Without heating, in our experience evaporation does not tend to be a significant issue over the course of 10s of minutes. For longer experiments, one option is to slightly increase the droplet size to 2+ electrodes to allow droplet movement after some evaporation. For more severe evaporation (e.g., experiments lasting hours; actively heating), there are several ways to potentially address the issue with differing trade-offs. For example:

• Remove DMF chips from the DropBot and place them in a humidified chamber when incubating for extended periods
• Apply a wax barrier or gasket around the chip with small gaps at reservoir electrodes
• Use silicone oil filler media. Moving droplets in oil can prevent evaporation, but some of the downsides are potential crosstalk between drops through the oil (e.g., analytes partitioning into the oil phase), prevention of gas exchange for cell-based applications, general messiness, etc.

We've worked with drops ranging from a couple of hundred nanoliters up to almost a milliliter. Theoretically, the physics should scale beyond this range, as long as the aspect ratio between the electrode pitch and gap spacing is maintained [1]. On the low end, the limitations are (1) fabrication capabilities (how small can you fabricate electrodes and gaps between electrodes) and (2) evaporation (see #1). On the upper end, electrostatic driving forces should continue to scale, but gravity increasingly becomes a factor with larger volumes.

MicroDrop does not currently support capturing video or images. As a workaround, you can disable you webcam in MicroDrop and use another application (e.g., Windows Camera) to record video. Alternatively, you can use screen capture software (e.g., OBS Studio, HyperCam, or the Windows Game bar) to record your screen while MicroDrop is running.

This is one of the more common failure modes for DMF chips. We test all chips to ensure that there are no major defects in the dielectric layer, but sometimes pinholes can develop over time. You can minimize the probability of this happening by only leaving electrodes on when necessary and limiting the voltage as much as possible. Leaving liquid in the reservoirs for extended periods of time also increases the risk of electrolysis. We have anecdotal evidence that certain liquids may be more prone to this (e.g., water + salt seems to be worse than DI water).

If you are careful and if there are no major issues with the dielectric coating, this issue should occur infrequently. But if you are using chips multiple times over a long period, they will eventually fail. Sometimes we have experienced a bad batch of Parylene for which these electrolysis events are more common, but this should be a rare occurrence that should be identified during our QC process. If you get electrolysis right away when using a new chip, please let us know and we will happily provide credit for a replacement.

a. Protein fouling: any liquid containing proteins will eventually slow down as proteins adsorb to the chip surface making it less hydrophobic. This effect can be minimized through the use of surfactants (e.g., Tetronic 90R4 or Pluronics F68, L64 or L92).

b. Over voltage: if you use too high of a voltage, it can cause the surface of the chip to become more hydrophilic. This effect is less well understood (it's rarely mentioned in the literature) but there have been recent studies suggest the effect may be related to the "contact angle saturation" phenomenon in electrowetting [2]. One of the proposed mechanisms is that when the voltage exceeds the "voltage saturation limit", it can cause air ionization and/or charge injection at the three-phase contact line. In any case, the best way to avoid this problem is to use the lowest voltage you can get away with for any given liquid/operation. For example, you generally need a higher voltage to split/dispense drops, buy you can use a lower voltage for moving/mixing drops.

If you are just doing protocol development and testing, we recommend using propylene glycol. Chips tend to last longer using propylene glycol relative to aqueous solutions (e.g., we experience less surface degradation and electrolysis). We've had some chips moving propylene glycol for for several months without any noticeable change in performance.

1. Fair, R. B. “Scaling Fundamentals and Applications of Digital Microfluidic Microsystems.” In Microfluidics Based Microsystems, edited by S. Kakaç, B. Kosoy, D. Li, and A. Pramuanjaroenkij, 285–304. NATO Science for Peace and Security Series A: Chemistry and Biology. Springer Netherlands, 2010. https://doi.org/10.1007/978-90-481-9029-4_16
2. Swyer, Ian, Ryan Fobel, and Aaron R. Wheeler. “Velocity Saturation in Digital Microfluidics.” Langmuir 35, no. 15 (April 16, 2019): 5342–52. https://doi.org/10.1021/acs.langmuir.9b00220