Evaporation of pure liquid sessile and spherical suspended drops: a review

Evaporation-induced flows inside a confined droplet of diluted saline solution

Flow patterns inside a droplet of diluted aqueous NaCl solution confined by two flat substrates under natural evaporation were investigated both experimentally and numerically. We focused on natural convection-driven flows inside confined droplets at high Rayleigh numbers (i.e., the ratio of buoyancy to diffusion, Ra), where the convection of solutes is strongly dominant, compared to diffusion. The evaporated water at the free surface of the droplet builds up a concentration gradient inside the solution, which induces the Rayleigh convection flow. Three-dimensional trajectories of tracer particles in the droplet were tracked, and axisymmetric flow motions induced by the Rayleigh convection were experimentally measured by using a digital in-line holographic microscopy technique. In addition, the effects of the confined droplet's aspect ratio and the liquid's molar concentration on the evaporation-induced flows were investigated. The convection velocity is found to be increased as molar concentration increases, because Rayleigh convection becomes significant at high the molar concentration is high (i.e. high Ra). Our numerical simulation based on the Boussinesq approximation fairly well predicted the velocity profiles of evaporating confined droplets at low concentrations. Consequently, evaporation kinetics inside the confined droplets can be controlled with varying droplet's aspect ratio and the liquid's molar concentration, which provides helpful information for the design of biochemical microplating with limited resources and for tuning self-assembly micro/nanoparticle clusters.

Robust Cassie state of wetting in transparent superhydrophobic coatings

This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a double-layer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows.

Constructing robust liquid marbles for miniaturized synthesis of graphene/Ag nanocomposite

Miniaturized synthesis is attracting much attention due to many potential applications; a challenge remains in exploring versatile microreactors capable of producing pure products. In this study, we reported a kind of thermally robust liquid marbles and their application for miniaturized synthesis of graphene/Ag nanocomposite. The liquid marbles were constructed by using superhydrophobic Fe3O4/C microsheets as encapsulating agents. Results revealed that the morphology of the encapsulating agent as well as the humidity of atmosphere strongly affected the robustness of liquid marbles at elevated temperature. The resulting graphene/Ag nanocomposite showed one of the best catalytic characteristics for 4-nitroaniline reduction among the reported catalysts. The findings of this study not only offer an alternative insight into the stability of liquid marbles at elevated temperature but also provide a facile strategy for miniaturized synthesis.

Evaporation of water droplets on soft patterned surfaces

The evaporation process of a sessile drop of water on soft patterned polydimethylsiloxane (PDMS) substrates is investigated in this study. Different softness of a regular pillar-like patterned PDMS substrate can be achieved by controlling the mixing ratio of a PDMS's prepolymer base and a curing agent at 10 : 1, 20 : 1 and 30 : 1. The receding contact angle is smaller for softer pillar-like patterned substrates. Consequently, the evaporation rate is faster on softer pillar-like substrates. A sessile drop on the regular pillar-like PDMS substrates, prepared at the mixing ratio of a base to a curing agent of 10 : 1 and 20 : 1, is observed to start evaporating in the constant contact radius (CCR) mode then switching to the constant contact angle (CCA) mode via stepwise jumping of the contact line, and finally shifting to the mixed mode sequentially. During the evaporation, a wetting transition from the Cassie to the Wenzel state occurs earlier for the softer substrate because softer pillars relatively cannot stand the increasingly high Laplace pressure. For the softest regular pillar-like PDMS substrate prepared at the mixing ratio of the base to the curing agent of 30 : 1 (abbreviated by PDMS-30 : 1 substrate), the pillars collapse irreversibly after the sessile drop exhibits the wetting transition into the Wenzel state. Furthermore, it is interesting to find out that the initial stage of evaporation of a sessile drop on the PDMS-30 : 1 substrate in the Cassie state is in the CCR mode followed by the CCA mode with stepwise retreatment of the contact line. Further evaporation would induce the wetting transition from the Cassie to the Wenzel state (due to the collapse of pillars) and resume the CCR mode followed by the CCA mode again sequentially.

