Lingling Shui

Microscopic structure influencing macroscopic splash at high Weber number

The dynamics of water drop impact at high impinging velocity onto superhydrophobic substrates is experimentally investigated. The solid substrate—comprised of regular and hydrophobic micropillars—is transparent, thereby facilitating close-up, top-or-bottom-view, high-speed imaging. With a sufficient impact velocity, instead of a completely-bouncing ‘‘Fakir’’ droplet, wetting splashing can occur, with an entrapped air bubble at the centre surrounded by a wetted area as well as an emission of satellite droplets during the advancing phase of spreading lamella. A large portion of the lamella travels upon air and subsequently recoils due to surface tension, forming a partial rebound on the central wet spot. We present and discuss quantitative results of the entrapped air bubble, the central wetted area, and the maximal spreading lamella as the impact velocity is increased. We further vary the lattice periodicity of the micro-patterns and find its profound influence on the macroscopic flow. More specifically, directional splashing can emerge, emitting secondary droplets in certain directions which are associated with the lattice. Directional splashing can be suppressed to a gentle spreading by decreasing the periodicity of the lattice and, furthermore, can be tuned to a completely-wetting splashing in the diagonal directions of the lattice by a larger periodicity, offering opportunities to control the wetting process. Finally, the elimination of directional splashing by reducing air pressure suggests that the underlying air is squeezed outwards by the falling droplet upon the solid boundary whereby the air flow is affected, leading to different splashing behavior.

Single-enzyme analysis in a droplet-based micro- and nanofluidic system

The kinetic activity of individual enzyme molecules was determined in aqueous droplets generated in a nano- and microfluidic device. To avoid high background noise, the enzyme and substrate solution was confined into femtoliter carriers, achieving high product concentrations from single-molecule encapsulation. The tiny droplets (φ ~ 2.5-3 μm) generated from this fluidic system were highly monodisperse, beneficial for an analysis of single enzyme activity. The method presented here allows to follow large numbers of individual droplets over time. The instrumental requirements are furthermore modest, since the small droplet size allows to use of standard microscope and standard Pyrex glass chips as well as the use of relatively high enzyme concentrations (nM range) for single molecule encapsulation.

Multiphase flow in lab on chip devices: a real tool for the future?

Many applications for lab on a chip (LOC) devices require the use of two or more fluids that are either not chemically related (e.g. oil and water) or in different phases (e.g. liquid and gas). Utilizing multiphase flow in LOC devices allows for both the fundamental study of multiphase flow and the development of novel types of pumping, mixing, reaction, separation, and detection technologies. Current examples of multiphase LOC applications include inkjet printers, separation of biochemical samples, manipulation of biomolecules, bio-sensing, enhanced mixing for bio-sample reactions, biomolecule detection, microelectronic cooling, drug delivery devices, explosives detection, dairy analysis, bubble computing and analysis of emulsions, foams, and bubble coalescence. In this focus article, we will briefly review the basics of multiphase flow with reference to microfluidic systems, describe some of the most promising flow control methods for multiphase fluid systems, and discuss our thoughts about future directions of microfluidic multiphase flow.

Breath Figure Method for Construction of Honeycomb Films

Honeycomb films with various building units, showing potential applications in biological, medical, physicochemical, photoelectric, and many other areas, could be prepared by the breath figure method. The ordered hexagonal structures formed by the breath figure process are related to the building units, solvents, substrates, temperature, humidity, air flow, and other factors. Therefore, by adjusting these factors, the honeycomb structures could be tuned properly. In this review, we summarized the development of the breath figure method of fabricating honeycomb films and the factors of adjusting honeycomb structures. The organic-inorganic hybrid was taken as the example building unit to discuss the preparation, mechanism, properties, and applications of the honeycomb films.

Geometry-controlled droplet generation in head-on microfluidic devices

We investigated the generation of droplets in a head-on microfluidic device operated with the two identical channels as inlets and the “long leg” as a constriction channel leading to a wider outlet section. For capillary numbers (Ca) of approximately 10−5 or less, we find a Ca-independent droplet volume equal to the volume of the constriction channel, which decreases at higher Ca when shear forces become relevant. The droplet generation mechanism is explained in terms of a global capillary instability involving surprisingly stable intermediate surface configurations.

