ELECTROLYTIC GENERATION OF TRAPPED NANOBUBBLES VIA
NUCLEATION CORE WITH PICOLITER PRECISION
Eugene Yoon and Ellis Meng
University of Southern California, Los Angeles, California, USAABSTRACT
A design configuration and methodology for control ofelectrolytic generation of gas bubbles within microfluidic
devices is presented that achieves decreased power
consumption and improved volume precision. Electrolytic
generation of single trapped nanoliter-microbubbles within
microchannels at picoliter precision (σx̅ down to 1.1 pL)
was achieved by employing a large surface area counterelectrode (CE). We also attained reduced energy
consumption (up to 44%). Micro and macro CEs with areas
spanning five orders of magnitude were evaluated while
holding the micro working electrode (WE) area constant.
To the best of our knowledge, this is the first investigation
into the relationship between CE size and nanobubble
volume repeatability with implications on electrochemical
bubble-based microfluidic devices.

to affect the repeatability of electrochemical reactions andthe additional surface area of a larger CE was hypothesized
to minimize such effects because the relative ratio ofcontaminated vs. nominal surface area is more favorable
than that of smaller CEs.

KEYWORDS
Nanobubble, microbubble, microfluidic, electrolysis,
precision, Parylene C, electrochemistry, Faradaic reaction

INTRODUCTION
Electrolytic bubbles can be generated on-demand andhave several advantages leading to their application in
MEMS pumps [1], valves [2], actuators [3], contrast agents[4], and sensors [5]. However, to date, these devices used
only microscale electrodes (<1 mm2) for the WE and CE.
While microelectrodes preserve a small device
footprint, this design choice results in several
consequences: increased power consumption anddecreased repeatability of generated gas volume. Since theelectrode-to-electrolyte
voltage (not electrode-toelectrode) drives Faradaic gas evolution [6], one strategy toimprove efficiency is to confine the voltage drop to theWE-electrolyte interface (Fig. 1).
This strategy can be implemented by using amacroscale CE with large surface area in lieu of a
microelectrode to increase the double layer capacitance(Cdl) of that electrochemical interface. Due toelectroneutrality, the same amount of charge is applied atthe WE and CE whenever a current/voltage is applied
between them. At the CE-electrolyte interface, most of thecharge is spent on charging Cdl so Faradaic reactions do notoccur appreciably. In contrast, the WE-electrolyte interface
Cdl is quickly charged beyond capacity and most of thecharge is therefore spent on Faradaic gas evolution. This
ideal configuration effectively and efficiently localizes
electrode polarization to the WE with only a simple
modification to the device design. If both WE and CE aresimilarly sized microelectrodes, energy is wasted inpolarizing both (non-ideal case).
To achieve precise nanobubble volume control, weinvestigated the effect of CE surface area on nanobubble
generation at a microscale WE within a microchannel.
Surface inhomogeneities on electrode surfaces are known

Figure 1: Electrochemical potential diagrams showing
voltage drop magnitude at the WE-electrolyte (green) andCE-electrolyte (red) interface. Ideally, the drop should be
localized at the WE-electrolyte interface. The total applied
voltage is indicated in blue.
For this work, generation of single nanobubbles withina microchannel was investigated (Fig. 2). The nanobubble
was generated within a nucleation core containing the WE
and directed into the adjacent microchannel. This format isnecessary to position the generated bubble away from any
exposed, catalytic Pt microelectrodes and prevent gas
recombination. The larger microchannel also facilitatedoptical analysis.

Figure 2: Schematic diagram showing main device
components. The working electrode (WE) is used withdifferent counter electrodes (CE) within and external to thedevice to generate the nanobubble.

MATERIALS AND METHODSParylene surface micromachining was used tofabricate microchannel structures containing exposed Pt
microelectrodes (Fig. 2). The left electrode in thenucleation core was designated as the WE. Different CEs
were used having areas that spanned five orders ofmagnitude and included the identical right electrode in thecore (XS), larger microfabricated electrode in the adjacentchannel (S), and two Pt wire macro electrodes placed
exterior to the channel but in contact with the electrolyte (Land XL, Fig. 3). Although CE-WE distances vary acrossthese cases, the associated iR drop is negligible comparedto the interfacial potential.

Figure 5: Testing setup for optical and electrochemical
data acquisition. The RE was Ag|AgCl (3M NaCl). Scalebar = 5 cm.

Figure 3: (Left) Two macroscale Pt wire CEs. The wire
diameters were 0.5 mm. (Right) Table showing thegeometric surface areas (GSA) of the tested CEs.
Room temperature 1 × phosphate buffered saline(PBS) served as the electrolyte and filled the microchannel
via fluid ports (Fig. 2 & 4). Devices were mounted in acustom acrylic test fixture which allowed them to be
immersed in PBS (Fig. 5). A potentiostat obtained
electrochemicalimpedance
spectroscopy(EIS)
measurements. A sourcemeter and oscilloscope wereconfigured as shown in Fig. 6 to control theelectrochemical cell with an Ag|AgCl reference electrode(RE). For voltage cycling experiments, WE-CE voltage
was cycled at ±100 mV/s; user input switched the ramp’s
polarity upon observation of H2/O2 evolution (3trials/device/CE size). For pulsing experiments, H2nanobubbles were generated (2H+ + 2e-  H2) undercurrent control (-0.6 µA, 6 s; 8 trials/device/CE size on 3devices). MATLAB scripts extracted nanobubble volume
from microscope images acquired at 10 fps (Fig. 9).