Patterns from drying drops

The objective of this review is to investigate different deposition patterns from dried droplets of a range of fluids: paints, polymers and biological fluids. This includes looking at mechanisms controlling the patterns and how they can be manipulated for use in certain applications such as medical diagnostics and nanotechnology. This review introduces the fundamental properties of droplets during evaporation. These include profile evolution (constant contact angle regime (CCAR) and constant radius regime (CRR)) and the internal flow (Marangoni and Capillary flow (Deegan et al. [22])). The understanding of these processes and the basic physics behind the phenomenon are crucial to the understanding of the factors influencing the deposition patterns. It concludes with the applications that each of these fluids can be used in and how the manipulation of the deposition pattern is useful. The most commonly seen pattern is the coffee-ring deposit which can be seen frequently in real life from tea/coffee stains and in water colour painting. This is caused by an outward flow known as capillary flow which carries suspended particles out to the edge of the wetted area. Other patterns that were found were uniform, central deposits and concentric rings which are caused by inward Marangoni flow. Complex biological fluids displayed an array of different patterns which can be used to diagnose patients.

Patterned deposition at moving contact lines

When a simple or complex liquid recedes from a smooth solid substrate it often leaves a homogeneous or structured deposit behind. In the case of a receding non-volatile pure liquid the deposit might be a liquid film or an arrangement of droplets depending on the receding speed of the meniscus and the wetting properties of the system. For complex liquids with volatile components as, e.g., polymer solutions and particle or surfactant suspensions, the deposit might be a homogeneous or structured layer of solute--with structures ranging from line patterns that can be orthogonal or parallel to the receding contact line via hexagonal or square arrangements of drops to complicated hierarchical structures. We review a number of recent experiments and modelling approaches with a particular focus on mesoscopic hydrodynamic long-wave models. The conclusion highlights open question and speculates about future developments.

Phase separation and the 'coffee-ring' effect in polymer-nanocrystal mixtures

The coupling between the 'coffee-ring' effect and liquid-liquid phase separation is examined for ternary mixtures of solvent, polymer and semiconductor nanocrystal. Specifically, we study mixtures of toluene, polystyrene (PS) and colloidal silicon nanocrystals (SiNCs) using real-space imaging and spectroscopic techniques to resolve the kinetic morphology of the drying front for varied molecular weight of the PS. Our results demonstrate that the size of the polymer has a significant impact on both phase-separation and drying, and we relate these observations to simulations, measured and predicted binodal curves, and the observed shape of the flow field at the contact line. The results inform a deposition process that reduces the influence of drying instabilities for low-molecular-weight polymers while paving the way for more detailed and generic computational descriptions of drying instabilities in complex fluids.

A rapid one-step fabrication of patternable superhydrophobic surfaces driven by Marangoni instability

We present a facile and inexpensive approach without any fluorinated chemistry to create superhydrophobic surface with exceptional liquid repellency, transportation of oil, selective capture of oil, optical bar code, and self-cleaning. Here we show experimentally that the control of evaporation is important and can be used to form superhydrophobic surface driven by Marangoni instability: the method involves in-situ photopolymerization in the presence of a volatile solvent and porous PDMS cover to afford superhydrophobic surfaces with the desired combination of micro- and nanoscale roughness. The porous PDMS cover significantly affects Marangoni convection of coating fluid, inducing composition gradients at the same time. In addition, the change of concentration of ethanol is able to produce versatile surfaces from hydrophilic to superhydrophobic and as a consequence to determine contact angles as well as roughness factors. In conclusion, the control of evaporation under the polymerization provides a convenient parameter to fabricate the superhydrophobic surface, without application of fluorinated chemistry and the elegant nanofabrication technique.

