminate the drop from the backside. The video movies were then extracted into single images by the Photron Fastcam Viewer 3 software (Photron) and further analyzed using an in-house Matlab code for determining the droplet contact base radius and contact angle. All the 0.5 μL droplets used in this study were much smaller than the capillary length (2.7 mm for water) and have the shapes of spherical caps; hence, the detected droplet profiles were very well fitted using an equation for circles. Our Matlab code was also modified to obtain the numerical solution of the Young−Laplace equation to fit the droplet profiles and gave the same results, confirming the assumption about the droplet shape of spherical caps. Under all conditions, the experiments were repeated 10 times, showing consistent dynamics of the droplet evaporation. The residual deposits of nanoparticle droplets were imaged using a metallurgical inverted microscope (L2003C, ProSciTech, Australia).

pin the TPCL, but the crystalline salt did not produce the pinned effect. Kuncicky,10 on another hand, reported that salt, i.e. potassium chloride, played an important role in modulating the interaction between polystyrene latex microspheres and solid surfaces. Bhardwaj et al.13 considered the influence of DLVO (Derjaguin−Landau−Verwey−Overbeek) intermolecular interactions, such as the electrostatic and van der Waals interactions, on the titania nanoparticle deposition from nanoparticle droplets onto the glass surface. However, the self-assembly mechanism of the colloids inside sessile droplets and its effect on the evaporation process are currently not well understood. Furthermore, the effects of colloidal properties such as the size, surface charge, and hydrophobicity of the solid particles and surfaces on the evaporation dynamic and residual deposit are still unclear. In this paper, we examine the evaporation kinetics and the particle deposits of nanoparticle suspension droplets on nanometer-scale smooth hydrophobic surfaces. Specifically, the focus is on understanding the influence of colloidal particles and salt on the pinning and receding of the TPCLs. A new type of nanoparticle deposit, termed the inner coffee ring deposit, on these surfaces will be presented and will be shown to depend on the colloid particle concentration, the salt concentration, and the pinning and receding characteristics of the TPCLs. Further progress on the pinning and receding criteria of the TPCL during the nanoparticle droplet evaporation will also be provided.

2. EXPERIMENTS 2.1. Materials and Solid Surfaces. Suspensions of silica nanoparticles were prepared by diluting a stock solution (20 wt %, Snowtex-20L, Nissan Chemicals) with deionized (DI) water. The Snowtex-20L particles were made by growing monodispersed, negatively charged, amorphous silica particles in water, which were stabilized by the electrical repulsion between the negatively charged particle surfaces. Another suspension containing organic pigment nanoparticles made from anthraquinone (an aromatic organic compound with formula C14H8O2), BI-ZR3 (Brookhaven Instruments Corp., Holtsville, NY), was also used in this study. These particles were chargestabilized and are referred to as BI particles in this paper. The negative surface charge stabilized the BI particles that are used as the reference material for zeta-potential measurements. The water used in the experiments was freshly purified using a setup consisting of a reverse osmosis RIO’s unit and an Ultrapure Academic Milli-Q system (Millipore). The measured pH = 6 of the water was slightly lower than pH = 7 expected for neutral condition because of dissolved carbon dioxide from ambient air. Different ionic strengths of the sessile droplet suspensions were controlled by potassium chloride (KCl) of concentrations up to 10 mM (99.5% ACS grade, Sigma-Aldrich, Australia). The surface (zeta) potential and size distribution of the silica nanoparticle and BI nanoparticles were determined using a ZetaPlus instrument (Brookhaven Instruments Corp., Holtsville, NY). The phase analysis light scattering (ZetaPALS) technique available within the ZetaPlus instrument was used for the measurements of low microelectrophoretic mobility of particles in high salt concentrations. The shape of the nanoparticles was determined by imaging the dry samples using a scanning electron microscope (SEM) (JEOL 6400, Japan). The solid surfaces used in the evaporation experiments of single sessile droplets were nanoscopically smooth silicon

