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Electrowetting on liquid-infused film (EWOLF): Complete reversibility and controlled droplet oscillation suppression for fast optical imaging
Chonglei Hao, Yahua Liu, Xuemei Chen, Yuncheng He, Qiusheng Li, K. Y. Li, Zuankai Wang
TL;DR
The paper asks whether EWOLF’s reversibility and droplet-oscillation suppression can be harnessed for fast optical focusing. It uses a liquid-infused dielectric film and reports complete reversibility and oscillation suppression, while exploring an adaptive liquid-lens application.
Problem
The paper examines whether EWOLF’s reversibility and droplet-oscillation suppression can be harnessed for fast optical focusing.
Source: Results · p. 6Method
The approach uses a liquid-infused film as the dielectric layer and investigates manipulating viscosity to tailor transient-response damping.
Source: Results · p. 2, Discussion · p. 7Results
EWOLF provides complete electrowetting reversibility and suppresses severe droplet oscillation; the liquid lubricant is described as a damper.
Source: Discussion · pp. 7–8Takeaways & Limitations
The authors explore EWOLF as an adaptive liquid lens for fast optical focusing.
Source: Discussion · p. 7The authors note that an unresolved issue poses challenges for subsequent operations.
Source: Discussion · p. 8
Abstract
from arXiv · showhide
Electrowetting on dielectric (EWOD) has emerged as a powerful tool to electrically manipulate tiny individual droplets in a controlled manner. Despite tremendous progress over the past two decades, current EWOD operating in ambient conditions has limited functionalities posing challenges for its applications, including electronic display, energy generation, and microfluidic systems. Here, we demonstrate a new paradigm of electrowetting on liquid-infused film (EWOLF) that allows for complete reversibility and tunable transient response simultaneously. We determine that these functionalities in EWOLF are attributed to its novel configuration, which allows for the formation of viscous liquid-liquid interfaces as well as additional wetting ridges, thereby suppressing the contact line pinning and severe droplet oscillation encountered in the conventional EWOD. Finally, by harnessing these functionalities demonstrated in EWOLF, we also explore its application as liquid lens for fast optical focusing.
Results
This section characterizes EWOLF’s reversible electrowetting and damped droplet response, and examines its optical transparency and focusing behavior.
Sample preparation
- Oil infiltration made the PTFE film optically transparent, with excellent visible-light transmittance versus the membrane’s nearly zero transmittance.Replacing air pockets with oil reduces light scattering at the air/liquid/solid interfaces.Source: Figure 1 · p. 2, Results · p. 3
Complete reversibility of EWOLF
- EWOLF switched between ~103° and ~53° over 10 voltage-on/off cycles with nearly perfect reversibility, whereas EWOD was irreversible and its switching angle fell below 5°.EWOLF’s spreading and receding curves showed nearly perfect fidelity; contact-angle hysteresis was ~3° on the infused film versus ~40° on PTFE.Source: Figure 2 · p. 3, Results · p. 4
Suppressing droplet oscillation in the transient response
- 45 ms, with over 90% response-time reduction versus EWOD, accompanied EWOLF’s progressive contact-diameter growth and substantially reduced droplet-height oscillation.The authors attribute suppression to viscous liquid-liquid interfaces beneath the droplet and additional wetting ridges at its periphery.Source: Figure 3 · p. 5, Results · p. 5
Results
- In CCL mode, droplet-height oscillation was substantially reduced, and steady state was reached in ~45 ms, over 90% faster than in EWOD.The comparison follows the MCL stage without retraction.Source: Results · p. 5
- Lubricant infiltration formed viscous liquid-liquid interfaces and flexible wetting ridges that damp oscillation while preserving reversibility as ridge anchoring releases during cycling.The liquid-liquid interface enabled reversibility, while the soft, flexible ridges released their contact-line anchoring during activation and deactivation.Source: Results · pp. 5–6
Adaptive liquid lens for fast optical focusing
- EWOLF produced sharp, stable liquid-lens images in ~50 ms, while EWOD took ~250 ms and continued oscillating between blurry and sharp states.EWOD images fluctuated between blurry and sharp states even after EWOLF images had stabilized.Source: Figure 4 · p. 6, Results · p. 6
- At 51.0 ms the normalized EWOD droplet height was 0.61905, but it rose to 0.66786 at 53.5 ms, deviating from the focusing equilibrium and blurring the image.The height change tracked EWOD's continuing droplet oscillation.Source: Results · p. 7
Discussion
EWOLF combines electrowetting reversibility with suppressed droplet oscillation, using a liquid-infused dielectric film to increase viscous dissipation. Lubricant properties tune the transient response, and the approach is also explored for liquid-lens focusing.