Temperature distribution along the surface of evaporating droplets

The surface temperature can significantly affect the flow field of drying droplets. Most previous studies assumed a monotonic temperature variation along the droplet surface. However, the present analyses indicate that a nonmonotonic spatial distribution of the surface temperature should occur. Three different patterns of the surface temperature distribution may appear during the evaporation process of liquid droplets: (i) the surface temperature increases monotonically from the center to the edge of the droplet; (ii) the surface temperature exhibits a nonmonotonic spatial distribution along the droplet surface; (iii) the surface temperature decreases monotonically from the center to the edge of the droplet. These surface temperature distributions can be explained by combining the evaporative cooling at the droplet surface and the heat conduction across the substrate and the liquid. Furthermore, a "phase diagram" for the distribution of the surface temperature is introduced and the effect of the spatial temperature distribution along the droplet surface on the flow structure of the droplet is discussed. The results may provide a better understanding of the Marangoni effect of drying droplets and provide a potential way to control evaporation-driven deposition as well as the assembly of colloids and other materials.

Assessment of water droplet evaporation mechanisms on hydrophobic and superhydrophobic substrates

Evaporation rates are predicted and important transport mechanisms identified for evaporation of water droplets on hydrophobic (contact angle ~110°) and superhydrophobic (contact angle ~160°) substrates. Analytical models for droplet evaporation in the literature are usually simplified to include only vapor diffusion in the gas domain, and the system is assumed to be isothermal. In the comprehensive model developed in this study, evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Conjugate heat and mass transfer are solved in the solid substrate, liquid droplet, and surrounding gas. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. The influences of evaporative cooling and convection on the evaporation characteristics are determined quantitatively. The liquid-vapor interface temperature drop induced by evaporative cooling suppresses evaporation, while gas-phase natural convection acts to enhance evaporation. While the effects of these competing transport mechanisms are observed to counterbalance for evaporation on a hydrophobic surface, the stronger influence of evaporative cooling on a superhydrophobic surface accounts for an overprediction of experimental evaporation rates by ~20% with vapor diffusion-based models. The local evaporation fluxes along the liquid-vapor interface for both hydrophobic and superhydrophobic substrates are investigated. The highest local evaporation flux occurs at the three-phase contact line region due to proximity to the higher temperature substrate, rather than at the relatively colder droplet top; vapor diffusion-based models predict the opposite. The numerically calculated evaporation rates agree with experimental results to within 2% for superhydrophobic substrates and 3% for hydrophobic substrates. The large deviations between past analytical models and the experimental data are therefore reconciled with the comprehensive model developed here.

Shape, shell, and vacuole formation during the drying of a single concentrated whey protein droplet

The drying of milk concentrate droplets usually leads to specific particle morphology influencing their properties and their functionality. Understanding how the final shape of the particle is formed therefore represents a key issue for industrial applications. In this study, a new approach to the investigation of droplet-particle conversion is proposed. A single droplet of concentrated globular proteins extracted from milk was deposited onto a hydrophobic substrate and placed in a dry environment. Complementary methods (high-speed camera, confocal microscopy, and microbalance) were used to record the drying behavior of the concentrated protein droplets. Our results showed that whatever the initial concentration, particle formation included three dynamic stages clearly defined by the loss of mass and the evolution of the internal and external shapes of the droplet. A new and reproducible particle shape was related in this study. It was observed after drying a smooth, hemispherical cap-shaped particle, including a uniform protein shell and the nucleation of an internal vacuole. The particle morphology was strongly influenced by the drying environment, the contact angle, and the initial protein concentration, all of which governed the duration of the droplet shrinkage, the degree of buckling, and the shell thickness. These results are discussed in terms of specific protein behaviors in forming a predictable and a characteristic particle shape. The way the shell is formed may be the starting point in shaping particle distortion and thus represents a potential means of tuning the particle morphology.

