Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport

Quantifying the Microstructure of Dried Deposits Using Height-Height Correlation Function

Colloidal self-assembly has garnered significant attention in recent research, owing to applications in medical and engineering domains. Understanding the arrangement of particles in self-assembled systems is crucial for comprehending the underlying physics and synthesizing complex nano- and microscale structures. In this study, we introduce a novel methodology for analyzing the spatial distribution of particles in colloidal assemblies, focusing specifically on quantifying the microstructure of deposits formed by the evaporation of colloidal particle-laden drops. Utilizing a height-height correlation-function-based approach, we quantify variations in the height profile of deposits in radial and azimuthal directions. This approach enables the classification of the patterns into typical examples encountered in an evaporation-driven assembly. The method is demonstrated to be robust for quantifying synthetic and experimentally obtained deposit patterns, exhibiting excellent agreement in the estimated parameters. The mapping developed between pattern morphology and the quantitative measures introduced in this work may be used in a variety of applications including disease diagnosis as well as in developing pattern recognition tools.

Versatile Deposition of Complex Colloidal Assemblies from the Evaporation of Hanging Drops

Existing strategies designed to produce ordered arrangements of colloidal particles on solid supports are of great interest for their wide range of applications, from colloidal lithography, plasmonic and biomimetic surfaces to tags for anti-counterfeiting, but they all share various degrees of complexity hampering their facile implementation. Here, a drastically simplified methodology is presented to achieve ordered particle deposition, consisting in adding micromolar amounts of cationic surfactant to a colloidal suspension drop and let it evaporate in an upside-down configuration. Confinement at the air/water interface enables particle assembly into monolayers, which are then transferred on the substrate producing highly ordered structures displaying vivid, orientation-dependent structural colors. The method is compatible with many particle types and substrates, while controlling system parameters allows tuning the deposit size and morphology, from monocrystals to polycrystalline disks and "irises", from single-component to crystal alloys with Moiré patterns, demonstrating its practicality for a variety of processes.

Dried blood drops on vertical surfaces

The analysis of structures in dried droplets has made it possible to detect the presence and conformational state of macromolecules in relevant biofluids. Therefore, the implementation of novel drying strategies for pattern formation could facilitate the identification of biomarkers for the diagnosis of pathologies. We present an experimental study of patterns formed by evaporating water-diluted blood droplets on a vertical surface. Three significant morphological features were observed in vertical droplet deposits: (1) The highest concentration of non-volatile molecules is consistently deposited in the lower part of the droplet, regardless of erythrocyte concentration. (2) The central region of deposits decreases rapidly with hematocrit; (3) At high erythrocyte concentrations (36-40% HCT), a broad coating of blood serum is produced in the upper part of the deposit. These findings are supported by the radial intensity profile, the relative thickness of the crown, the aspect ratio of the deformation, the relative area of the central region, and the Entropy of the Gray Level Co-occurrence Matrix Entropy (GLCM). Moreover, we explore the pattern formation during the drying of vertical blood drops. We found that hematocrit concentration has a significant impact on droplet drying dynamics. Finally, we conducted a proof-of-concept test to investigate the impact of vertical droplet evaporation on blood droplets with varying lipid concentrations. The results revealed that it is possible to differentiate between deposits with normal, slightly elevated, and moderately elevated lipid levels using only the naked eye.

Morphologies of electric-field-driven cracks in dried dispersions of ellipsoids

We report an experimental and theoretical study of the morphology of desiccation cracks formed in deposits of hematite ellipsoids dried in an externally applied alternating current (ac) electric field. A series of transitions in the crack morphology is observed by modulating the frequency and the strength of the applied field. We also found a clear transition in the morphology of cracks as a function of the aspect ratio of the ellipsoid. We show that these transitions in the crack morphology can be explained by a linear stability analysis of the equation describing the effective dynamics of an ellipsoid placed in an externally applied ac electric field.