3. EXPERIMENTAL RESULTS 3.1. Properties of Solid Surfaces and Nanoparticles. The initial contact angles (described as θ0) of 0.5 μL nanoparticle droplets freshly deposited onto the Oct-silicon and OTS-silicon surfaces are 80.7 ± 1.4° and 102.3 ± 1.6°, respectively. Therefore, the Oct-silicon surface is considered as a moderately hydrophobic, while the OTS-silicon surface as strongly hydrophobic. AFM images over 1 μm × 1 μm areas of the hydrophobized surfaces show that both modified silicon surfaces are nanoscopically smooth: The RMS (root-mean4708

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noted that after drying the BI particles aggregated strongly as shown in Figure 1, but they readily redispersed in solutions as confirmed by the size measurements. 3.2. Residual Deposits of Nanoparticles on the Smooth Hydrophobic Solid Surfaces. Evaporation of nanoparticle droplets of different particle concentrations and salt concentrations were studied under the same environment conditions. Characteristic deposit patterns of the same volume (0.5 μL) nanoparticle droplets on the smooth surfaces are shown in Figures 3 and 4. The “coffee ring” effect on the deposit formation is evident for both solid surfaces, where the majority of particles preferentially accumulated at the wetted periphery and formed the ringlike patterns. We note that the initial contact angle of the studied droplets on the Oct-silicon surface is smaller than 90°, while it is larger than 90° on the OTS-silicon surface. This difference in the hydrophobicity (as measured by the initial contact angles) of solid surfaces would be critical to the formation of the “coffee ring” deposits as well as many other factors could also play an important role. Significantly, we discovered that, despite the nanoscopically smooth surfaces used in the experiments, the droplets were always pinned to the surface from the beginning and the nanoparticles were always collected in the single rings with radius (R) smaller than radius (R0) of the initially pinned droplet contact areas, i.e., R ≤ R0. Here we refer these small ring deposits to as the inner coffee ring deposits or the ICRDs to distinguish them from the deposit rings of the initial base radius as observed and reported in the literature previously.1,2,5,6,15,16 Specifically, the droplets of 1 wt % SiO2 consistently formed the ICRs at ∼38.36 ± 3.34% and ∼35.48 ± 2.26% of the initial wetted radius on the Oct-silicon and OTS-silicon surfaces, respectively. These diameters of the ICRDs are incidentally comparable with the droplet base diameters where the late (additional) pinning of the TPCLs occurred during the evaporation. This aspect will be discussed in details in section 3.3. The experiments were carried out with whole range of particle concentrations and showed the significant influence of concentration of the two particle types on the ring diameter. Screening suitable concentration of the particles for compatible ICRD formation and sizes led to the suitable concentration range of 0−0.01 and 0−1 wt % for the BI and SiO2 particles as reported here. By considering the particle size ratio (16) and the mass ratio (10) for producing similar ICRDs, it appears that the ratio of BI to SiO2 particle numbers required to make similar ICRDs is about 160. This huge difference in the particle mass and number required for making similar ICRDs can be attributed to the difference in the particle-surface friction coefficients of the irregular BI and spherical SiO2 particles and many other factors as discussed in section 4. Significant differences in the radial position and size of the deposit rings are observed, depending on the component of the fluid and solid surface properties. Adding salt, KCl 10 mM, can enlarge the radius of SiO2 deposits on the Oct-silicon surface to R/R0 = 46%. Increasing concentration of particles and/or salts also enlarges the normalized radius of ICRDs (Figures 5). For the same salt concentration, normalized radius of ICRDs linearly increases with increasing the BI particle concentration on both the Oct-silicon and OTS-silicon surfaces. Comparing the ICRD radii of SiO2 particles and BI particles deposits reveals that the BI particles have a significantly stronger pinned effect on the ICRD formation and size than do the SiO2 particles. For instance, on the Oct-silicon surfaces, the TPCLs