Physics-based agent to simulant correlations for vapor phase mass transport

Chemical warfare agent simulants are often used as an agent surrogate to perform environmental testing, mitigating exposure hazards. This work specifically addresses the assessment of downwind agent vapor concentration resulting from an evaporating simulant droplet. A previously developed methodology was used to estimate the mass diffusivities of the chemical warfare agent simulants methyl salicylate, 2-chloroethyl ethyl sulfide, di-ethyl malonate, and chloroethyl phenyl sulfide. Along with the diffusivity of the chemical warfare agent bis(2-chloroethyl) sulfide, the simulant diffusivities were used in an advection-diffusion model to predict the vapor concentrations downwind from an evaporating droplet of each chemical at various wind velocities and temperatures. The results demonstrate that the simulant-to-agent concentration ratio and the corresponding vapor pressure ratio are equivalent under certain conditions. Specifically, the relationship is valid within ranges of measurement locations relative to the evaporating droplet and observation times. The valid ranges depend on the relative transport properties of the agent and simulant, and whether vapor transport is diffusion or advection dominant.

Droplet evaporation dynamics on a superhydrophobic surface with negligible hysteresis

We report on experiments of droplet evaporation on a structured superhydrophobic surface that displays very high contact angle (CA ∼ 160 deg), and negligible contact angle hysteresis (<1 deg). The droplet evaporation is observed to occur in a constant-contact-angle mode, with contact radius shrinking for almost the entire duration of evaporation. Experiments conducted on Teflon-coated smooth surface (CA ∼ 120 deg) as a baseline also support an evaporation process that is dominated by a constant-contact-angle mode. The experimental results are compared with an isothermal diffusion model for droplet evaporation from the literature. Good agreement is observed for the Teflon-coated smooth surface between the analytical expression and experimental results in terms of the total time for evaporation, transient volume, contact angle, and contact radius. However, for the structured superhydrophobic surface, the experiments indicate that the time taken for complete evaporation of the droplet is greater than the predicted time, across all droplet volumes. This disparity is attributed primarily to the evaporative cooling at the droplet interface due to the high aspect ratio of the droplet and also the lower effective thermal conductivity of the substrate due to the presence of air gaps. This hypothesis is verified by numerically evaluating the temperature distribution along the droplet interface. We propose a generalized relation for predicting the instantaneous volume of droplets with initial CA > 90 deg, irrespective of the mode of evaporation.

Evaporation of drops on two parallel fibers: influence of the liquid morphology and fiber elasticity

We investigate experimentally the evaporation of liquid accumulated on a pair of parallel fibers, rigid or flexible. The liquid wetting the fibers can adopt two distinct morphologies: a compact drop shape, whose evaporation dynamics is similar to that of an isolated aerosol droplet, or a long liquid column of constant cross-section, whose evaporation dynamics depends upon the aspect ratio of the column. We thus find that the evaporation rate is constant for drops, while it increases strongly for columns as the interfiber distance decreases, and we propose a model to explain this behavior. When the fibers are flexible, the transition from drops to columns can be induced by the deformation of the fibers because of the capillary forces applied by the drop. Thus, we find that the evaporation rate increases with increasing flexibility. Furthermore, complex morphology transitions occur upon drying, which results in spreading of the drop as it evaporates.

Drop drying on surfaces determines chemical reactivity - the specific case of immobilization of oligonucleotides on microarrays

BACKGROUND

Drop drying is a key factor in a wide range of technical applications, including spotted microarrays. The applied nL liquid volume provides specific reaction conditions for the immobilization of probe molecules to a chemically modified surface.

RESULTS

We investigated the influence of nL and μL liquid drop volumes on the process of probe immobilization and compare the results obtained to the situation in liquid solution. In our data, we observe a strong relationship between drop drying effects on immobilization and surface chemistry. In this work, we present results on the immobilization of dye labeled 20mer oligonucleotides with and without an activating 5'-aminoheptyl linker onto a 2D epoxysilane and a 3D NHS activated hydrogel surface.

