Evaporation Of Sessile Droplets Research Articles

Modeling the elastic deformation of polymer crusts formed by sessile droplet evaporation

Evaporating droplets of polymer or colloid solution may produce a glassy crust at the liquid-vapor interface, which subsequently deforms as an elastic shell. For sessile droplets, the known radial outward flow of solvent is expected to generate crusts that are thicker near the pinned contact line than the apex. Here we investigate, by nonlinear quasistatic simulation and scaling analysis, the deformation mode and stability properties of elastic caps with a nonuniform thickness profile. By suitably scaling the mean thickness and the contact angle between crust and substrate, we find that data collapse onto a master curve for both buckling pressure and deformation mode, thus allowing us to predict when the deformed shape is a dimple, Mexican hat, and so on. This master curve is parameterized by a dimensionless measure of the nonuniformity of the shell. We also speculate on how overlapping time scales for gelation and deformation may alter our findings.

Self-Assembly of Spherical Particles on an Evaporating Sessile Droplet

Particles adsorbed on the surface of a droplet form three-dimensional packings when the droplet evaporates. We study the final packings when the liquid droplet is attached to a solid substrate. In contrast to a droplet evaporating away from a substrate, here the final packings are highly dependent on both the number of particles and the contact angle between the droplet and the surface. Simple geometrical constraints quantitatively determine the parameter regions that particular packings can form.

Analysis of the Effects of Marangoni Stresses on the Microflow in an Evaporating Sessile Droplet

We study the effects of Marangoni stresses on the flow in an evaporating sessile droplet, by extending a lubrication analysis and a finite element solution of the flow field in a drying droplet, developed earlier. The temperature distribution within the droplet is obtained from a solution of Laplace's equation, where quasi-steadiness and neglect of convection terms in the heat equation can be justified for small, slowly evaporating droplets. The evaporation flux and temperature profiles along the droplet surface are approximated by simple analytical forms and used as boundary conditions to obtain an axisymmetric analytical flow field from the lubrication theory for relatively flat droplets. A finite element algorithm is also developed to solve simultaneously the vapor concentration, and the thermal and flow fields in the droplet, which shows that the lubrication solution with the Marangoni stress is accurate for contact angles as high as 40 degrees. From our analysis, we find that surfactant contamination, at a surface concentration as small as 300 molecules/microm(2), can almost entirely suppress the Marangoni flow in the evaporating droplet.

Analysis of the Microfluid Flow in an Evaporating Sessile Droplet

The axisymmetric time-dependent flow field in an evaporating sessile droplet whose contact line is pinned is studied numerically and using an analytical lubrication theory with a zero-shear-stress boundary condition on the free surface of the droplet at low capillary and Reynolds numbers. A finite element algorithm is developed to solve simultaneously the vapor concentration and flow field in the droplet under conditions of slow evaporation. The finite element solution confirms the accuracy of the lubrication solution, especially when terms of higher order in the droplet flatness ratio (the ratio of droplet height to radius, h/R) are included in the lubrication theory to account more accurately for the singular flow near the contact line.

Self-organised marangoni motion at evaporating drops or in capillary menisci – thermohydrodynamical model

Thermocapillary convection has been observed both in capillaries and at the surface of evaporating sessile droplets. The present paper aims to give a model of the onset of these convective patterns. The model accounts for the self-induced surface gradient of temperature linked to the stronger mass transfer observed near the triple line region. The role of the apparent contact angle is also evidenced. Dimensionless quantities such as the Marangoni number and the partition coefficient are the key factors of the present analysis. The calculations show that for the same environmental conditions the convection can be influenced by the geometrical considerations. Indeed, the calculated Marangoni numbers are found to be greater in the capillary than in the drop for a wetting contact angle of less than π /4 ; for an angle greater than π /4 , the results are inverted.

Application of the Quartz Crystal Microbalance to the Evaporation of Colloidal Suspension Droplets

An investigation into the evaporation of sessile droplets of latex and clay particle suspensions is presented in this work. The quartz crystal microbalance (QCM) has been used to study the interfacial phenomena during the drying process of these droplets. Characteristic changes of the crystal oscillating frequency and crystal resistance (damping of the oscillating energy) have been observed and related to the different stages of the evaporation process. Measurements have been made for latex particle sizes from 1.9 to 10 microm and for rough and polished crystals using drops from 0.3 to 1.5 microL. The behavior of the QCM is shown to depend strongly on the size of particles present and on the morphology of the crystal surface. One of the most striking features is a drastic damping of the oscillation energy and corresponding rise in frequency observed during the final stages of evaporation, particularly for the clay suspensions.

Solute Deposit during Evaporation of a Sessile Binary Liquid Micro-Droplet on a Substrate

Formation of a ring-like deposit of solute during the evaporation of a binary liquid sessile droplet on a substrate was studied both experimentally and numerically. The effects of the droplet spread radius and substrate surface temperature on ring-like deposit of solute were investigated. The ring width increased with a decrease in the spread radius and the substrate temperature. The evaporation rate distributions on the droplet surface were numerically obtained to predict the shape of the thin film of solute after the evaporation using a theoretical model for solute transfer in a sessile droplet. When the substrate temperature was low, the increase in the evaporation rate near the contact line was inhibited. As a result, the velocity in the droplet toward the contact line, which caused the solute transfer, was small and then the ring width increases.

Evaporation of Sessile Drops: Application of the Quartz Crystal Microbalance

The application of the quartz crystal microbalance (QCM) to the study of the evaporation of sessile droplets is reported. The evaporation of a homologous series of light alcohols, from the surface of an oscillating quartz crystal, has been investigated. The droplet evaporation process is observed to cause reproducible, characteristic changes in crystal oscillation frequency that are indicative of the complex thermophysical phenomena occurring at both the liquid−vapor and crystal−fluid interfaces. The influence of surface morphology on the frequency responses during the evaporation process is understood in the framework of the perturbation theory of surfaces of slight roughness and random corrugation in the low viscosity limit. The experimental data are understood in terms of the radial sensitivity of the QCM S(r,) via the deduction of the areal retreat speed vr. The extreme modes of droplet evaporation, associated with constant contact area and constant contact angle, are identifiable from the observed frequency responses. The trend of characteristic frequency responses observed is consistent with surface-tension driven convection effects, which are often responsible for fluid−vapor interface phenomena.