Do Jin Im

Electrophoresis of a Charged Droplet in a Dielectric Liquid for Droplet Actuation

Electrophoretic motion of a charged droplet in a dielectric fluid under an electric field has been investigated experimentally for use as a microdroplet actuation method. The effects of the droplet size, electric field strength, and electrolyte concentration and ion species on the charging of an aqueous droplet have been examined. The amount of electrical charging has been measured by two different methods: indirect measurement using the image analysis of droplet motion and direct measurement using the electrometer. Quantitative comparison of the droplet charge measured experimentally and the theoretical value of a perfectly conductive sphere shows that an aqueous droplet is less charged than the corresponding perfectly conductive sphere. The limiting effect on electrical charging is more significant for an electrolyte droplet, and the effect is positively correlated to the electrolyte concentration rather than the ion species. This implies that the low electrical conductivity of water is not a major cause of the limiting effect. The scaling law of the charging amount for a deionized water droplet nearly follows that of the perfect conductor, whereas for an electrolyte droplet, the scaling law exponent is slightly higher. Some advantages and potentials of the current droplet actuation method are also discussed in comparison with the conventional ones.

Submillisecond organic synthesis: Outpacing Fries rearrangement through microfluidic rapid mixing

Rapid mixing to race past rearrangement Chemistry relies on encounters between reactive partners. Sometimes one of the partners changes shape during the wait, spoiling the desired outcome. Kim et al. designed a microfluidic device to keep such botched encounters from happening. The device operates at low temperatures to keep individual reactants from isomerizing. It also achieves fast flow rates to maximize encounters between reactants on a microsecond time scale. The authors showcase the device by achieving bimolecular carbon-carbon coupling before one of the reagents can undergo a Fries rearrangement that would shift a neighboring group to the coupling site. Science, this issue p. 691 Bimolecular coupling can outpace a fast unimolecular rearrangement via extremely rapid low-temperature microfluidic mixing. In chemical synthesis, rapid intramolecular rearrangements often foil attempts at site-selective bimolecular functionalization. We developed a microfluidic technique that outpaces the very rapid anionic Fries rearrangement to chemoselectively functionalize iodophenyl carbamates at the ortho position. Central to the technique is a chip microreactor of our design, which can deliver a reaction time in the submillisecond range even at cryogenic temperatures. The microreactor was applied to the synthesis of afesal, a bioactive molecule exhibiting anthelmintic activity, to demonstrate its potential for practical synthesis and production.

Spontaneous electrical charging of droplets by conventional pipetting

We report that a droplet dispensed from a micropipette almost always has a considerable electrical charge of a magnitude dependent on the constituents of the droplet, on atmospheric humidity and on the coating material of pipette tip. We show that this natural electrification of a droplet originates from the charge separation between a droplet and pipette tip surface by contact with water due to the ionization of surface chemical groups. Charge on a droplet can make it difficult to detach the droplet from the pipette tip, can decrease its surface tension, can affect the chemical characteristics of solutions due to interactions with charged molecules, and can influence the combination and localization of charged bio-molecules; in all cases, the charge may affect results of experiments in which any of these factors is important. Thus, these findings reveal experimental parameters that should be controlled in experiments that use micropipettes.

Circulating flows inside a drop under time-periodic nonuniform electric fields

The circulating flows formed inside a spherical drop under time-periodic nonuniform electric fields are considered. For simplicity, it is assumed that there are axisymmetric electric fields and that the flow fields are in the Stokes flow regime. An analytical solution of the streamfunction distribution inside and outside the drop is obtained. The flow field is found to be dependent on the frequency of the time-periodic electric field and the ratios of the material properties such as the viscosity, the electrical conductivity, and the electrical permittivity. As part of the solution, an analytical expression of the dielectrophoretic migration velocity of a drop under a time-periodic electric field is also obtained. The result shows an interesting physics—that dielectrophoretic migration is possible in a time-periodic electric field even in the situation where dielectrophoresis would be impossible in a static electric field. By using the analytical solution of the streamfunction, fluid mixing inside a drop is analyzed based on the Poincare maps. The mass transfer enhancement factor due to fluid mixing has also been computed by solving the unsteady mass transfer equation numerically. The existence of an optimal frequency has been confirmed as in other mass transfer enhancement processes by time-periodic forcing.

Discrete electrostatic charge transfer by the electrophoresis of a charged droplet in a dielectric liquid.