CONCLUSIONS

Our experiments identified two basic processes determining immobilization. First, the rate of drop drying that depends on the drop volume and the ambient relative humidity. Oligonucleotides in a dried spot react unspecifically with the surface and long reaction times are needed. 3D hydrogel surfaces allow for immobilization in a liquid environment under diffusive conditions. Here, oligonucleotide immobilization is much faster and a specific reaction with the reactive linker group is observed. Second, the effect of increasing probe concentration as a result of drop drying. On a 3D hydrogel, the increasing concentration of probe molecules in nL spotting volumes accelerates immobilization dramatically. In case of μL volumes, immobilization depends on whether the drop is allowed to dry completely. At non-drying conditions, very limited immobilization is observed due to the low oligonucleotide concentration used in microarray spotting solutions. The results of our study provide a general guideline for microarray assay development. They allow for the initial definition and further optimization of reaction conditions for the immobilization of oligonucleotides and other probe molecule classes to different surfaces in dependence of the applied spotting and reaction volume.

Portable self-contained cultures for phage and bacteria made of paper and tape

In this paper, we demonstrate that a functional, portable device for the growth of bacteria or amplification of bacteriophage can be created using simple materials. These devices are comprised of packing tape, sheets of paper patterned by hydrophobic printer ink, and a polydimethyl siloxane (PDMS) membrane, which is selectively permeable to oxygen but non-permeable to water. These devices supply bacteria with oxygen and prevent the evaporation of media for a period over 48 h. The division time of E. coli and the amplification of the phage M13 in this device are similar to the rates measured on agar plates and in shaking cultures. The growth of bacteria with a fluorescent mCherry reporter can be quantified using a flatbed scanner or a cell phone camera. Permeating devices with commercial viability dye (PrestoBlue) can be used to detect low copy number of E. coli (1-10 CFU in 100 μL) and visualize microorganisms in environmental samples. The platform, equipped with bacteria that carry inducible mCherry reporter could also be used to quantify the concentration of the inducer (here, arabinose). Identical culture platforms can, potentially, be used to quantify the induction of gene expression by an engineered phage or by synthetic transcriptional regulators that respond to clinically relevant molecules. The majority of measurement and fabrication procedures presented in this report have been replicated by low-skilled personnel (high-school students) in a low-resource environment (high-school classroom). The fabrication and performance of the device have also been tested in a low-resource laboratory setting by researchers in Nairobi, Kenya. Accordingly, this platform can be used as both an educational tool and as a diagnostic tool in low-resource environments worldwide.

Embedded adaptive optics for ubiquitous lab-on-a-chip readout on intact cell phones

The evaluation of disposable lab-on-a-chip (LOC) devices on cell phones is an attractive alternative to migrate the analytical strength of LOC solutions to decentralized sensing applications. Imaging the micrometric detection areas of LOCs in contact with intact phone cameras is central to provide such capability. This work demonstrates a disposable and morphing liquid lens concept that can be integrated in LOC devices and refocuses micrometric features in the range necessary for LOC evaluation using diverse cell phone cameras. During natural evaporation, the lens focus varies adapting to different type of cameras. Standard software in the phone commands a time-lapse acquisition for best focal selection that is sufficient to capture and resolve, under ambient illumination, 50 μm features in regions larger than 500 × 500 μm². In this way, the present concept introduces a generic solution compatible with the use of diverse and unmodified cell phone cameras to evaluate disposable LOC devices.

Evaporation of water droplets on "lock-and-key" structures with nanoscale features

Highly ordered poly(dimethylsiloxane) microbowl arrays (MBAs) and microcap arrays (MCAs) with "lock-and-key" properties are successfully fabricated by self-assembly and electrochemical deposition. The wetting properties and evaporation dynamics of water droplets for both cases have been investigated. For the MBAs case, the wetting radius of the droplets remains unchanged until the portion of the droplet completely dries out at the end of the evaporation process. The pinning state extends for more than 99.5% of the total evaporation time, and the pinning-shrinking transition is essentially prevented whereas in the case of the MCAs the contact radius exhibits distinct stages during evaporation and the contact line retreats significantly in the middle of the evaporation process. We explain the phenomenon by a qualitative energy balance argument based on the different shrinkage types of the nanoscale-folded contact line.