Discussion
- Reconfigurable lubricant forms wetting ridges that add anchoring and viscous energy dissipation during the MCL and CCL stages, helping the lubricant damp severe oscillation.The paper contrasts this with EWOD, where dissipation is minimal and droplet oscillation is considerable.Source: Discussion · p. 7
- In a 50 µm film, the MCL and CCL damping ratios are 1.01 and 0.124, respectively, both larger than in EWOD.The paper reports that MCL damping depends considerably on lubricant thickness and viscosity, whereas CCL damping is almost insensitive to them but remains larger than EWOD's.Source: Discussion · pp. 7–8
- For surface 5080, the MCL damping ratio reaches 4.0—one order of magnitude larger than EWOD—while its steady-state response takes ~78 ms, versus ~65 ms on 2012 and ~50 ms on 2080.The paper identifies surface 5080 with the larger lubricant viscosity and thickness, and describes the response times as evidence of the importance of these properties for damping control.Source: Discussion · p. 8
- Unlike EWOLF, full oil encapsulation is reported to make electrowetting prone to over-damping and longer response times, and to make droplets challenging to address for subsequent operations.The longer response time is attributed to the enlarged liquid-liquid contact area in that configuration.Source: Discussion · p. 8
- EWOLF provides complete electrowetting reversibility and droplet-oscillation suppression together, with the liquid-infused film reported as the basis for both.The paper also reports exploring the approach as a liquid lens for fast optical focusing.Source: Discussion · pp. 7–8
- The paper describes EWOLF as scalable and low-power, and notes that its soft liquid-liquid interfaces make flexible electrowetting devices practical to develop.Its actuation principle does not involve moving mechanical parts or current flow.Source: Discussion · p. 8
Methods
The methods characterize wetting on PTFE and liquid-infused films, then measure electrowetting under changing potentials, repeated cycles, and step actuation.
Characterization of wetting properties and electrowetting responses
- The PTFE membrane and liquid-infused surface had contact angles of ~152° and ~103°, respectively; the infused film’s hysteresis was ~3°, much smaller than PTFE’s ~40°.Source: Methods · p. 9
Electrowetting experiment
- Potential was applied between a tungsten tip and ITO electrode, altered in 50 V increments every 5 s, with current monitored and the maximum limited to avoid dielectric breakdown.Apparent contact angle was examined under increasing and decreasing potentials.Source: Methods · p. 9
- Electrowetting cycles turned voltage on and off 10 times at 10 s intervals, and the PTFE membrane was also tested as the dielectric layer.The cycle and PTFE experiments are described as EWOD responses.Source: Methods · p. 9
Transient electrowetting responses
- At a 500 V step, a 2000 fps camera captured spreading and receding, while ImageJ measured normalized droplet height and contact diameter over time.Source: Methods · p. 9
Supporting information
Supplementary tests describe oil wetting on smooth Teflon and the resulting floating, cloaked state of water on the lubricant-infused film.
Drop stable configuration on lubricant infused film
- The stable configuration was investigated by measuring FC-70 and Krytox oil-drop contact angles on smooth Teflon in air and water.The notation uses o, s, a, and w for oil, solid, air, and water, respectively.Source: Supporting information · p. 13
- FC-70's final contact angles on smooth Teflon were nearly 0° in both air and water, suggesting preferential oil wetting and a floating deposited drop.Figure S1 shows the drop-morphology time evolution in the two environments.Source: Figure S1 · p. 13, Supporting information · p. 13
- Both oils had positive spreading parameters on water, indicating that a thin oil film cloaks the water drop at zero voltage.Figure S2 depicts the water drop's floating state on the lubricant-infused film.Source: Supporting information · p. 13, Figure S2 · p. 14