One-Pot Preparation of Hydrophilic Polylactide Porous Scaffolds by Using Safe Solvent and Choline Taurinate Ionic Liquid

Polylactides (PLAs) are a class of polymers that are very appealing in biomedical applications due to their degradability in nontoxic products, tunable structural, and mechanical properties. However, they have some drawbacks related to their high hydrophobicity, lack of functional groups able to graft bioactive molecules, and solubility in unsafe solvents. To circumvent these shortcomings, porous scaffolds for tissue engineering were prepared by vigorously mixing a solution of isotactic and atactic PLA in nontoxic ethyl acetate at 70 °C with a water solution of choline taurinate. The partial aminolysis of the polymer ester bonds by taurine -NH2 brought about the formation of PLA oligomers with surfactant activity that stabilized the water-in-oil emulsion. Upon drying, a negligible shrinking occurred, and mechanically stable porous scaffolds were obtained. By varying the polymer composition and choline taurinate concentration, it was possible to modulate the pore dimensions (30–50 µm) and mechanical properties (Young’s moduli: 1–6 MPa) of the samples. Furthermore, the grafted choline taurinate made the surface of the PLA films hydrophilic, as observed by contact angle measurements (advancing contact angle: 76°; receding contact angle: 40°–13°). The preparation method was very simple because it was based on a one-pot mild reaction that did not require an additional purification step, as all the employed chemicals were nontoxic.

A Molecular Dynamics Investigation on Methane Flow and Water Droplets Sliding in Organic Shale Pores with Nano-structured Roughness

Roughness of surfaces significantly influences how methane and water flow in shale nanopores. We perform molecular dynamics simulations to investigate the influence of surface roughness on pore-scale transport of pure methane as well as of two-phase methane–water systems with the water sliding as droplets over the pore surface. For single-phase methane flow, surface roughness shows a limited influence on bulk methane density, while it significantly reduces the methane flow capacity. In methane–water systems, the mobility of water is a strong function of surface roughness including a clear transition between immobile and mobile water droplets. For cases with mobile water, droplet sliding speeds were correlated with pressure gradient and surface roughness. Sliding water droplets hardly deform, i.e., there is little difference between their advancing and receding contact angle with structured roughness.

A New Approach To Improve The Assessments of CO2 Geo-Sequestration Capacity of Clay Minerals

CO2 geological storage (CCS)isconsidered as the most promising technique to reduce atmospheric CO2emissions. However, due to the density variation between the injected supercritical CO2 and the formation water,CO2 tends to move vertically toward the air. This vertical CO2 leakage can be prevented by four trapping mechanisms (i.e. structural trapping,capillary trapping, solubility trapping, and mineral trapping). The capacities of structural and residual trapping are highly affected by rock wettability. Clay wettability is one of the crucial parametersin evaluation of CO2 geo-sequestration. However, the literature data show that there are many uncertainties associated with experimental measurements. One of these uncertainties is the influenceof the effect of gas density on the clay mineral wettability. Thus, here, we compared the wettability of a clay mineral (i.e. illite) of three different gas densities scenarios (i.e. low (Helium), moderate (Nitrogen), and high (CO2) gas densities). To do so, we measured the advancing and receding contact angle (i.e. wettability) of illite for CO2/water, nitrogen/water, and Helium/water systems at a constant (333 K) and four different pressures (5, 10, 15, and 20 MPa). The brine composition used was 4 wt% NaCl, 4 wt% CaCl2, 1 wt% MgCl2 and 1 wt% KCl, for all gas density scenarios. The results indicate that gas density has a significant effect on the clay mineral wettability and that both advancing and receding contact angles increase with an increase in gas density. The results show that a higher density gas scenario has a higher contact angle of illite, measured at the same temperature and pressure. For instance, the advancing contact angle of illite at 333 K and 20 MPa was 65° for the CO2/water system, 53° for the nitrogen/water system, and 50° for Helium/water Helium/water system. Thus, we conclude that the gas density affects the Clay wettability measurement and that the higher gas density leads to a higher contact angle measurements (i.e. a more CO2-wet system) of the clay and thus reduces the estimated CO2 geo-sequestration capacity and containment security.

Driving Scheme Optimization for Electrowetting Displays Based on Contact Angle Hysteresis to Achieve Precise Gray-Scales

As a new display technology, electrowetting display (EWD) has many excellent display characteristics, such as paper-like, low power consumption, quick response and full color. These characteristics make EWD devices very suitable for portable devices. However, the gray-scale distortion caused by the contact angle hysteresis of EWDs seriously affects the accuracy of gray-scale display. To improve this phenomenon, the hysteresis curve of an EWD panel was studied according to the motion characteristics of advancing contact angle and receding contact angle of oil in a pixel. Then, a driving scheme for EWDs using alternating current (AC) voltage instead of direct current (DC) voltage was proposed in this paper. And the advantages and disadvantages of the driving scheme at different AC frequencies from 90 to 2,700 Hz were analyzed through experiments. According to the stability of aperture ratio in EWDs, a 470 Hz AC driving scheme was determined. Experimental results showed that the aperture ratio distortion of EWDs could be reduced from 35.82 to 5.97%, which significantly improved the display performance of pixel units.