Figure 6: Circuit diagram of the device testing setup. A
sourcemeter applied and measured voltage or current
between the WE and CE. A 2-channel oscilloscope withunity gain voltage buffers measured voltage of the WE andCE with respect to RE.

Figure 4: a) SEM image of fabricated device. b) Close up
detail of the bottom fluid port, showing the opening to themicrochannel.

Figure 7. The EIS reveals that the WEs across the 4 tested
devices have similar electrochemical impedance
magnitude and phase.

RESULTS AND DISCUSSION
Electrochemical Impedance Spectroscopy
EIS prior to bubble generation revealed low interdevice variability. WEs displayed similar electrochemical
impedance magnitude and phase (Fig. 7). This ensures that
all tested devices were comparable.

Figure 8: Representative voltage vs. time plots. Applied voltage was ramped at -100 mV/s until H2 evolution was observedin real time. At that instant, voltage was ramped at +100 mV/s until O2 evolution was observed and then ramped at -100mV/s to 0 V. For larger CE sizes, power consumption is reduced because less applied voltage is required to generate gas
when the voltage drop occurs mainly at the WE-electrolyte interface.
Voltage Cycling
For voltage cycling experiments (Fig. 8), the hydrogen
evolution reaction (HER) and oxygen evolution reaction(OER) were consistently observed across all trials. The
total applied voltage (blue) and the WE-RE voltage (green)
were more closely matched when a larger CE were used,confirming that electrode polarization was successfully
localized at the WE-electrolyte interface. The timepoints at
which HER or OER reactions occurred revealed that lower
total applied voltage magnitude was required to generate
gas for larger counter electrodes. (Fig. 8). Larger CE size
experiments also exhibited less noise.
Pulse Tests: Bubble Energy
For pulsing experiments, the average total applied
voltage between the WE-CE pair, average current during
the pulse, and pulse duration were multiplied to computethe energy required to generate a single nanobubble. Table
1 reveals that larger CEs have the capability to reduceenergy consumed per bubble generated.
This energy reduction benefit was obtained between
sizes S and L (2.5 × 104 to 1.75 × 106 µm2), illustrating thatlarger CEs are more energy efficient. Up to 44% in energy
savings was achieved when comparing between XS and L(1.99 and 1.12 µJ) CEs.
Pulse Tests: Bubble Volume
Bubble growth and dissolution dynamics weresuccessfully tracked by using high frame rate image
acquisition software and custom MATLAB algorithms(Fig. 9 and 10).

Table 1: During the current pulse, the applied voltage wasmeasured. The energy per bubble was tabulated bymultiplying the average total applied voltage throughout
the pulse by the constant -0.6 μA current applied for 6 s.

Figure 9: Representative sequential frames showing
bubble volume experiments. Custom MATLAB algorithms
were used to process raw images into binary images inorder to quantify bubble area.

ACKNOWLEDGEMENTS
This work was supported in part by the National
Science Foundation under award numbers ECCS-1231994,IIP-1601340, and IIP-1827773 and the Viterbi Mousetrap
Fund. The authors would like to thank Drs. Dan Merrill andAlex Baldwin for the fruitful discussions which helped
frame this work. In additional, many thanks go out tomembers of the USC Biomedical Microsystems
Laboratory for their assistance.
Figure 10: A representative time course of H2 bubble
volume for a single current pulse. An approximation of thetotal bubble volume can be obtained by multiplying thebubble area as determined via the MATLAB algorithm bychannel height (as measured by a profilometer). Themaximum can then be easily extracted.

REFERENCES

The average maximum bubble volumes were 0.09,0.12, 0.11, and 0.08 nL for the various CE sizes (XS, S, L,and XL; respectively). Associated standard error (SE)averaged between datasets with appropriate statisticalmethods revealed that larger CEs yielded smaller SE inbubble volume. The best SE in one XL dataset consistingof 8 trials was 1.1 pL which corresponds to approximately
5 × improvement over the 5.1 pL average SE in the XS case(Fig. 10). These results suggest the advantage of increased
bubble volume precision when employing larger CEs for
microscale electrochemical devices. Although Pt wires
were used here, equivalent area thin-film electrodes should
result in a similar benefit.

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CONTACT*E. Yoon; tel: +1-213-821-3897; eugenejy@usc.edu*E. Meng; tel: +1-213-821-3949; ellismeng@usc.edu
Figure 11: From maximum bubble volume data, thestandard error was computed and plotted by CE size.
Larger CE provided greater precision in nanobubble
control than the smaller micro CEs.

CONCLUSION
This work examined the effect of CE size on theperformance of electrolytic nanobubble generators used in
microfluidic devices. Basic electrochemical principles
suggest the use of a large surface area counter electrode toimprove control and efficiency. A systematic study wasconducted and supported increase of CE size. Specifically,larger area CEs resulted in decreased power consumption
per generated nanobubble and improved precision of
bubble volume control.