squared) roughness factors obtained by the AFM technique are 0.149 ± 0.007 and 0.189 ± 0.007 nm for the Oct-silicon and OTS-silicon surfaces, respectively.14 The CAHs are relatively small, i.e., are approximately 16° for Oct-silicon and 7° for OTS-silicon surfaces. Therefore, both the solid surfaces used in the experiments can be considered as morphologically and chemically uniform. These surface characteristics of the hydrophobic surfaces present a distinctive feature of our studies into evaporation of nanoparticle sessile droplets. Surface charge of the colloidal particles in the suspensions is an important parameter that determines the electrostatic interactions between the colloidal particles and the silicon surfaces. The surface potentials of the SiO2 and BI nanoparticles measured at two salt concentrations of 1 and 10 mM KCl are described in Table 1. Both SiO2 and BI nanoparticles Table 1. Surface Potential (mV) of the Colloidal Particles and the Solid Surfaces KCl concentration 1 mM SiO2 nanoparticles BI nanoparticles Oct-silicon OTS-silicon

−47.45 −56.39 −31.07 −30.18

± ± ± ±

1.34 0.59 0.34 (pH 6) 0.31 (pH 6)

10 mM −24.68 −33.93 −14.20 −9.95

± ± ± ±

1.67 0.61 0.26 (pH 6) 0.29 (pH 6)

possess negatively charged surfaces in the KCl solutions. When the salt concentration increases from 1 to 10 mM, the magnitude of surface potential of the particles decreases. The SEM images (Figure 1) and ZetaPlus size results (Figure 2)

Figure 1. SEM images of silica (left) and BI (right) nanoparticle (dry) samples. The bars show the scales of 100 and 1000 nm for the left and right images, respectively.

Figure 2. Particle size distribution of silica (Snowtex-20L) nanoparticles and BI nanoparticles. The median diameter of SiO2 and BI particles is about 57 and 147 nm, respectively.

show that the SiO2 particles have a spherical shape with the median diameter of about 57 nm, while BI particles have an irregular shape with the median diameter of about 147 nm. It is 4709

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Figure 3. Effect of salt (KCl) concentration and surface hydrophobicity on the size, shape, and pattern of residual deposits produced by evaporation of 0.5 μL droplets of SiO2 and BI nanoparticle suspensions on hydrophobized surfaces of silica wafers.

were pinned from the beginning (R/R0 = 100%) by a very small amount of BI particles, as low as 0.01 wt %, together with 10 mM KCl. However, no such “coffee ring” of the SiO2 particles on this surface was formed by increasing both the SiO2 concentration and salt concentration. Even with very high particle concentration, as high as 20 wt % SiO2 particles, the droplets only formed ICRDs at R/R0 = 80%. It is emphasized here that in this comparison the BI concentration used was about 2 orders of magnitude smaller than the SiO2 particle concentration. Similar behaviors of the SiO2 and BI nanoparticle droplets were observed on the more hydrophobic OTSsilicon surfaces. However, the ICRD radii and their normalized radii, R/R0, were always smaller than the corresponding ICRD radii on the Oct-silicon surfaces (Figure 5). Besides the different sizes of the deposit rings, the formation of complex structures within the rings, including the dendrite and crystalline structures, was also observed, but only when KCl salt was added to the droplets (Figures 3−5). At low salt concentrations (1 mM KCl), the fractal patterns, which can be explained by the well-known diffusion-limited-aggregation mechanism,9 normally occupied the whole area inside the rings. At 10 mM KCl, both fractal patterns and single salt crystals were observed. It is noted that the fractal patterns are just developed around single crystals in this case. The

needlelike crystals were also formed at high salt concentration and low particle concentration. It is noted that the fractal patterns were formed not only inside the rings but also on the top surface of the rings, as in the case of 1 wt % SiO2 particles in 10 mM KCl (Figure 6). This result confirms that the TPCL can climb over the surface of the deposit rings when it starts receding. 3.3. Evaporation Dynamics of Nanoparticle Sessile Droplets. To understand how evaporation dynamics affects the residual deposit patterns, we filmed the evolution of pure water and nanoparticle droplets from the side view at the rate of 15 fps. The time dependences of contact angle and base radius of these droplets are shown in Figure 7, 8, and 9. Evaporation of a pure water droplet on the Oct-silicon surface involves three distinctive modes: pinned (constant contact radius or CCR), receding (constant contact angle or CCA), and simultaneously mixed mode, where neither contact radius nor contact angle is constant. The contact angle decreases during the first mode, while the base radius decreases during the second mode. After contact angle reaches a critical value (θ*), the TPCL starts receding and shrinking toward the droplet center. Near the end of the droplet lifetime (∼0.5% of the droplet lifetime or less) the mixed mode of evaporation was observed when both contact angle and base radius simulta4710

