Teck Neng Wong

Thermally mediated droplet formation in microchannels

Precise dispensing of microdroplets is an important process for droplet-based microfluidics. The dropletformation by shear force between two immiscible fluids depends on their flow rates, the viscosities, and the interfacial tension. In this letter, the authors report the use of integrated microheater and temperature sensor for controlling the dropletformation process. The technique exploits the dependency on temperature of viscosities and interfacial tension. Using a relatively low heating temperature ranging from 25 to 70 ° C , the droplet diameter can be adjusted to over two times of its original value. The relatively low temperature range makes sure that this concept is applicable for droplets containing biological samples.

Thermally mediated breakup of drops in microchannels

The authors used thermally induced surface tension gradients to manipulate aqueous droplets in microchannels. Control of the droplet breakup process was demonstrated. Droplet sorting can be achieved with temperatures above a critical value. Numerical simulation using a two-dimensional model agrees qualitatively well with the experimental results. The used control temperature of less than 55°C shows that this active control concept is suitable for biochemical applications. Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.

Thermally controlled droplet formation in flow focusing geometry : formation regimes and effect of nanoparticle suspension

This paper reports experimental investigations on the droplet formation of deionized water and a nanofluid in a heat-induced microfluidic flow focusing device. Besides the effect of temperature, the effects of nanoparticle suspension (nanofluid) and the flow rate of aqueous fluid on the droplet formation and size manipulation were studied. At constant flow rates of the two liquids, three different droplet breakup regimes were observed and their transition capillary numbers as well as temperatures were identified. The heat generated by an integrated microheater changes the droplet formation process. Increasing the temperature enlarges the size of the droplets significantly. These results also demonstrate that the titanium oxide (15 nm)/deionized water-based nanofluid exhibits similar characteristics in droplet formation at different temperatures and any small change in the flow rate of this nanofluid has little impact on the size of the droplets formed in a flow focusing geometry.

Thermally mediated control of liquid microdroplets at a bifurcation

The ability to precisely control the motion of droplets is essential in droplet-based microfluidics. It serves as the basis for various droplet-based devices. This paper presents a thermal control technique for microdroplets at a bifurcation. Control was achieved using an integrated microheater that simultaneously induces a reduction in fluidic resistance and thermocapillarity. The temperature of the heater was monitored by an integrated temperature sensor. At a bifurcation with symmetric branches, a droplet can be split into two daughter droplets of controllable sizes or entirely switched into a desired branch. The physics of this phenomenon was investigated with the help of a numerical model. Splitting and switching were demonstrated within an operational temperature range 25‐38 ◦ C. The relatively low operational temperature range allows this technique to be used for droplets containing biological samples. The present control concept is not limited to bifurcations, but can be employed in other geometries.

ACTIVE CONTROL FOR DROPLET-BASED MICROFLUIDICS

Active control of microdroplets in microchannels is an important task in droplet-based microfluidics. The breakup process of droplets at an T-junction is usually controlled passively by the fluidic resistance of the branches. We used thermal control to actively manipulate aqueous droplets in microchannels. The temperature affects both viscosity and interfacial tension between the phases. The concept was first simulated with a two-dimensional model. The simulation results show that increasing temperature at a branch can change the size ratio of the two daughter droplets from 0 to 1. That means, droplet switching is possible with this concept. Control of droplet size during the formation process and splitting process was demonstrated experimentally by varying the temperature of the branches. At a critical temperature, droplet switching can be achieved. The used control temperature of less than 40 ◦C shows that this active control concept is suitable for biochemical applications. Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.

Interface control of pressure-driven two-fluid flow in microchannels using electroosmosis

This paper presents an experimental investigation of the pressure-driven two-fluid flow in microchannels with electroosmosis effect. Two fluids, aqueous NaCl solution and aqueous glycerol, were introduced by syringe pumps to flow side by side in a straight microchannel. The external electric field was applied on the NaCl side. Under the same inlet volumetric flowrate condition, the applied electric field was varied. The interface position between the two fluids with the electroosmosis effect was studied in the first part of the experiment using the fluorescence imaging technique. In the second part, the velocity field was measured using the micro-PIV technique. The parameters, flowrate and electric field which affect the interface position and velocity field were investigated. The measured velocity field from the experiment agrees well with that of theoretical analysis.

Electro-osmotic control of the interface position of two-liquid flow through a microchannel

This paper presents theoretical and experimental investigations of the pressure-driven two-liquid flow in microchannels with the AN electro-osmosis effect. For fully developed, steady state, laminar flow of two liquids under the combined effects of pressure gradient, electro-osmosis and surface charges at the liquid–liquid interface, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field. It was shown that adjusting the externally applied electric field could control the fluid interface position precisely. The prediction from the proposed model compares very well with measured data.

Fabrication of heat sinks by Selective Laser Melting for convective heat transfer applications

ABSTRACT In this paper, the forced convective heat transfer performance of heat sinks produced by Selective Laser Melting (SLM) was experimentally investigated. Three heat sinks comprising pin fins of circular, rectangular-rounded and aerofoil geometries were fabricated by SLM from aluminium alloy AlSi10Mg powder. The heat sinks were tested in a rectangular air flow channel for convective heat transfer performance. Experiments performed for Reynolds numbers ranging from 3400 to 24,000 show that the heat transfer performances of the aerofoil and rectangular-rounded heat sinks exceeded those of the circular heat sink. Using the cylindrical heat sink as a benchmark, the average enhancements in the normalised Nusselt numbers were computed to be 15.0% and 21.4% for the rectangular-rounded and aerofoil heat sinks, respectively. It was demonstrated that SLM can be employed to design and fabricate heat sinks of customised geometries for heat sink applications.

Study on the viscosity of the liquid flowing in microgeometry

While micro-fabrication technologies are under extensive exploitation to fully realize the applications of micromachines etc, until the present time the basic mechanisms governing the underlying fundamental principles of these micro- and nano-scale systems have not been fully understood. As the characteristic dimensions of these devices approach micro- and nano-scales, attempts to understand the working phenomena pose great challenges. Under such situations, the commonly applied theories in continuum mechanics often become limiting cases or even completely unsuitable. For example, one of the commonly used properties, viscosity, seems to play an important role in characterizing the behavior of these micro- or nano-scale devices. In this paper, a theoretical study, employing a molecular theory to investigate the effects of the geometrical dimensions on the viscosity and flow characteristics for polar and non-polar liquids will be presented. The results indicate that the effects of the geometrical dimensions on the viscosity of liquids become significant as the dimensions approach the micro- or nano-scale, and these effects are more significant in polar than in non-polar fluids.

Hydrodynamically mediated breakup of droplets in microchannels

A flow focusing junction is integrated into a microchannel to break up droplets into a controllable number and size of daughter droplets. High speed images of the breakup at the flow focusing junction show that the breakup depends on the interplay between interfacial tension, shear force on the interface, and the confinement of the microchannel. Phase diagrams of the splitting performance show that the breakup is controllable by varying the flow rate of the continuous phase or by varying the size of the mother droplet.