Universal scaling law for electrified sessile droplets on a lyophilic surface

Electrified sessile droplets on solid surfaces are ubiquitous in nature as well as in several practical applications. Although the influence of electric field on pinned sessile droplets and soap bubbles has been investigated experimentally, the theoretical understanding of the stability limit of generic droplets remains largely elusive. By conducting a theoretical analysis in the framework of lubrication approximation, we show that the stability limit of a sessile droplet on a lyophilic substrate in the presence of an electric field exhibits a universal power-law scaling behavior. The power-law exponent between the critical electric field and the droplet volume is found to be -1. The existence of this scaling law is further explained by virtue of minimization of the total free energy of the electrified droplet.

Kinetics of evaporation of colloidal dispersion drops on inclined surfaces

Evaporation of colloidal dispersion drops leaves a deposit pattern where more particles are accumulated at the edge, popularly known as the coffee-ring effect. Such patterns formed from dried sessile drops are azimuthally symmetric. When the substrate is inclined, the symmetry of the patterns is altered due to the influence of gravity. This is reflected in the changes in (i) pinning/depinning dynamics of the drop, (ii) the strength of the evaporation-driven flows, and (iii) ultimately, the lifetime of the drop. We present a systematic investigation of the kinetics of evaporation of particle-laden drops on hydrophilic inclined solid substrates. The angle of inclination of the substrate (ϕ) is varied from 0° to 90°. The temporal analysis of the drop shape profile is carried out to unearth the contribution of different processes to kinetics of evaporation of drops on inclined surfaces. The influence of particle concentration, drop volume, and angle of inclination on the kinetics of evaporation and the resulting deposit patterns are discussed.

Drying of bio-colloidal sessile droplets: Advances, applications, and perspectives

Drying of biologically-relevant sessile droplets, including passive systems such as DNA, proteins, plasma, and blood, as well as active microbial systems comprising bacterial and algal dispersions, has garnered considerable attention over the last decades. Distinct morphological patterns emerge when bio-colloids undergo evaporative drying, with significant potential in a wide range of biomedical applications, spanning bio-sensing, medical diagnostics, drug delivery, and antimicrobial resistance. Consequently, the prospects of novel and thrifty bio-medical toolkits based on drying bio-colloids have driven tremendous progress in the science of morphological patterns and advanced quantitative image-based analysis. This review presents a comprehensive overview of bio-colloidal droplets drying on solid substrates, focusing on the experimental progress during the last ten years. We provide a summary of the physical and material properties of relevant bio-colloids and link their native composition (constituent particles, solvent, and concentrations) to the patterns emerging due to drying. We specifically examined the drying patterns generated by passive bio-colloids (e.g., DNA, globular, fibrous, composite proteins, plasma, serum, blood, urine, tears, and saliva). This article highlights how the emerging morphological patterns are influenced by the nature of the biological entities and the solvent, micro- and global environmental conditions (temperature and relative humidity), and substrate attributes like wettability. Crucially, correlations between emergent patterns and the initial droplet compositions enable the detection of potential clinical abnormalities when compared with the patterns of drying droplets of healthy control samples, offering a blueprint for the diagnosis of the type and stage of a specific disease (or disorder). Recent experimental investigations of pattern formation in the bio-mimetic and salivary drying droplets in the context of COVID-19 are also presented. We further summarized the role of biologically active agents in the drying process, including bacteria, algae, spermatozoa, and nematodes, and discussed the coupling between self-propulsion and hydrodynamics during the drying process. We wrap up the review by highlighting the role of cross-scale in situ experimental techniques for quantifying sub-micron to micro-scale features and the critical role of cross-disciplinary approaches (e.g., experimental and image processing techniques with machine learning algorithms) to quantify and predict the drying-induced features. We conclude the review with a perspective on the next generation of research and applications based on drying droplets, ultimately enabling innovative solutions and quantitative tools to investigate this exciting interface of physics, biology, data sciences, and machine learning.