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Figure 4. Effect of BI nanoparticle concentration and KCl salt concentration on the size and patterns of residual deposits produced by evaporation of 0.5 μL nanoparticle droplets on a hydrophobized surfaces of Oct-silicon wafer with θ0 = 80.7°.

droplets on a solid surface with a significant contact angle hysteresis (CAH). The Oct-silicon wafer surface has a low CAH due to its surface smoothness. Consequently, the duration of the pinned mode of the pure water droplet TPCLs was quite short, less than 20% of the total evaporation time. Evaporation dynamics is different in the presence of nanoparticles and salts. Figure 7 shows that adding 1 wt % SiO2 nanoparticles prolonged the first (pinned) mode and reduced the receding rate of the TPCLs during the second (receding) mode. Remarkably, we also observed an additional (third) pinned mode after the (second) receding mode. It is called the “late pinned mode” to distinguish with the first pinned mode. This late pinned mode occurred earlier in the presence of salts in the nanoparticle droplet than in the absence of the salts. For instance, a droplet of 1 wt % SiO2 could be repined at about 90% of droplet lifetime, while a 1 wt % SiO2 in 10 mM KCl could repin the TPCLs as earlier as at about 80% of droplet lifetime. In the presence of the BI particles, the evaporation dynamics of the sessile droplets also includes the early (first) pinned, receding, late (second) pinned, and simultaneously mixed mode of evaporation as described earlier for the SiO2 nanoparticle suspension droplets (Figure 7B and D). Noticeably, there is a significant prolonging of the early pinned mode of the evaporation of the BI nanoparticle suspension droplets. The TPCL is pinned at its initial mode up to 40% of the total evaporation time of 0.005 wt % BI particle suspension droplets, while it is just about 15% for a pure water droplet. The extension of the early pinned mode was also observed in the case of SiO2 nanoparticles, but the BI nanoparticles displays a much stronger impact on the prolonging of the early pinned

Figure 5. Normalized radius of the BI nanoparticle ring deposits formed on the Oct-silicon (solid line) and OTS-silicon (dashed line) surfaces vs particle concentration and salt concentrations.

Figure 6. Dendrite structures and a single crystal formed by evaporation of 0.5 μL nanoparticle droplets of 1.0% SiO2 particles (left) and 0.01% BI particles (right) on the hydrophobized Oct-silicon wafer surfaces.

neously decreased. However, this last mode sometime could only be detected and/or measured with great care due to limitations of the optical system.17 These combined modes of evaporation are consistent with the evaporation of sessile 4711

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Figure 7. Evaporation dynamics of nanoparticle droplets on the Oct-silicon wafer surfaces. Diagrams A and C on the left and diagrams B and D on the right are for the droplet base radius and contact angle of the SiO2 nanoparticle suspension droplets and of the BI nanoparticle suspension droplets, respectively. Evaporation time and base radius are respectively normalized by the total evaporation time and the initial base radius. The color scheme is uniquely applied to the symbols and the text.

Figure 8. Effect of nanoparticle concentration and salt concentration on the prolonging of the early (first) pinned mode of nanoparticle droplets on the Oct-silicon surface. The normalized pinned time of TPCLs increases with increasing the concentrations of the BI and SiO2 particles and KCl salt.

pinned mode took place from the beginning and ended at about 25% of the droplet lifetime, which is significantly shorter than 100% early pinning on the Oct-silicon surface. Additionally, the effect of 1 wt % SiO2 nanoparticles on the late pinning on the OTS-silicon surface became evident at longer time (>70% of the total evaporation time) than on the Oct-silicon surface. Evidently, the experimental results show that the concentration of nanoparticles, concentration of salts, and surface hydrophobicity as measured by contact angle and contact angle hysteresis significantly influence the formation of particle deposits, the deposit size and structures, and evaporation dynamics. It is very likely that the colloidal interactions between the nanoparticles and the solid surfaces are critical to the nanoparticle droplet evaporation and deposit formation. Shown below is a detailed analysis to unveil the role of the colloidal interactions.

