Evaporation of saline colloidal droplet and deposition pattern

The dynamic process of the evaporation and the desiccation of sessile saline colloidal droplets,and their final deposition are investigated.During the evaporation,the movement of the colloidal particles shows a strong dependence on the salt concentration and the droplet shape.The final deposition pattern indicates a weakened coffee-ring effect in this mixed droplet system.The microscopic observation reveals that as evaporation proceeds,the particle motion trail is affected by the salt concentration of the droplet boundary.The Marangoni flow,which is induced by surface tension gradient originating from the local evaporative peripheral salt enrichment,suppresses the compensation flow towards the contact line of the droplet.The inhomogeneous density and concentration field induced by evaporation or crystallization can be the major reason for various micro-flows.At last stage,the distribution and crystallization of Na Cl are affected by the colloidal particles during the drying of the residual liquid film.

Hydrodynamic Pattern in Drying Saline Droplet with Suspended Nanoparticles

Evaporation of multicomponent droplets has gained much attention nowadays because of their complex flow fields and various deposition patterns.Here we observe strong flows in evaporating sodium chloride saline droplets with suspended alumina oxide nanoparticles.The evolution of flow pattern was studied by tracking the trajectories of particles and the velocity field was investigated with Particle Image Velocimetry analysis.The non-uniform evaporation rate along the droplet surface leads to a concentration gradient which induces the convection flow.During the evaporation process before crystallization happens,evolution of the flow can be divided into two regimes.In Regime I,a centrosymmetric convection recirculation is formed gradually.In RegimeⅡ,the convection recirculation migrates to the droplet edge and evolves into several small vortexes.At the late evaporation stage,crystallization could induce strong convection flows.It is shown that the flow tends to become more chaotic with a lower salt concentration.

Investigation of Droplet Evaporation on Copper Substrate with Different Roughness

In the present study,we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40℃ambient temperature.These four substrates are classified as smooth surface and rough surface based on their droplet contact angles.It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness.The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface.The below factors contribute to the result.On the smooth surface,the contact angle of droplet increases as the roughness decreases.On the rough surface,the contact angle increases as the roughness increases.With contact angle decreasing,the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate.Moreover,the capillary flow is hindered by increasing the substrate roughness,while the Marangoni flow remains constant,resulting in more particles remain in the center of the droplet on the rough surface.To sum up,the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40℃temperature.