A scaling law of particle transport in inkjet-printed particle-laden polymeric drops

Hydrogels with embedded functional particulates are widely used to create soft materials with innovative functionalities. In order to advance these soft materials to functional devices and machines, critical technical challenges are the precise positioning of particulates within the hydrogels and the construction of the hydrogels into a complex geometry. Inkjet printing is a promising method for addressing these challenges and ultimately achieving hydrogels with voxelized functionalities, so-called digital hydrogels. However, the development of the inkjet printing process primarily relies on empirical optimization of its printing and curing protocol. In this study, a general scaling law is proposed to predict the transport of particulates within the hydrogel during inkjet printing. This scaling law is based on a hypothesis that water-matrix interaction during the curing of inkjet-printed particle-laden polymeric drops determines the intra-drop particle distribution. Based on the hypothesis, a dimensionless similarity parameter of the water-matrix interaction is proposed, determined by the hydrogel's water evaporation coefficient, particle size, and mechanical properties. The hypothesis was tested by correlating the intra-drop particle distribution to the similarity parameter. The results confirmed the scaling law capable of guiding ink formulation and printing and curing protocol.

Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications

The uneven deposition at the edges of an evaporating droplet, termed the coffee-ring effect, has been extensively studied during the past few decades to better understand the underlying cause, namely the flow dynamics, and the subsequent patterns formed after drying. The non-uniform evaporation rate across the colloidal droplet hampers the formation of a uniform and homogeneous film in printed electronics, rechargeable batteries, etc., and often causes device failures. This review aims to highlight the diverse range of techniques used to alleviate the coffee-ring effect, from classic methods such as adding chemical additives, applying external sources, and manipulating geometrical configurations to recently developed advancements, specifically using bubbles, humidity, confined systems, etc., which do not involve modification of surface, particle or liquid properties. Each of these methodologies mitigates the edge deposition via multi-body interactions, for example, particle–liquid, particle-particle, particle–solid interfaces and particle–flow interactions. The mechanisms behind each of these approaches help to find methods to inhibit the non-uniform film formation, and the corresponding applications have been discussed together with a critical comparison in detail. This review could pave the way for developing inks and processes to apply in functional coatings and printed electronic devices with improved efficiency and device yield.

An Experimental Investigation of Evaporation of Ethanol-Water Droplets Laden with Alumina Nanoparticles on a Critically Inclined Heated Substrate

We experimentally investigate the evaporation of water-ethanol binary sessile droplets loaded with alumina nanoparticles on a critically inclined heated surface and compare it to the no-loading condition. In contrast to a droplet of pure fluids, several distinct and interesting phenomena observed in a binary-nanofluid droplet on a critically inclined substrate are reported for the first time. The critical angle at which a droplet begins to slide increases for ethanol-rich binary droplets up to 0.6 wt % nanoparticle loading. The critical angle for binary droplets also increases as the substrate temperature increases and as the ethanol concentration decreases for modest loading conditions. It is observed that the advancing side of a binary droplet is pinned in both the loading and no-loading scenarios, whereas the receding side is pinned in the loading case but shrinks continuously in the no-loading case. The pinning effect caused by nanoparticles results in a larger perimeter and surface area for the nanoparticle-laden droplets, enhancing the evaporation rates and significantly decreasing the lifetime of the nanoparticle-containing droplets compared to the no-loading case. Increasing the ethanol percentage in the binary droplet placed on an inclined substrate produces complex thermosolutal Marangoni convection, which becomes more affluent in the case of nanoparticles loading than the no-loading condition. The radial symmetry of the circular coffee ring structure observed on a horizontal surface is shattered in the inclined case because the droplet elongates and preferentially deposits toward the advancing side of the triple line due to the action of the body force. Despite its fundamental nature, the present study can contribute to understanding many practical applications.

An experimental and theoretical study of the inward particle drift in contact line deposits

The coffee ring effect, which refers to the formation of a ring-like deposit along the periphery of a dried particle laden sessile drop, is a commonly observed phenomenon. The migration of particles from the interior to the edge of a drying drop as a result of evaporation driven flow directed outwards, is well studied. In this article, we document the inward drift of a coffee stain, which is governed by the descent of the water-air interface of the drying drop due to solvent evaporation. A combination of experimental study and model predictions is undertaken to elucidate the effect of the diameter of particles in the drying drop, the wettability of the substrate on which the drop resides, and the concentration of particles on the inward drift of the coffee stain. This work also suggests a novel method to estimate the coefficient of friction between the particles and the substrate.

