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.

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.

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Invertase catalyzes the hydrolysis of sucrose producing glucose and fructose. However, its mechanism of action is not well understood as other studies have shown that there is an initial delay before the enzyme reaches peak activity. This indicates that there may be another step for the active form of invertase to be activated. To better understand this mechanism, isothermal titration calorimetry (ITC) was used to measure the enzymatic activity of invertase. In this study, varying concentrations of invertase between 0.0125 to 0.125mg/mL were used to react with 1M and 2M sucrose solutions. Results show an exponential relationship between varying concentrations of invertase and the time to reach peak height. Increasing 0.0125mg/mL of invertase by tenfold, decreases the time required for the enzyme to reach peak activity by 85%. This suggests at increased invertase concentrations there is more active invertase, which may be because invertase forms a type of multimer for full activity. The effect of temperature was also explored, 1.2M sucrose was titrated into 0.016mg/mL invertase at temperatures from 25 to 55 °C. By comparing 25 and 45°C, we observed that the time required to reach peak activity decreased by 62%. However, at 55°C, the time increased by 24%. Also contributing to the delay is the viscosity of sucrose. Concentrations of sucrose ranging from 0.5 to 2M were titrated into 0.0125mg/mL of invertase. The time for the enzyme to reach peak activity increases by 23% when increasing sucrose concentration from 0.5M to 2M. However, this increase in time was also observed in the control (0.5 to 2M sucrose concentration titrated into 0.1M sodium acetate buffer). Therefore, the viscosity of sucrose does not completely account for the delay. In conclusion, the delay in peak maximal activity is in part due to increased sucrose concentration, probably due to inefficient mixing. Additionally, the delay in maximal activity is decreased in the presence of increasing invertase concentration indicating that there are inter‐invertase interactions that are required for maximal invertase activity.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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