mode. Comparing with silica nanoparticles, the BI nanoparticles require much lower (by 2 orders of magnitude) particle concentration to affect the initial pinned mode (Figure 8). The effect of salt (KCl) concentration on the pinned mode is also as strong as the particle concentration. Figure 9 shows the details of evaporation kinetics on the more hydrophobic OTS-silicon wafer surface. The evaporation also undergoes the fours typical modes as reported for the less hydrophobic Oct-silicon surface shown in Figure 7. However, since the OTS-silicon surface has a smaller CAH than the Octsilicon surface, the early pinned mode on the OTS surface is also shorter than on the Oct-silicon surface. The TPCLs of the pure water droplets on the OTS surface are only pinned during the first 10% of the total evaporation time. Nanoparticles and salts also significantly impact the dynamics of droplet contact angle and base radius. The late pinned mode was also observed in the presence of nanoparticles and salts. However, the impact of nanoparticles on the evaporation dynamics was weaker than on the Oct-silicon surface. It can be seen from Figure 9 that in the case of 0.005 wt % BI nanoparticles in 10 mM KCl the early 4712

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Figure 9. Evaporation dynamics of nanoparticle droplets on the strongly hydrophobic OTS-silicon wafer surfaces. Diagrams A and C on the left and diagrams B and D on the right are for the droplet base radius and contact angle of the SiO2 nanoparticle suspension droplets and of the BI nanoparticle suspension droplets, respectively.

Figure 10. Effect of salt concentration on reducing repulsion of colloidal (surface) force as the sum of van der Waals force given by eq 1 and electrical double-layer force given by eq 2 between the particle and the solid surface as a function of separation distance in 1 mM (solid lines) and 10 mM (dashed lines) KCl salt solutions. The red lines and blue line describe the two extreme cases of the net force with the edl interactions at the constant surface charge and potential conditions, respectively. The real interactions (e.g., the surface charge regulation) occur in the shaded areas between the two lines. The details are given in the Supporting Information.

lecular forces.7,18 The significant effect of salt on the nanoparticle deposit size as demonstrated in Figures 3 and 4 can only be explained by considering the change in colloidal attractive and repulsive forces between the particle and surface by increasing the salt concentrationthe attractive forces can

4. DISCUSSION The pinning and receding of TPCLs are due to the balance of competitive forces acting on the fluid molecules and particles at the three-phase contact zone. These forces include the hydrodynamic forces, frictional forces, and colloidal/intermo4713