Patterns from drops drying on inclined substrates

The coffee ring effect results from the migration of particles in a drying particle laden drop and their subsequent deposition at the three phase contact line. The evaporative flux during the drying of sessile drops and the spatial distribution of particles in the coffee ring patterns exhibit azimuthal symmetry. It is possible to break this symmetry with the help of gravity by simply manipulating the inclination of the substrate on which the colloidal droplet undergoes drying. However, the effect of particle size, substrate wettability and inclination angle on the extent of asymmetry in the spatial distribution of particles over the deposit patterns has not been explored and is the subject of the current work. Our experiments on the drying of aqueous dispersions of polystyrene particles show that (i) asymmetry in the deposition of particles is observed irrespective of the diameter of the dispersed particles in the drying drop (ii) the degree of asymmetry increases with a decrease in wettability of the drop on the substrate and (iii) it is a non-monotonic function of the inclination angle of the substrate. These results indicate the possibility of additional particle transport mechanisms working in tandem with evaporation driven capillary flows and demand further investigation of the physics of pattern formation in drops drying on oriented substrates.

Evaporation and deposition of inclined colloidal droplets

Colloidal droplets on flat solid substrates commonly leave symmetric ring-like deposits due to coffee-ring flows during evaporation. On inclined substrates, droplet shapes may become asymmetric by gravity. On this basis, it is not clear how their evaporation dynamics and final deposits are changed depending on inclination. Here we explore evaporation and deposition dynamics of colloidal droplets on inclined substrates, mainly by controlling colloidal particle size, substrate inclination, and relative humidity, which are crucial to gravitational intervention and evaporation dynamics. We experimentally investigate two different flows with opposite directions: downward sedimentation flows by gravity (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$v_s$$\end{document}vs) and upward capillary flows by evaporation (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$v_c$$\end{document}vc). We find that the competition of two flows determines the formation of final deposits with a flow speed ratio of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha = v_s/v_c$$\end{document}α=vs/vc. Notably, for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha$$\end{document}α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ll$$\end{document}≪ 1, evaporation-driven upward flows overwhelm sedimentation-driven downward flows, resulting in accentuated particle movement towards the top ring, which seems to defy gravitational intervention. We suggest a possible explanation for the flow speed dependence of final deposits in evaporating colloidal droplets. This study offers a framework to understand the intervention of inclination to the formation of final deposits and how to overcome the deposit pattern radial asymmetry, achieving symmetric deposit widths from inclined colloidal droplets.

Further Insights into Patterns from Drying Particle Laden Sessile Drops

The evaporation of colloidal dispersions is an elegant and straightforward route to controlled self-assembly of particles on a solid surface. In particular, the evaporation of particle laden drops placed on solid substrates has received considerable attention for more than two decades. Such particle filled drops upon complete evaporation of the solvent leave behind a residue, commonly called particulate deposit pattern. In these patterns, typically, more particles accumulate at the edge compared to the interior, a feature observed when coffee drops evaporate. Consequently, such evaporative patterns are called coffee stains. In this article, the focus is on the evaporation of highly dilute suspension drops containing particles of larger diameters ranging from 3 to 10 μm drying on solid substrates. This helps us to investigate the combined role of gravity-driven settling of particles and capillary flow-driven particle transport on pattern formation in drying drops. In the highly dilute concentration limit, the evaporative patterns are found to show a transition, from a monolayer deposit that consists of a single layer of particles, to a multilayer deposit as a function of particle diameter and initial concentration of particles in the drying drop. Moreover, the spatial distribution of particles as well as the ordering of particles in the deposit patterns are found to be particle size dependent. It is also seen that the order-disorder transition, a feature associated with the organization of particles at the edge of the deposit, observed typically at moderate particle concentrations, disappears at the highly dilute concentrations considered here. The evaporation of drops containing particles of 10 μm diameter, where the effect of gravity on the particle becomes significant, leads to uniform deposition of particles, i.e, suppression of the coffee-stain effect and to the formation of two-dimensional percolating networks.

