Say Hwa Tan

Oxygen plasma treatment for reducing hydrophobicity of a sealed polydimethylsiloxane microchannel

Rapid prototyping of polydimethylsiloxane (PDMS) is often used to build microfluidic devices. However, the inherent hydrophobic nature of the material limits the use of PDMS in many applications. While different methods have been developed to transform the hydrophobic PDMS surface to a hydrophilic surface, the actual implementation proved to be time consuming due to differences in equipment and the need for characterization. This paper reports a simple and easy protocol combining a second extended oxygen plasma treatments and proper storage to produce usable hydrophilic PDMS devices. The results show that at a plasma power of 70 W, an extended treatment of over 5 min would allow the PDMS surface to remain hydrophilic for more than 6 h. Storing the treated PDMS devices in de-ionized water would allow them to maintain their hydrophilicity for weeks. Atomic force microscopy analysis shows that a longer oxygen plasma time produces a smoother surface.

Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics

Interfacial tension and viscosity of a liquid play an important role in microfluidic systems. In this study, temperature dependence of surface tension, interfacial tension and viscosity of a nanofluid are investigated for its applicability in droplet-based microfluidics. Experimental results show that nanofluids having TiO2 nanoparticles of 15?nm diameter in deionized water exhibit substantially smaller surface tension and oil-based interfacial tension than those of the base fluid (i.e. deionized water). These surface and interfacial tensions of this nanofluid were found to decrease almost linearly with increasing temperature. The Brownian motion of nanoparticles in the base fluid was identified as a possible mechanism for reduced surface and interfacial tensions of the nanofluid. The measured effective viscosity of the nanofluid was found to be insignificantly higher than that of the base fluid and to decrease with increasing fluid temperature. The dependence on the temperature of the droplet formation at the T-junction of a microfluidic device is also studied and the nanofluid shows larger droplet size compared with its base fluid.

Magnetowetting and sliding motion of a sessile ferrofluid droplet in the presence of a permanent magnet

Motion of a droplet on a planar surface has applications in droplet-based lab on a chip technology. This paper reports the experimental results of the shape, contact angles, and motion of ferrofluid droplets driven by a permanent magnet on a planar homogeneous surface. The water-based ferrofluid in use is a colloidal suspension of single-domain magnetic nanoparticles. The effect of the magnetic field on the apparent contact angle of the ferrofluid droplet was first investigated. The results show that an increasing magnetic flux decreases the apparent contact angle of a sessile ferrofluid droplet. Next, the dynamic contact angle was investigated by observing the shape and the motion of a sessile ferrofluid droplet. The advancing and receding contact angles of the moving ferrofluid were measured at different moving speeds and magnetic field strengths. The measured contact angles were used to estimate the magnitude of the forces involved in the sliding motion. Scaling analysis was carried out to derive the critical velocity, beyond which the droplet is not able to catch up with the moving magnet.

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.

Formation and manipulation of ferrofluid droplets at a microfluidic T-junction

This paper reports the formation and manipulation of ferrofluid droplets at a microfluidic T-junction in the presence of a permanent magnetic field. A small circular permanent magnet with a diameter of 3 mm is used to control the size of the ferrofluid droplets within a microfluidic device. In the absence of a magnetic field, the size of the ferrofluid droplets decreases linearly with the increase of the flow rate of the continuous phase. In the presence of a magnetic field, the size of the droplets depends on the magnetic field strength, magnetic field gradient and the magnetization of the ferrofluid. The magnetic field strength is adjusted in our experiment by the location of the magnet. The induced attractive magnetic force affects the droplet formation process leading to the change in the size of the formed droplets. Experimental observation also shows that the relative change in the size of the droplet depends on the flow rate of the continuous phase. Furthermore, the paper compares the evolving shape of the ferrofluid droplet during the formation process with and without the magnetic field.

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.

The Microfluidic Jukebox

Music is a form of art interweaving people of all walks of life. Through subtle changes in frequencies, a succession of musical notes forms a melody which is capable of mesmerizing the minds of people. With the advances in technology, we are now able to generate music electronically without relying solely on physical instruments. Here, we demonstrate a musical interpretation of droplet-based microfluidics as a form of novel electronic musical instruments. Using the interplay of electric field and hydrodynamics in microfluidic devices, well controlled frequency patterns corresponding to musical tracks are generated in real time. This high-speed modulation of droplet frequency (and therefore of droplet sizes) may also provide solutions that reconciles high-throughput droplet production and the control of individual droplet at production which is needed for many biochemical or material synthesis applications.

Tunable micro-optofluidic prism based on liquid-core liquid-cladding configuration.

The integration of optical components into microfluidic systems has the potential to reduce the amount of bulky external devices and thus reduce the cost. However, one of the challenges of this concept is the accurate alignment of the optical path among multiple optical components inside a chip. We propose a tunable micro-optofluidic prism based on the liquid-core liquid-cladding structure formed in a sector-shape chamber. The optical interface of the prism is maintained in a straight line shape by distributing a row of pressure barriers in the chamber. By adjusting the flow rate ratio between core and cladding streams, the apex angle of the prism can be tuned accordingly. As a consequence, the deviation angle of the light beam refracted by the prism can be changed continuously. This tunability of our optofluidic prism can be utilized for the alignment of the optical path inside a chip or for the development of optical switches.

Self-Aligned Interdigitated Transducers for Acoustofluidics

The surface acoustic wave (SAW) is effective for the manipulation of fluids and particles at microscale. The current approach of integrating interdigitated transducers (IDTs) for SAW generation into microfluidic channels involves complex and laborious microfabrication steps. These steps often require full access to clean room facilities and hours to align the transducers to the precise location. This work presents an affordable and innovative method for fabricating SAW-based microfluidic devices without the need for clean room facilities and alignment. The IDTs and microfluidic channels are fabricated using the same process and thus are precisely self-aligned in accordance with the device design. With the use of the developed fabrication approach, a few types of different SAW-based microfluidic devices have been fabricated and demonstrated for particle separation and active droplet generation.