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predicted by Langmuir.23 (It is noted that in the limit of small κh the force described by eq 1 and its disjoining pressure asymptotically approach respectively πrεε0(ψp + ψs)2/h and πrεε0(ψp + ψs)2/h2 as in the case of the Langmuir equation.23) Given that the contact period between the particles and the solid surface is relatively long, it is very likely that the actual edl interaction within the droplets is close to the interaction at the constant surface potential. The attractive surface force at short separation distances between the BI particles and the solid surfaces is stronger than the force between the silica particles and the surfaces as supported by the blue lines in Figure 10. Therefore, the ICRDs of the BI particles are larger than those of the silica particles (Figures 3 and 5). Also, the ICDRs on the Oct-silicon surfaces being larger than those on the OTS-silicon surfaces can be due to the faster reduction rate of the edl repulsion between the particles and the Oct-silicon surfaces since the salt concentration (the evaporation rate) increases faster in the droplets evaporating on the less hydrophobic Octsilicon surfaces than in the droplets evaporating on the more hydrophobic OTS-silicon surfaces. In addition to the colloidal forces, the formation of the particle deposits can be affected by the frictional force which is proportional to the net of the normal (perpendicular to the substrate surface) forces being able to pin the particles to the surface. The negative (attractive) net of the colloidal forces and the number of particles in contact with the solid surface are the important components of the frictional force. (The weight of the nanoparticles in water is of the order of 1 pN, which is smaller than the net of colloidal forces at contact (e.g., h = 0.1− 0.2 nm, where the colloidal forces can be balanced by the Born repulsion and can be neglected in the force balance.) It is very likely that the nanoparticle aggregates or layers enhance the initial pining of TPCL, but the friction force between the particle aggregates and the surface is not sufficiently large to pin the particles to the surface to form the particle deposits during the initial pinning of TPCL. Since the friction force increases with increasing the net of the colloidal forces at the contact between the particle and the solid surface and number of particles and/or their aggregates, increasing the initial particle concentration can lead to an increase in the ICRD radius as shown in Figures 3−5. Significantly, the friction force (being proportional to the friction coefficient) between the BI particles (of irregular shape) and the substrate surface can be much larger than that between the SiO2 spherical particles and the substrate surfaces. The difference in the friction coefficient between the BI and SiO2 particles may explain the different effects of the particle concentration on the ICRD radius (Figures 3). Indeed, in order to create an ICRD of SiO2 particles having a similar radius of 0.01 wt % BI particle deposits, the wt % concentration of SiO2 particles required would be higher than the BI concentration by 1 order of magnitude (Figures 3−5), while the particle number concentration required would be different by 2 orders of magnitude as discussed in section 3.2. It is argued in the literature1,24,25 that the coffee rings occur on the hydrophilic surfaces, i.e., when the initial contact angle is smaller than 90°. However, many of these experimental results have shown that the formation of coffee rings does not critically depend on the initial contact angle. So, how does the surface hydrophobicity affect the coffee ring formation? Our previous study14 shows that the evaporative flux at the air−liquid interface is not constant but changes along the interface and depends on the actual contact angle of the droplet with the

pin the particles to the surface, while the repulsive forces can expel the particles from the surface for further transportation by the water flow. Specifically, since both the nanoparticles and flat surfaces are negatively charged as quantified by the surface potentials in Table 1, the electrical double-layer interaction is repulsive and can be weakened by increasing the salt concentration. The van der Waals (vdW) interaction between the particles and surfaces is attractive but can also be affected by increasing the salt concentration as per the microscopic Lifshitz theory based on quantum mechanics.18 The edl and vdW forces are the two key components of the celebrated DLVO theory on colloid stability.19,20 Many other non-DLVO forces are also known but less affected by salts. Therefore, only the DLVO forces are analyzed here to examine the significant effect of salts. For simplicity, the well-known Hogg−Healy−Fuerstenau approximation is used to calculate the electrostatic double layer (edl) interaction force as a function of the shortest surface-tosurface separation distance, h, as follows:21 Fedl = 2πrεε0κ

2ψpψs exp(κh) ∓ (ψp2 + ψs 2) exp(2κh) − 1

(1)

where ψp and ψs are the particle and substrate surface potentials (Table 1), respectively, ε0 = 8.85418 × 10−12 C2 J−1 m−1 is the dielectric constant of vacuum, ε = 78 is the water dielectric constant, κ is the reciprocal of the Debye length, r is the particle radius, and the minus (plus) sign is used for the interaction at the constant surface potential (charge). At room temperature (25 °C), we have κ [nm−1] = 3.288√c, where c is the KCl concentration in mol/L.18 The van der Waals force is derived from the combined Hamaker−Lifshitz theory as follows:18 r FvdW = −A(κ , h) 2 (2) 6h where the Hamaker−Lifshitz function, A, in high dielectric constant medium of water22 is a function of the salt concentration and separation distance and can be determined using the electromagnetic properties (the refractive index) of the particle and the substrate as shown in the Supporting Information. It is noted that the Hamaker−Lifshitz function reduces to the Hamaker constant at very small (zero) separation distances, which is about 8.91 × 10−21 and 4.49 × 10−21 J for the BI and SiO2 particle−surface interactions, respectively. Figure 10 shows the net, Fedl + FvdW, of the colloidal forces between the nanoparticles and the surfaces as a function of short intersurface separation distance in the salt solutions. Increasing the salt concentration reduces the edl repulsion via the reduced magnitude of the surface potentials shown in Table 1 and the thickness of the double layers (κ−1), leading to decreasing the repulsive force between the particles and the substrate surfaces. The Hamaker−Lifshitz function is also reduced by the salt (κ) because its zero-frequency term is essentially of electrostatic origin. For simplicity, only the colloidal edl interactions under the condition of either constant surface potential or constant surface charge are considered. The actual interaction occurs between the two extreme cases, depending on the contact time between surfaces and surface charge regulation.18 For example, if the contact between the surfaces (such as colliding particles in a mixing tank) is very brief, the interaction is closer to constant surface charge as first 4714

