Reversing Coffee-Ring Effect by Laser-Induced Differential Evaporation (original) (raw)
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Langmuir, 2020
The present study experimentally and numerically investigates the evaporation and resultant patterns of dried deposits of aqueous colloidal sessile droplets, when the droplets are initially elevated to a high temperature before being placed on a substrate held at ambient temperature. The system is then released for natural evaporation without applying any external perturbation. We employed digital controlled heating and temperature measurement scheme of colloidal particle laden liquid, high-speed side visualization, high-speed infrared thermography, optical microscopy of fluorescent particle tracers and optical profilometry as essential tools for data acquisition. Initially, a significant temperature gradient exists along the liquid-gas interface as soon as the droplet is deposited on the substrate-the droplet remains the hottest at the top and the coolest near the contact line. The temperature gradient induces a Marangoni recirculation flow, which is directed from the top of the droplet towards the contact line along the liquidgas interface-thus, the flow is in reverse direction to that seen in conventional substrate heating case. Interestingly, this temperature gradient decays rapidly-within the first 10% of the total evaporation time and the droplet-substrate system reaches thermal equilibrium with ambient thereafter. Despite fast decay of the temperature gradient, the coffee-ring dimensions significantly diminish, leading to an inner deposit. The reproducibility of the observations has been confirmed by varying particle concentrations in the aqueous suspension. This suppression of the coffee-ring effect is attributed to the fact that the recirculation flow, generated by initial temperature gradient induced Marangoni stress, continues till the last stage of the evaporation, even after the interfacial temperature gradient vanishes. This is essentially a consequence of liquid inertia. Thus, the suspended colloidal particles are advected by this recirculating flow towards the inner region of the droplet, thereby suppressing the coffee-ring effect. Finally, a finite-element based twodimensional modeling in axisymmetric geometry has been developed and employed to verify the measurements. The modeling and measurements are in reasonable agreement and the hypothesis considered in the present study corroborates well with a first approximation qualitative scaling analysis. Overall, together with a new experimental condition, the present investigation discloses a distinct nature of Marangoni stress induced flow in the droplet and its role in influencing the associated colloidal deposits, which was not explored previously. The insights gained from this study are useful to advance technical applications such as spray cooling, ink-jet printing, bioassays, etc.
Overcoming the “Coffee-Stain” Effect by Compositional Marangoni- Flow-Assisted Drop-Drying
Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the "coffee-stain" effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles.
Soft Matter, 2011
We study the influence of electrowetting on the formation of undesired solute residues, so-called coffee stains, during the evaporation of a drop containing non-volatile solvents. Electrowetting is found to suppress coffee stains of both colloidal particles of various sizes and DNA solutions at alternating (AC) frequencies ranging from a few Hertz to a few tens of kHz. Two main effects are shown to contribute to the suppression: (i) the time-dependent electrostatic force prevents pinning of the three phase contact line and (ii) internal flow fields generated by AC electrowetting counteract the evaporation driven flux and thereby prevent the accumulation of solutes along the contact line.
Dynamics of liquid droplets in an evaporating drop: liquid droplet ''coffee stain'' effect
In this paper, we demonstrate the dynamics of bidispersed oil droplets in an evaporating water sessile drop. This phenomenon is therefore equivalent to a unique liquid droplet based ''coffee stain'' effect, with the depositing colloidal particles (of a classical ''coffee stain'' problem) being replaced by the oil droplets partially wetting the substrate. The important difference with respect to the classical ''coffee stain'' problem, as revealed by our experiments, is that the oil droplets, unlike the colloidal particles, cannot reach the contact line; rather the aversion of the oil droplets to the air ensures that the oil droplets always remain at a finite distance from the contact line. We call this effect an ''enclosure'' effect, characterized by this distance. We provide a theoretical model to explain this phenomenon, and our theoretical results match well with the experimental observations. The ''enclosure'' effect depends on the droplet size, thereby allowing an automatic size-based separation of the oil droplets. Additionally, this effect depends on the wettability of the oil droplets and the sessile drop, as well as the relative velocity of the oil droplets with respect to the rate of decrease of the sessile drop contact angle. Our identification of this new phenomenon in a liquid-droplet based ''coffee stain'' problem will have a huge impact on microscale control and manipulation of liquid droplets in a two phase system.
Toward Controlling Evaporative Deposition: Effects of Substrate, Solvent, and Solute
Understanding evaporative deposition from a colloidal suspension and on-demand control over it are important due to its industrial and biomedical applications. In particular, it is known that interactions among substrate, solute, and solvent have important consequences on evaporative depositions; however, how these are affecting the deposition patterns and at which conditions these interactions are prominent need detailed investigations. Here we report that the total time of deposition (t d) and the geometric shape of the droplet (L c = initial footprint diameter/height) have a significant role in determining the evaporative deposition patterns. We have identified four zones based on t d and L c , and found that with longer deposition time (high t d) and larger available space for particle motion within a liquid droplet (high L c), deposition patterns were governed by the interactions among the substrate, solute, and solvent. We also experimentally demonstrated that the pinned contact line is indispensable for the "coffee ring" effect by comparing the deposition on surfaces with and without hysteresis. The effect of the Marangoni flow is also discussed, and it is shown that by controlling Marangoni flow, one can manipulate the droplet deposition from uniform disk-like to coffee ring with a central deposition.
Amplifying and attenuating the coffee-ring effect in drying sessile nanofluid droplets
Physical Review E, 2013
Experiments and simulations to promote or attenuate the "coffee-ring effect" for pinned sessile nanofluid droplets are presented. The addition of surfactant inside a water suspension of aluminum oxide nanoparticles results in coffee-ring formation after the pinned sessile droplets are fully dried on a substrate, while droplets of the same suspension without the surfactant produce a fine uniform coverage. A mathematical model based on diffusion-limited cluster-cluster aggregation has been developed to explain the observed difference in the experiments. The simulations show that the particle sticking probability is a crucial factor on the morphology of finally dried structures.