We have experimentally investigated the electrostatic charging of a water droplet on an electrified electrode surface to explain the detailed inductive charging processes and use them for the detection of droplet position in a lab-on-a-chip system. The periodic bouncing motion of a droplet between two planar electrodes has been examined by using a high-resolution electrometer and an image analysis method. We have found that this charging process consists of three steps. The first step is inductive charge accumulation on the opposite electrode by the charge of a droplet. This induction process occurs while the droplet approaches the electrode, and it produces an induction current signal at the electrometer. The second step is the discharging of the droplet by the accumulated induced charge at the moment of contact. For this second step, there is no charge-transfer detection at the electrometer. The third step is the charging of the neutralized droplet to a certain charged state while the droplet is in contact with the electrode. The charge transfer of the third step is detected as the pulse-type signal of an electrometer. The second and third steps occur simultaneously and rapidly. We have found that the induction current by the movement of a charged droplet can be accurately used to measure the charge of the droplet and can also be used to monitor the position of a droplet under actuation. The implications of the current findings for understanding and measuring the charging process are discussed.

Digital Electrophoresis of Charged Droplets

A digital microfluidic system based on a direct electric charging and subsequent electrophoretic manipulation of droplets is made by simple fabrication at low cost. Digitally controlled two-dimensional droplet motions are realized by digital polarity control of an array of electrodes. By independent control of droplets and colorimetric detection, the coalescence and mixing of droplets is analyzed quantitatively. The gelation of sodium alginate and the crystallization of calcium carbonate by multiple droplet translations and coalescence and the actuation of glassy carbon beads are demonstrated to show the versatile manipulation capability of the proposed technology. Finally, we discuss the implications and potentials of the present technology.

Influences of electric field on living cells in a charged water-in-oil droplet under electrophoretic actuation.

We experimentally investigate the effects of high electric field on living cells inside a charged droplet under electrophoretic actuation. When an aqueous droplet suspended in a dielectric liquid contacts with electrified electrode, the droplet acquires charge. This charged droplet undergoes electrophoretic motion under strong electric field (1-3 kV/cm), which can be used as a droplet manipulation method in biomicrofluidic applications. However, because strong electric field and use of dielectric oil can be a harmful environment for living cells, the biological feasibilities have been tested. Trypan blue test and cell growth test have been performed to check the viability and proliferation of cells in a droplet under various electric field strengths and actuation times. We have not observed any noticeable influence of electric field and silicone oil on the viability and proliferation of cells, which indicates that electrophoresis could be safely used as a manipulation method for a droplet containing living biological system.

Electrophoretic motion of a charged water droplet near an oil-air interface

The trajectory of a charged droplet near an oil-air interface is investigated. When a charged droplet is translated in oil by electrophoresis, the droplet follows a straight line between two electrodes. However, if an oil-air interface is close to a droplet, the droplet follows an upwardly concave parabolic pathway. By using the leaky dielectric model, we have verified that this phenomenon is caused by the distorted electric field due to the difference in permittivities. Furthermore, it is enhanced by the accumulated free charges on the oil-air interface due to the difference in the electric conductivities of oil and air. Finally, we suggest a complementary arrangement of electrodes for realizing a straight pathway of the droplet near the interface.

Next generation digital microfluidic technology: Electrophoresis of charged droplets

Contact charging of a conducting droplet in a dielectric medium is introduced as a novel and useful digital microfluidic technology as well as an interesting scientific phenomenon. The history of this phenomenon, starting from original observations to its interpretations and applications, is presented. The basic principle of the droplet contact charging is also presented. Several fundamental aspects of the droplet contact charging from view points of electrochemistry, surface science, electrocoalescence, and electrohydrodynamics are mentioned. Some promising results for future applications and potential features as a next generation digital microfluidic technology are discussed, especially for 3D organ printing. Finally, implications and significance of the proposed technology for chemical engineering community are discussed.

Digital Microfluidic Approach for Efficient Electroporation with High Productivity: Transgene Expression of Microalgae without Cell Wall Removal

A unique digital microfluidic electroporation (EP) system successfully demonstrates higher transgene expression than that of conventional techniques, in addition to reliable productivity and feasible integrated processes. By systematic investigations into the effects of the droplet EP conditions for a wild-type microalgae, 1 order of magnitude higher transgene expression is accomplished without cell wall removal over the conventional bulk EP system. In addition, the newly proposed droplet EP method by a droplet contact charging phenomena shows a great potential for the integration of EP processes and on-chip cell culture providing easy controllability of each process. Finally, the implications of the accomplishments and future directions for development of the proposed technology are discussed.