Particle size and substrate wettability dependent patterns in dried pendant drops

The particle laden sessile drops when dried on solid surfaces under certain conditions leave a deposit pattern wherein all the particles are confined to a narrow region close to the edge of the deposit. Such patterns which often form when coffee drops dry are referred to as the coffee ring patterns or the coffee stains. Recent research points to the formation of intriguing patterns when colloidal particle laden drops are dried in configurations other than sessile mode. In this article, the combined effect of particle size and wettability of the substrate on the patterns formed by drying drops in sessile and pendant configurations is investigated via experiments. Our results demonstrate a transition from coffee ring to central dome-like deposit morphology with decrease in wettability of the substrates when drops containing 3 μm diameter particles are dried in pendent mode. A similar transition in the deposit morphology is observed with increase in the diameter of the particles in pendant drops dried on substrates of near neutral wettability (θ = 86 ± 3°). The influence of particles size, substrate wettability and drop configuration on the kinetics of deposition of particles at the three phase contact line will also be discussed. We compare our experimental observations with particle based simulations wherein the dried patterns are generated by accounting for three particle transport modes, namely, advective particle transport resulting from capillary flow, gravity driven settling of particles and particle capture by descending interface.

Disk-Ring Deposition in Drying a Sessile Nanofluid Droplet with Enhanced Marangoni Effect and Particle Surface Adsorption

The present study is to explore the central particle deposition from drying a sessile nanofluid droplet experimentally and theoretically. Normally, a pinned colloidal droplet dries into a coffee-ring pattern as a result of moving the particles to a three-phase line by the radial direction capillary flow. However, the strong evaporation can generate the nonuniform temperature at the evaporating droplet interface and the droplet periphery temperature is higher than that close to the droplet centerline. The induced Marangoni flow would reversibly transport the particles at the periphery toward the centerline. We have thus designed the experiments to increase the droplet evaporation rate in vacuum conditions and accordingly to enhance the Marangoni effect. We have observed distinguishable disk deposition inside the outer coffee ring. A three-dimensional diffusion-limited cluster-cluster aggregation Monte Carlo model has been developed to simulate the deposition process. With modeling the Marangoni effect, particle adsorption at the liquid-air interface and particle aggregation behaviors, the formation of the disk pattern inside a coffee ring has been simulated. The qualitative agreement has been found in the comparison of local deposition distribution between the related experiment and simulation.

Evaporation of Inclined Drops: Formation of Asymmetric Ring Patterns

Evaporation of colloidal drops on horizontal surfaces deposits the contained particles at the drop-edge producing radially symmetric ring-like stains. The symmetry in the particle deposition is broken when the drop is placed on a tilted surface due to the influence of gravity on the suspended particles and the drop itself. Using extremely small drops generated by electrospray, we explore cases where different mechanisms of particle transport dominate. We show that the asymmetric residues are formed as the gravity-induced effects compete with the capillary flow. Our results give a broad insight into the pattern formation of evaporating inclined drops.

Beyond Coffee Rings: Drying Drops of Colloidal Dispersions on Inclined Substrates

The patterns resulting from drying particle-laden sessile drops (for example, coffee rings, where the particles are concentrated more at the edge, and their complete suppression, where the particles are uniformly distributed throughout the pattern) have been well studied for more than two decades. For the ubiquitous instance of occurrence of drying of drops containing nonvolatile species (either dissolved or dispersed) on substrates oriented at different angles with respect to gravity, the investigation of resulting evaporative patterns has not received much attention. This mini-review addresses the need to investigate the drying of drops residing on inclined surfaces and highlights recent advances in this field.

Modulation of Central Depletion Zone in Evaporated Sessile Drops via Substrate Heating

In this article, we report the influence of substrate temperature (T_sub) on the evaporation driven patterning of colloids on solid substrates. When the drops are dried in an environment maintained at temperature, T_env, lower than T_sub, the temperature difference between the drop apex and the three-phase contact line leads to thermal Marangoni flow. We show that the interplay between the radial capillary flow, the thermal Marangoni flow, and the descending rate of the drop surface can be tuned to modulate the spatial distribution of colloids in the dried deposits. At ΔT (=T_sub - T_env) ≥ 45 °C, the distribution of particles in the interior region of the pattern is nearly uniform with a significant decrease in concentration of particles in the ring-like deposit at the edge. The deposits formed at 15 °C ≤ ΔT ≤ 40 °C are accompanied by a particle depleted zone in the center, which has not been reported to date. The formation of the central depletion zone arises from the suppression of the thermal Marangoni flow at the penultimate stage of drying and the interplay between the radial capillary flow and the descending rate of the drop surface. At ΔT < 15 °C, the dried deposits are found to exhibit coffee-ring-like stains.