Altering the coffee-ring effect by adding a surfactant-like viscous polymer solution OPEN
A uniform deposition of the suspended particles in an evaporating droplet is necessary in many research fields. Such deposition is difficult to achieve, because the coffee-ring effect dominates the internal flow in a droplet. The present study adopts a biocompatible, surfactant-like polymer (Polyethylene glycol, PEG) to break the coffee-ring effect and obtain a relatively uniform deposition of the microparticles with yielding multi-ring pattern over a droplet area. Movements of the suspended particles in evaporating droplets and deposition patterns of them on a glass substrate were analyzed with microscopic images and video files. The PEG in the droplets successfully altered the coffee-ring effect because of the surface tension variation, which induced a centripetal Marangoni flow. Balancing these two phenomena apparently generated the Marangoni vortex. For PEG solution droplets, the pinning–depinning process during evaporation was periodically repeated and multiple rings were regularly formed. In conclusion, adding a surfactant-like viscous polymer in a droplet could provide a uniform coating of suspended particles, such as cells and various biomaterials, which would be essentially required for droplet assays of biomedical applications. The evaporation of a sessile droplet, which is by itself an interesting encounter in everyday life, causes a complex , non-equilibrium fluid dynamic phenomenon 1. The fluid dynamic complexity of a drop is caused by the non-uniform evaporating flux over its gas–liquid interface and the thermal gradient–induced Marangoni stress 2. The liquid within the droplet will flow outward the edge when the Marangoni effect is negligible to compensate the large evaporation loss of liquid at the edge 3. Subsequently, the suspended particles are driven to the edge and deposited in a ring-like pattern, known as the coffee-ring effect 1, 2, 4. The coffee-ring phenomenon occurs when the droplet is pinned at its contact line 1, 2, 4. Occasional depinnings of the contact line may result in concentric rims on the solid surface 5. Poly-disperse particles are orderly deposited from the edge to the center in a size-dependent manner 6. The coffee-ring effect is observed for various particle sizes ranging from microparticles to nanoparticles 6, 7 , molecules 5 , and ions 5. However, the coffee-ring stain phenomenon is a crucial defect in fields of inkjet printing 8 , microdot array 9 , and other coating technologies that need uniform depositions. Uniform coating is also important for the sensing and detection assay in biotechnology. Probe materials, such as antibodies for virus detection, are commonly coated on a substrate as a droplet pattern. However, most probe materials are highly concentrated near an edge, and the surface contact area is highly restricted because of the coffee-ring effect. The probe sensitivity would then significantly decrease. Therefore, many research groups are making efforts to obtain a uniform deposition by suppressing the coffee-ring effect. A typical example is the Yunker et al. 's report 10 which demonstrated interaction between anisotropic particles suppresses the coffee-ring effect and lead to a uniform deposition of particles. This report was uniquely done without any chemical manipulation. In fact, there have been many notable reports to overcome coffee-ring effect with controlling liquid properties such viscosity and pH and altering temperatures of substrate and droplet. Cui et al. 11 demonstrated when an added hydrosoluble polymer increased the solution viscosity, resulting in a large resistance to the radially outward flow and subsequently a small amount of spheres deposited at droplet edge. Also, Bhardwaj et al. 12 demonstrated the effect of the pH of the solution on the deposit pattern, which is the results of force interactions between substrate and particles such as the electrostatic and van der Waals forces. Parsa et al. 13 reported that depending on the substrate temperature, three distinctive deposition patterns are observed: a nearly uniform coverage
Coalescence, evaporation and particle deposition of consecutively printed colloidal drops
The particle deposition dynamics of two consecutively printed evaporating colloidal drops is examined using a fluorescence microscope and a synchronized side-view camera. The results show that the relaxation time of the water–air interface of the merged drop is shorter than that of a single drop impacting on a dry surface. It is also found that both morphology and particle distribution uniformity of the deposit change significantly with varying jetting delay and spatial spacing between two drops. As the drop spacing increases while keeping jetting delay constant, the circularity of the coalesced drop reduces. For the regime where the time scale for drop evaporation is comparable with the relaxation time scale for two drops to completely coalesce, the capillary flow induced by the local curvature variation of the air–water interface redistributes particles inside a merged drop, causing suppression of the coffee-ring effect for the case of a high jetting frequency while resulting in a region of particle accumulation in the middle of the merged drop at a low jetting frequency. By tuning the interplay of wetting, evaporation, capillary relaxation, and particle assembly, the deposition morphology of consecutively printed colloidal drops can hence be controlled.
Beyond the coffee-ring effect: Pattern formation by wetting and spreading of drops
Physical Review E
Drying of colloidal drops on solid surfaces is the widely known method to form self-assembled patterns. The underlying principle of this method is the phenomenon known as the coffee-ring effect. Here, we report a phenomenon of pattern formation involving not drying but conversely wetting and spreading of drops on a solid surface containing a thin layer of dispersed particulates. Fascinating ringlike patterns are formed in a subsecond timescale by the interplay between the dynamics of spreading and imbibition. Occasionally, such patterns can be observed when, say, water spills on dusted floors and they are often misidentified as those formed by the coffee-ring effect. In the highly wetting scenario, we found that this pattern formation is independent of the liquid properties, however it is strongly dependent on the powder properties. Our findings have both fundamental and technological importance.
Beyond Coffee Rings: Drying Drops of Colloidal Dispersions on Inclined Substrates
ACS Omega
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.