Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

Characterization of the Anisotropic Wetting Property in Liquid Transportation on the Surface With Ratchet Structures

In this paper, the samples with ratchet structures surface, are set vertically and horizontally in the liquid transportation experiments. Both static and dynamic directional property has been recorded by a high-speed camera. The variation of static contact angles and the shape of the drop in a ratchet structure are measured. The geometric evolutions of drop shed off along different direction on ratchet structure surface and the smooth surface under dynamic condition, are observed for comparison. The advancing contact angle and velocity of drop shed off the ratchet structure are analyzed in detail. The directional liquid transportation mechanism of anisotropic wettability property is demonstrated. Results show that the static drop moves parallel along ridges quickly, and the tooth limit the anisotropic drop spread to the other teeth along the vertical direction, this anisotropic wettability behavior in ratchet structure, which is related to the velocity of liquid transport, the velocity of liquid drop along the backward direction is slower than that of forward direction. The anisotropic wettability property along the horizontal direction of groove limits the transportation ability of the drop. The observed drop deformation includes rotating, rolling, and shaking-induced movements. The surface tension, adhesive behavior, and curvature difference cause the hysteresis effect, which presents a major barrier for liquid transportation.

Numerical Simulation of Residual Oil Flooded by Polymer Solution in Microchannels

This paper establishes a flow equation using non-Newtonian fluid mechanics and defines the deformation of residual oil using numerical computation in order to conduct a study on the flow law of residual oil in microchannels of rock during polymer flooding, the influence of flooding fluid elasticity on the deformation of residual oil, and flooding mechanism of viscoelastic displacing fluid. Computation shows that advancing contact angle increases and receding contact angle decreases as the viscosity ratio decreases. The higher elasticity of polymer solution with higher concentration or molecular weight leads to significantly more obvious deformation of residual oil and benefits migration and stripping of residual oil. The impact of the initial wetting angle of residual oil film on deformation is analyzed. A smaller initial wetting angle corresponds to a bigger change of advancing contact angle and smaller change of receding contact angle. A better understanding of the flooding process is gained via a study on residual oil deformation in polymer flooding. Consequently, oil flooding efficiency and oil recovery can be enhanced. This is the hydrodynamic mechanism of enhanced oil recovery (EOR) by polymer flooding.

Wettability modification of heat-treated wood (HTW) via cold atmospheric-pressure nitrogen plasma jet (APPJ)

The modification of heat-treated wood (HTW) wettability by cold atmospheric-pressure nitrogen plasma jet (APPJ) for several treatment durations has been investigated. The effects of the modification were assessed by measurement of the advancing contact angle (ACA) of water along with determination of surface free energy. Additionally, the morphology and chemical changes of the HTW surface were characterized by scanning electron microscope (SEM) and FTIR spectroscopy. As expected, the measurements demonstrated that the ACA decreased proportionally with treatment time of APPJ. The optimal treatment time was 20 s. Clear etching traces are visible on the SEM images of HTW surfaces. The roughness of HTW increased after plasma treatment. FTIR spectra demonstrate that OH, C=O, and COOH groups are formed on the HTW surfaces. All these modifications are beneficial for the HTW wettability, which leads to better bonding strength of HTW.