Sessile Drop Evaporation Research Articles

Simple analytical model of capillary flow in an evaporating sessile drop

An analytical expression of hydrodynamic potential inside an evaporating sessile drop with pinned contact line is found. The problem is considered for a hemispherical drop (with the contact angle of 90 degrees ) at the very early stages of the evaporation process when the shape of the drop is still a hemisphere and the evaporation field is uniform. The capillary flow carries a fluid from the drop apex to the contact line. Comparison with the published calculations performed using lubrication approximation (very thin drop) suggests that qualitative picture of the capillary flow is insensitive to the ratio of initial drop height to the drop radius.

Controlling Arrangement of 60 nm Nanospheres in Evaporating Sessile Drops with Low Level Laser Light

The interplay between liquid flow and surface interaction can arrange colloidal particles in drops evaporating on substrates into crystalline rings. Recently, it was reported that magnetic fields can affect the crystallinity in rings formed by suspensions containing paramagnetic particles. Here, it is demonstrated that laser light, applied at intensities as low as 1000 W m -2 , has a manifest impact on the attachment and self-organization of 60 nm particles on wet substrates. Results indicate that the physical state of water layers attached to substrates can be modulated by low level laser light even in an aqueous milieu, and imply the subaquatic persistence of an ordered interlayer formed by molecules with less degrees of freedom, as compared to those in the bulk liquid. Low level laser light has already been shown to change the height of nanoscopic water layers on substrates in air. Nanoscopic water layers are not only important in determining the arrangement of nanoparticles into two-dimensional crystals, they also control the lifetime of scanning near-field optical microscopy sensors, adhesion processes on biochips, nonspecific interactions at biomaterial/cell interfaces, charge transfer between ice crystals and graupel pellets in thunderclouds, and presumably the attachment of nanobacteria and proteins to tissues in blood. Modalities by which laser light and nanoscopic water layers cooperatively control the attachment of nanoparticles to substrates are illustrated in representative scenarios. Results could be converted into various practical applications, especially in nanotechnology and biomedicine, and unlock a novel and promising field of research.

Simultaneous Determination of Toluene Diffusion Coefficient in Air from Thin Tube Evaporation and Sessile Drop Evaporation on a Solid Surface

In this work, we simultaneously determined the diffusion coefficient of toluene vapor in air by following the toluene evaporation in two different geometries: in a thin glass tube and a sessile drop on poly(tetrafluoroethylene) (Teflon) substrate in the same cell by inducing a quasi-steady liquid−vapor phase transition. A glass cell whose walls were lined with activated carbon was used in both experiments. Photography was applied to monitor the sessile drop evaporation, which occurred with constant contact angle mode (approximately 43°). When the restriction of evaporation space by the presence of the substrate surface [f(θ) factor] was considered in the sessile drop evaporation, the toluene vapor diffusion coefficient in air was found to be close in both methods. The diffusion coefficient was found to vary linearly with the temperature for both methods, and there is a 3.17 power dependence when the absolute temperatures are used. However, this figure is higher than the 1.75−2.00 power dependence, which is generally used for evaporating liquid vapors in the existing literature. In addition, an equation was derived showing the decrease of the square of the height of the sessile drop should vary linearly with time and was confirmed with the experimental results. Similarly, the decrease of the square of the contact radius of the drops was found to vary linearly with time in the experiments as expected from the theory, which was previously derived.

Effects of Evaporation and Thermocapillary Convection on Volatile Liquid Droplets

Results of an experimental investigation of evaporating sessile drops on a glass-slide surface for three volatile liquids show that both evaporation and thermocapillary convection in the sessile drop strongly affect the drop spreading and contact angle. The evolution of contact diameter of the drops can be divided into four stages: 1) initial spreading, 2) spreading-evaporation balance, 3) evaporation-dominating contraction, and 4) final rapid contraction. Molecular-kinetic spreading always occurs in the early first stage and is rapidly restrained and then taken over by the effects of evaporation. Thermocapillary convection, induced by the evaporation, promotes the competition of evaporation over the spreading and shortens the spreading-evaporation balance stage to become undetectable. Evaporation may increase or decrease the contact angle of the evaporating sessile drops, depending on the evaporation rate

Pattern formation in drying drops

Ring formation in an evaporating sessile drop is a hydrodynamic process in which solids dispersed in the drop are advected to the contact line. After all the liquid evaporates, a ring-shaped deposit is left on the substrate that contains almost all the solute. Here I show that the drop itself can generate one of the essential conditions for ring formation to occur: contact line pinning. Furthermore, I show that when self-induced pinning is the only source of pinning an array of patterns-that include cellular and lamellar structures, sawtooth patterns, and Sierpinski gaskets-arises from the competition between dewetting and contact line pinning.

