Investigation of the Dynamic Contact Angle Using a Direct Numerical Simulation Method

A Multi-Electrode Pixel Structure for Quick-Response Electrowetting Displays

A new reflective display technology, the electrowetting display (EWD), has the advantages of high contrast ratio, high reflectivity, and ultralow power consumption. The response speed of EWDs has an important influence on optical performance, and pixel structure is one of the key factors affecting the response speed of EWDs. In order to improve the response speed, a new multi-electrode pixel structure is proposed in this paper. This structure was realized by dividing the pixel into four square-shaped sub-electrodes, and a three-dimensional EWD simulation model was established. In this model, a driving voltage was first applied to one of these sub-electrodes, and the oil was ruptured. Then, its two adjacent sub-electrodes were also supplied with driving voltages, so as to spur the oil to move to a pixel corner quickly. Simulation results showed that the response speed of EWDs can be effectively improved by using the proposed multi-electrode pixel structure. Compared with a single-electrode pixel structure, the oil rupture response time of the multi-electrode pixel structure was advanced by 0.6 ms. The pixel achieved a 2.7 ms faster response time than the single-electrode pixel for reaching a 50% aperture ratio in an opening process, and the maximum aperture ratio was increased by 6.2%.

Phase-Field Simulation of Imbibition for the Matrix-Fracture of Tight Oil Reservoirs Considering Temperature Change

Injection water temperature is often different from that of the reservoir during water injection development in the tight reservoir. Temperature change causes different fluid properties and oil-water interface properties, which further affects the imbibition process. In this paper, a matrix-fracture non-isothermal oil-water imbibition flow model in tight reservoirs is established and solved by the finite element method based on the phase-field method. The ideal inhomogeneous rock structure model was used to study the influence of a single factor on the imbibition. The actual rock structure model was used to study the influence of temperature. The mechanism of temperature influence in the process of imbibition is studied from the micro-level. It is found that the imbibition of matrix-fracture is a process in which the water enters the matrix along with the small pores, and the oil is driven into the macropores and then into the fractures. Temperature affects the imbibition process by changing the oil-water contact angle, oil-water interfacial tension, and oil-water viscosity ratio. Reducing oil-water contact angle and oil-water viscosity ratio and increasing oil-water interfacial tension are conducive to the imbibition process. The increase in injection water temperature is usually beneficial to the occurrence of the imbibition. Moreover, the actual core structure imbibition degree is often lower than that of the ideal core structure. The inhomogeneous distribution of rock particles has a significant influence on imbibition. This study provides microscale theoretical support for seeking reasonable injection velocity, pressure gradient, injection temperature, and well-shutting time in the field process. It provides a reference for the formulation of field process parameters.

Pressure Drop with Moving Contact Lines and Dynamic Contact Angles in a Hydrophobic Round Minichannel: Visualization via Synchrotron X-ray Imaging and Verification of Experimental Correlations

In hydrophobic mini- and microchannels, slug flow with moving contact lines is typically generated under various two-phase flow conditions. There is a significant pressure drop in this flow pattern with moving contact lines, which is closely related to the dynamic contact angles. Researchers have investigated dynamic contact angles experimentally for decades, but due to the limitations of visualization techniques, these experiments have typically been conducted in low Weber number regions (We < 10^-3). In this study, we clearly visualized the dynamic contact angles of a liquid slug in high Weber number regions (10^-3 < We <1) via synchrotron X-ray imaging with high temporal (∼1000 fps) and spatial (∼2 μm/pixel) resolutions. We precisely measured the pressure drop with moving contact lines in a hydrophobic minichannel (inner diameter = 1.018 mm). On the basis of our experimental data, we verified previous correlations for dynamic contact angles and explored the relationship between pressure drop with moving contact lines and dynamic contact angles.

Wettability of Calcite Surfaces: Impacts of Brine Ionic Composition and Oil Phase Polarity at Elevated Temperature and Pressure Conditions

The interactions among the polar components of oil, aqueous phase ions, and carbonate minerals, as well as their subsequent effects on surface wettability, can significantly impact the fluid distribution and recovery in a hydrocarbon reservoir. In this study, we investigate the adsorption/desorption of molecules from oils with different levels of polarity on calcite surfaces during different displacement processes under elevated pressure and temperature conditions. We measured dynamic contact angles (CA) on untreated and aged calcite substrates using brines with different salinities and compositions and model oils, that is, mixtures of varying concentrations of stearic acid (SA) and n-decane. In particular, the impacts of the concentrations of Ca²⁺, SO₄^2-, and OH^- ions on the adsorption phenomena were explored. For the nonpolar oil, increasing brine salinity or removing Ca²⁺ ions from the aqueous phase impacted the potentials of oil-brine and brine-mineral interfaces and shifted the wettability of calcite surface toward more water-wet conditions. In the presence of polar oil, the adsorption of the polar components controls the surface wettability. Higher concentrations of Ca²⁺/SO₄^2- could facilitate/obstruct the polar component adsorption and thus increase/decrease the dynamic oil-water CAs. It is also observed that the brine salinity does not impact the wettability if excess SA is added to the oil phase, that is, if the oil phase is strongly polar. Moreover, the adsorption of SA on the calcite surface under experimental conditions is found to be reversible during the displacement events. The surface energy calculation for the adsorption process indicates that the surface coverage of calcite by SA is more sensitive to the presence of Ca²⁺ in brine than the concentration of polar components in oil. We also conducted several experiments on calcite substrates aged with SA. The measurements demonstrate that the adsorbed SA molecules are detached when the aged mineral surface is exposed to a lower-salinity brine at high temperatures, and the SA molecules could be adsorbed back on the surface once the displacement is halted.

Modeling of Oil/Water Interfacial Dynamics in Three-Dimensional Bistable Electrowetting Display Pixels

Electrowetting has drawn significant interest because of the potential applications of displays, lab-on-a-chip microfluidic devices, electro-optical switches, and so forth. However, electrowetting display (EWD) is monostable, which needs extra continuous voltage supply to keep contracting the oil. This paper is concerned with the simulation of two-phase liquid flow in three-dimensional EWD pixels with two electrodes (E1 and E2) demonstrating bistability, where power is only needed to move the oil droplet between two stable states. The effects of E1 geometry, E2 geometry, and E2 pulse characteristics on the dynamics of the oil droplet motion have been analyzed. Also, predictions of the transient states in four stages of the reversible bistable operation process have been carried out by employing the finite element method, in qualitative agreement with our experimental results of the monostable EWD and the existing literature. We seek to shed more light on the fundamental two-phase liquid flow in three-dimensional pixels exhibiting bistability for low power EWD and guide optimizing the electrodes to the perfect patterns with the aid of rigorous modeling.