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solid surface.27 For instance, the evaporation is faster at the TPCL zone than at the center of the gas−liquid interface of sessile droplets when θ0 < 90°. This nonuniform evaporative flux is believed to be the origin of the outward radial liquid flow inside the droplets. However, the evaporative flux is different when θ0 > 90°. Although the evaporation rate decreases toward the droplet edge at the TPCL zone, the exact analytical solutions also show similar outward flow patterns for θ0 > 90°.26 Therefore, the coffee rings seem to form regardless of whether or not the initial contact angle θ0 < 90° as observed in this paper. However, the strongly hydrophobic OTS-silicon surface with θY > 90° requires a higher concentration of particles to successfully pin the TPCL than the Oct-silicon surface (Figure 3). Consequently, the effect of nanoparticles on the droplet evaporation dynamics and the ICRD size is not as strong as on the less hydrophobic Oct-silicon surface as we have observed in the experiments.

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ASSOCIATED CONTENT

S Supporting Information *

Details about the Hamaker−Lishfitz function. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +61 7 336 53665; Fax +61 7 336 54199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the University of Queensland for the international postgraduate scholarship awarded to T.A.H.N. This research was supported under Australian Research Council’s Linkage Projects funding scheme (project number LP0989217).

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5. CONCLUSIONS The evaporation of nanoparticle droplets and their residual depositions on atomically smooth hydrophobic surfaces have been studied. The inner coffee ring deposits (ICRDs) with different sizes, patterns, and structures (dendrites and single crystals) have been determined using the silica nanoparticles and BI nanoparticles and the hydrophobic Oct-silicon surfaces (θ0 = 80.7 ± 1.4°) and OTS-silicon surfaces (θ0 = 102.3 ± 1.6°). The hydrophobic solid surfaces were nanoscopically smooth as revealed by AFM measurements showing the surface roughness being smaller than 0.2 nm. Both the hydrophobic surfaces have also exhibited a small contact angle hysteresis. The silica nanoparticles were spherical and of the median diameter of 57 nm, while the BI particles with the median diameter of 147 nm had an irregular and platelike shape. Both the particles and solid surfaces were negatively charged in water and KCl solutions. With increasing the salt concentration, the surface potentials and the repulsive surface forces reduced. In place of the classical coffee rings (on rough surfaces) whose base contact radius would be unchanged with time, we have observed the ICRDs with the base radius being smaller than the initial contact base radius of the nanoparticle suspension droplets. The size of ICRDs strongly depended on the solid surface hydrophobicity, the particle concentration, and the KCl salt concentration: the radius of the ICRDs increased with increasing the particle concentration and the salt concentration. The kinetics of the droplet evaporation was also influenced by the particle and salt concentrations. Increasing the particle and salt concentrations prolonged the early pinned mode. In the presence of the nanoparticles and salt, up to four modes of evaporation could be observed, namely, the early pinned mode (CCR), the receding mode (CCA), the late pinned mode (CCR), and the simultaneously mixed mode, where neither the contact base radius nor the contact angle was constant. The detailed analysis has shown the important role of the solid surface roughness, the solid surface contact angle and contact angle hysteresis, the particle and salt concentrations, the particle shape, and the colloidal surface forces such as the electrical double-layer forces. Finally, it can be argued that the size and structure of nanoparticle deposits can be made and controlled by a proper choice of the constituents of particles and salt concentration. These principles demonstrated here allow for a more precise method for making nanoparticle covered surfaces for many important applications.

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