The Leidenfrost Point: Experimental Study and Assessment of Existing Models

This study presents a detailed and thorough parametric study of the Leidenfrost point (LFP), which serves as the temperature boundary between the transition and film boiling regimes. Sessile drop evaporation experiments were conducted with acetone, benzene, FC-72, and water on heated aluminum surfaces with either polished, particle blasted, or rough sanded finishes to observe the influential effects of fluid properties, surface roughness, and surface contamination on the LFP. A weak relationship between surface energies and the LFP was observed by performing droplet evaporation experiments with water on polished copper, nickel, and silver surfaces. Additional parameters which were investigated and found to have negligible influence on the LFP included liquid subcooling, liquid degassing, surface roughness on the polished level, and the presence of polishing paste residues. The accumulated LFP data of this study was used to assess several existing models which attempt to identify the mechanisms which govern the LFP. The disagreement between the experimental LFP values and those predicted by the various models suggests that an accurate and robust theoretical model which effectively captures the LFP mechanisms is currently unavailable.

Determination of the Receding Contact Angle of Sessile Drops on Polymer Surfaces by Evaporation

The receding contact angles of water drops on PMMA and PET surfaces were determined by using video microscopy to follow the time-dependent evaporation of sessile drops. Depending on the initial drop size, receding angles of θr = 54−64° for PMMA and θr = 64−66° for PET were found with an average hysteresis of 23.5 ± 1.5 and 19.5 ± 1.5°, respectively. Advancing and receding angles, θa and θr, were also determined by the needle-syringe and the inclined plane methods for comparison. The discrepancies from the mean of the maximum and minimum contact angle results of both the needle-syringe and the inclined plane methods were larger than expected for all the polymer surfaces. A general trend was seen with samples giving a larger hysteresis also producing a larger discrepancy for all the samples. The major cause of this discrepancy is the variation of the rate of liquid introduction and withdrawal with the syringe in the needle-syringe method and the drop size effect in the inclined plane method. In this respect...

Influence of Evaporation on Contact Angle

The evaporation of sessile drops of water and n-decane placed on various substrates has been studied using a projection method. Drop dimensions and contact angle have been measured as a function of time. The first stage of the experiment corresponds to a saturated atmosphere ; then, when evaporation is allowed to occur, two or three different stages appear (depending on the surface roughness). For the first of these, a model is proposed which allows us to calculate the coefficient of diffusion of the liquid vapor in air.

NUMERICAL ANALYSIS OF NATURAL CONVECTION IN LIQUID DROPLETS BY PHASE CHANGE

A numerical analysis is performed on thermocapillary buoyancy convection induced by phase change in a liquid droplet. A finite-difference code is developed using an alternating-direction implicit (ADI) scheme. The intercoupling relation between thermocapillary force, buoyancy force, fluid property, heat transfer, and phase change, along with their effects on the induced flow patterns, are disclosed. The flow is classified into three types: thermocapillary, buoyancy, and combined convection. Among the three mechanisms, the combined convection simulates the experimental observations quite well, and the basic mechanism of the observed convection inside evaporating sessile drops is thus identified. It is disclosed that evaporation initiates unstable convection, while condensation always brings about a stable density distribution which eventually damps out all fluid disturbances. Another numerical model is presented to study the effect of boundary recession due to evaporation, and the “peeling-off” effect (the removal of the surface layer of fluid by evaporation) is shown to be relevant.

Theory and experiments on evaporating sessile drops of binary liquid mixtures

Theoretical and experimental studies are performed to determine the size, history and lifetime of evaporating sessile drops of binary liquid mixtures. The minimum and maximum drop lifetimes (equivalent to the maximum and minimum vaporization rates, respectively) occur at the azeotropic compositions of the positive and negative azeotropes, respectively. The mechanisms leading to the extremum drop lifetime are due to the changes in composition of the more volatile component during the vaporization process. This composition change tends to suppress vaporization. A new dimensionless parameter is found to determine the intensity of vaporization of a binary liquid drop. Theoretical predictions of the drop size, history and lifetime agree satisfactorily with experimental results.

Interfacial Flow and Evaporation of Sessile Drops on a Vertical Surface

Etude par ombroscopie laser de la dynamique d'evaporation de gouttes de differents liquides a bas point d'ebullition (acetone, methanol, ethanol, cyclohexane, tetrachlorure de carbone, etc...) placees sur une plaque verticale

The evaporation of sessile or pendant drops in still air

In the evaporation of drops on surfaces two modes of evaporation are distinguished: that at constant contact angle and that at constant contact area. A theoretical analysis of each mode is developed from which predictions can be made of evaporation rate and residual mass at any time in the life of the drop for contact angles ranging from near zero up to 180°. These predictions have been compared with known theoretical values and with experimental measurements, and it is considered that the theory is reasonably accurate for drops of mass between 1 pg and an upper limit somewhat greater than 40 mg.