Patterning of colloids into spirals via confined drying

Drying of complex fluids is a fascinating subject of interest to several growing fields, for example, forensic science, lithography, printing and coating technologies. In this article, we report that the drying of charge stabilized colloidal dispersions between two parallel plates is a route to intriguing self-assembly patterns. We show that when the dispersions are dried in parallel plate confinement, particles deposit as spiral patterns after the complete evaporation of the solvent irrespective of the confinement spacing. The formation of such patterns is understood by analyzing the underlying three phase contact line dynamics during the drying process. Compared to the usual discrete stick-slip motion of the contact line, typically observed in several drying configurations, in the parallel plate drying configuration, the contact line is found to exhibit continuous stick-slip motion. The de-pinning of the contact line is found to occur only locally and is observed to propagate in both clockwise and anticlockwise directions, leading to the patterning of colloids as spirals. Furthermore, we show that while the number of turns in the spiral deposit is influenced by the dispersion volume and particle concentration, the spiral patterns form irrespective of the shape of the particles in the dispersion.

Influence of the drying configuration on the patterning of ellipsoids - concentric rings and concentric cracks

Evaporation of colloidal dispersions leading to patterning of particles is a simple and elegant route for controlling the self-assembly of particles on a solid surface. In this article, we demonstrate that the configuration in which a colloidal dispersion is dried greatly influences the patterning of particles on a solid surface after complete evaporation of the solvent. Evaporation experiments are carried out using well-characterized stable aqueous dispersions of hematite ellipsoids and polystyrene spheres. The drying of particle laden sessile drops always give a "coffee-ring" deposit irrespective of the particle concentration. At a particle concentration ≥0.3 wt% circular cracks appear in the annular region of the coffee-ring deposit owing to the ordered arrangement of ellipsoids. In stark contrast, the deposits formed by drying the dispersion of ellipsoids in the sphere-on-plate configuration show a transition from "concentric rings" to "concentric cracks" in the micro-structure of the particulate film with an increase in the concentration of particles. Further, our experimental findings reveal that long-range circular cracks and long-range assemblies of particles can be achieved by drying of the dispersion in the sphere-on-plate configuration. While the nature of patterns - that is - coffee-rings and concentric rings - is independent of the shape of the particles, a strikingly different crack morphology is shown to be dictated by the shape of the particles in the dispersion. The results presented show that the drying of colloidal dispersions in the sphere-on-plate configuration enables the fabrication of a long range ordered assembly of particles over an area as large as few square millimeters.

Reply to "Comment on 'Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport'"

Hodges and Tangparitkul (Hodges, C. S.; Tangparitkul, S. M. Langmuir 2019, 35, doi: 10.1021/acs.langmuir.9b01442) in their Comment on "Patterns in Drying Drops Dictated by Curvature Driven Particle Transport" argue that the coffee-eye deposits in dried pendant drops can also be formed if the particles or particle clusters in the drying drop are large enough to sediment during the course of evaporation. In our reply to this comment, we compare these two different mechanisms, namely, gravity settling and curvature-driven interfacial migration of particles in the drying particle-laden drops, with an aim towards placing them in a correct perspective.

Comment on "Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport"

In a recent article, Mondal et al. (Mondal, R.; Semwal, S.; Kumar, P. L.; Thampi, S. P.; Basavaraj, M. G. Langmuir 2018, 34, 11473-11483) demonstrated different patterns (coffee-rings and coffee-eyes) in dry deposits from solutions of concentrated well-stabilized nanofluids. Coffee rings created from dried sessile droplets result mainly from internal radial flow as proposed by Deegan et al. (Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829). To generate coffee-eyes from pendent droplets, Mondal et al. have proposed a new particle transport route involving particle adsorption at the interface and its consequent curvature-driven settling along the interface due to gravity acting on the droplet. In this comment, we demonstrate that coffee-eyes can also be formed from pendent droplets by increasing the nanoparticle size to destabilize the colloidal liquid, causing nanoparticle accumulation at the water droplet apex without particle adsorption at the interface.