Study of Fluids Motion in a Microchannel under Heat Source

In this study, the numerical computation is used to investigate the transient thermocapillary migration of a water droplet in a Microchannel. For tracking the evolution of the free interface between two immiscible fluids, we employed the finite element method with the two-phase level set technique to solve the Navier-Stokes equations coupled with the energy equation. Both the upper wall and the bottom wall of the microchannel are set to be an ambient temperature. The heat source is placed at the left side of a water droplet. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet and the thermocapillary on the receding side is smaller than that on the advancing side. The temperature gradient inside the droplet increases quickly at the initial times and then decreases versus time. Therefore, the actuation velocity of the water droplet first increases significantly, and then decreases continuously. The dynamic contact angle is strongly affected by the oil flow motion and the net thermocapillary momentum inside the droplet. The advancing contact angle is always larger than the receding contact angle during actuation process.

Numerical and Experimental Investigation on Evaporation of Water Droplet on Surfaces With Mixed Wettability

Droplet evaporation is one of the most commonly observed phenomena and plays an important role in many applications such as in spray cooling, coating, and inkjet printing. Mechanisms such as dynamics of the contact line, evaporation-induced phase transitions, and formation of patterns on the substrate interact with each other in the evaporation of droplets. In this study, we investigated the effect of surface mixed wettability on water sessile droplet evaporation. The transient contact angle, center-height, contact radius, surface area, and droplet volume were experimentally measured and numerically estimated. Surfaces with mixed wettability consisting of hydrophilic islands surrounded by less hydrophilic area were fabricated. Visualization was conducted to capture droplet dynamics during evaporation using two high-speed cameras. According to the obtained results, there were three distinct stages in the water evaporation process: a constant contact radius mode, a constant contact angle mode, and a mixed-mode. The COMSOL 5.4 software was used for numerical analysis. According to the results, the receding contact angle and Marangoni instability in the droplet are two main factors that alter droplet dynamics and droplet evaporation.

Investigation of Single and Binary of “Sandwich” Type Convex Liquid Capillary Bridges, Stretched between Two Flat Surfaces (Experimental Approach)

The interest to monophasic liquid capillary bridges (CB) has a long history. These shapes are attractive not only because of their interesting surface properties but also because of the possibility of their behavior to be analytically predicted by the equations of differential geometry. In the current paper we extend our previous studies by implementation of an approach for prediction of liquid gravityless CB behavior during their quasi-static stretching. It was found, that a simple linear relation, h r m ~ ln R r m , is valid the case of good wetting, 0° ≤ θ ≤ 90°, where h is the height of CB, R is the radius at the contact surface, rm is the CB waist radius, and θ is the solid/liquid (static, receding) contact angle. We experimentally studied the geometrical properties evolution of monophasic cedar oil and water CBs between two glass plates during their quasi-static (stepwise with equilibration after each step for 1–2 min.) stretching. In addition, we investigated a binary CB of a new type, resembling “sandwich”. There, due to the stronger glass wetting by the water, the oil phase is adhered at the water/gas interface, partially engulfed with a tendency to stand in the zone around the waist (minimal surface energy). During the stretching, it tends to replace the water in the CB waist region. A simple mechanism for interaction of the two immiscible liquids leading to creation of “sandwich” like binary structures, is proposed. Experiments of capillary bridges (CB) stretching between two flat surfaces have been carried for all liquids at different volume proportions. The investigation is extended also to identification of CB profile generatrix shape. We experimentally found that for monophasic CB, it can be described by a circle during the quasi-static stretching. If the CB height is increased, before the rupture, the shape evolves consecutively to an ellipse, parabola, or possibly to a hyperbola. The investigated binary CB evolves a similar way. Conclusions are drawn and directions for further investigations are given.

Significance of the receding contact angle in the determination of surface free energy

<p>Surface free energy measurements of solids are a very important issue in various fields of science. Many functional, chemical and physical properties of a given material depend on its surface free energy. The basic method of the surface free energy determination are the contact angle measurements. There are several empirical methods useful to calculate the surface free energy of solids. They are based on the measurements of the contact angle of liquids with the defined surface tension. The aim of this paper is to examine the significance of the receding contact angle measurements in the determination of surface free energy of solids.</p>

Optimization of Hybrid Hydrophilic-Hydrophobic Surfaces for Dropwise Condensation Enhancement

A langrangian-based phenomenological model, following the evolution of the individual droplets nucleating at random positions, is applied to the simulation and optimization of hybrid, mixed hydrophobic and hydrophilic surfaces. The proposed mathematical model was originally developed to simulate droplets pattern evolution in the framework of in-flight icing problems and takes into account the surface tension effects (via the advancing and receding contact angle values), the external force balance on the single droplets, as well as the both the condensing and coalescence process. Here, the model is extended to the case of an arbitrary curved substrate surface and a parametric analysis of different cases is carried out, looking for the parameters that help to identify the optimal design for a given set of wettability properties, nucleation site density and wet surface condensation rates. A discussion on the sensitivity of the solutions with respect to the expected high uncertainties on the estimate of some of these parameters in actual practical problems is also presented, in order to assess the effectiveness of the simulation as a design tool. The analysis is carried out for condensation enhancement on both plane surfaces and mini or micro tubes. Comparison with experimental, open literature data ensure the reliability of the approach for both geometries.