A new method of UV-patternable hydrophobization of micro- and nanofluidic networks

This work reports a new method to hydrophobize glass-based micro- and nanofluidic networks. Conventional methods of hydrophobizing glass surfaces often create particulate debris causing clogging especially in shallow nanochannels or require skilful handling. Our novel method employs oxygen plasma, silicone oil and ultraviolet (UV) light. The contact angle of the modified bare glass surface can reach 100° whilst the inner channels after treatment facilitate stable and durable water-in-oil droplet generation. This modified surface was found to be stable for more than three weeks. The use of UV in principle enables in-channel hydrophobic patterning.

Screen-printing fabrication of electrowetting displays based on poly(imide siloxane) and polyimide

Abstract We report a screen-printing fabrication process for large area electrowetting display (EWD) devices using polyimide-based materials. The poly(imide siloxane) was selected as hydrophobic insulator layer, and relatively hydrophilic polyimide as grids material. EWD devices that use poly(imide siloxane) as hydrophobic insulator fabricated with conventional methods showed good and reversible electrowetting performance on both single droplet level and device level, which showed its potential application in EWDs. The compatibility of polyimide-based materials (hydrophobic poly(imide siloxane) and hydrophilic polyimide) guarantee the good adhesion between two layers and the capability of printable fabrication. To this end, the hydrophilic grids have been successfully built on hydrophobic layer by screen-printing directly. The resulting EWD devices showed good switch performance and relatively high yield. Compared to conventional method, the polyimide-based materials and method offer the advantages of simple, cheap and fast fabrication, and are especially suitable for large area display fabrication.

Microfluidics for electronic paper-like displays

Displays are ubiquitous in modern life, and there is a growing need to develop active, full color, video-rate reflective displays that perform well in high-light conditions. The core of display technology is to generate or manipulate light in the visible wavelength. Colored fluids or fluids with particles can be used to tune the light intensity (greyscale) or wavelength (colors) of reflective displays by different actuation methods. Microfluidic technology plays an increasing role in fluidic manipulation in microscale devices used in display areas. In this article, we will review microfluidic technologies based on different actuation methods used for display applications: pressure-driven flow, electrophoresis, electroosmosis, electrowetting, magnetic-driven flow, and cell-actuation principles.

Design and Application of an Alkaline‐Surfactant‐Polymer Flooding System in Field Pilot Test

We have developed an inexpensive sort of flooding reagent whose major components are natural mixed carboxylates from proportionally mixed saponification residuals of cottonseed oil, tea oil, vegetable oil, and soybean oil. An orthogonal test design method was used to select one form of this flooding reagent (which is named SD) by measuring the phase behavior and another form of the flooding system: alkaline/surfactant/polymer (named SD‐ASP). The optimal composition was obtained by measuring the transient interfacial tension (IFT) at the crude oil/aqueous interface. Results of the tests show that the SD‐ASP flooding systems are tolerant to salt up to 1.5×105 mg/L and to Ca2+ and Mg2+ to 5.0×103 mg/L at 90°C, the IFTmin (the minimum of IFT) is 10−3 mN/m, and the oil recovery ratio reaches 22.77% by using the core flood experiment. The pilot tests, including a putting in and sending out in a single oil well, a group oil well and a larger well region, were all very successful, with an average input‐output ratio of 1∶4.15. All these results demonstrate that the SD‐ASP formulation is a new, very inexpensive, and highly effective flooding system.

Component Effects on the Phase Behavior of Monoglyceride–Water Mixtures Studied by FT‐IR and X‐Ray Diffraction

Abstract The unit cell dimensions of the cubic monoglyceride (MO)–water phases were studied by x‐ray diffraction (XRD) when pure water was replaced by Iopamiro (Iopamidol aqueous solution). The XRD results suggested that there were no significant differences when pure water was replaced by Iopamiro, only at 30 wt % composition, the lattice parameter of the cubic phase (space group Ia3d) became slightly smaller when Iopamiro was used. The interactions among Iopamidol, Paclitaxel, PF‐127 and MO were also studied by Fourier‐transform infrared (FT‐IR) spectroscopy. The FT‐IR data indicated a somewhat disordered conformation of the MO acyl chain and structural rearrangements of the polar head‐group region in the cubic MO–water phases when the several components were added to this system. All findings in this report revealed that three components studied here could be added to this cubic phases and did not induce significant changes in the phase behavior of cubic MO–water phases.