Ionic-surfactant-mediated electro-dewetting for digital microfluidics

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Abstract

The ability to manipulate droplets on a substrate using electric signals1—known as digital microfluidics—is used in optical2,3, biomedical4,5, thermal6 and electronic7 applications and has led to commercially available liquid lenses8 and diagnostics kits9,10. Such electrical actuation is mainly achieved by electrowetting, with droplets attracted towards and spreading on a conductive substrate in response to an applied voltage. To ensure strong and practical actuation, the substrate is covered with a dielectric layer and a hydrophobic topcoat for electrowetting-on-dielectric (EWOD)11-13; this increases the actuation voltage (to about 100 volts) and can compromise reliability owing to dielectric breakdown14, electric charging15 and biofouling16. Here we demonstrate droplet manipulation that uses electrical signals to induce the liquid to dewet, rather than wet, a hydrophilic conductive substrate without the need for added layers. In this electrodewetting mechanism, which is phenomenologically opposite to electrowetting, the liquid–substrate interaction is not controlled directly by electric field but instead by field-induced attachment and detachment of ionic surfactants to the substrate. We show that this actuation mechanism can perform all the basic fluidic operations of digital microfluidics using water on doped silicon wafers in air, with only ±2.5 volts of driving voltage, a few microamperes of current and about 0.015 times the critical micelle concentration of an ionic surfactant. The system can also handle common buffers and organic solvents, promising a simple and reliable microfluidic platform for a broad range of applications.

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Fig. 1: The proposed ionic-surfactant-mediated electrodewetting mechanism, studied with a sessile drop on a conductive, hydrophilic substrate.
Fig. 2: Effect of surfactant concentration and actuation voltage on the electrodewetting.
Fig. 3: Droplet generation, transportation and splitting realized by the proposed electrodewetting.
Fig. 4: The proposed electrodewetting confirmed for a variety of liquids on bare silicon.

Data availability

Most data generated or analysed during this study are included in the published article. The rest will be available from the corresponding author on reasonable request.

Code availability

The custom-written code that detects the droplet position and defines the horizontal reference to assist measuring contact angles will be available on reasonable request. The code also allows one to measure very low contact angles (<10°).

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Acknowledgements

This work was supported by the National Science Foundation (grants 1711708 and 1720499), by the National Institute on Aging (grant R21 AG049918), by the Volgenau Endowed Chair in Engineering (C.-J.K.), by Ralph and Marjorie Crump for the UCLA Crump Institute for Molecular Imaging (R.M.v.D.), by the Simons Math + X Investigator Award (award 510776) (J.L.) and by the University of Massachusetts Amherst startup package (T.L.). We thank S. Seidlits and W. Xiao for helping with the fluorescence microscopy, S. Sadeghi and M. Balandeh for discussions on electrochemical measurements, J. Wang for help with buffer solutions, and A. L. Bertozzi for discussions on modelling.

Author information

C.-J.K. conceived the project and J.L. designed most of the work and performed all experiments. N.S.H and J.L. carried out the fluorescent experiments. T.L. and J.L. conducted the contact-angle analysis. C.-J.K., R.M.v.D. and T.L. provided overall guidance and data interpretation. C.-J.K. and J.L. prepared the manuscript and all authors contributed to it.

Correspondence to Chang-Jin ‘CJ’ Kim.

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Competing interests

The authors declare no competing interests.

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Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Peer review information Nature thanks Frieder Mugele and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Contact angle measurement setup with the wire–droplet system used in the current study.

The silicon substrate sits on an XY positioning stage; the wire electrode is attached to a Z positioning stage; and two cameras, each mounted on their own XYZ stage, view two orthogonal sides of the droplet. Not drawn to scale, for clarity. All the stages are fixed on a vibration-isolation plate.

Extended Data Fig. 2 An ideal electric circuit model of the proposed electrodewetting compared with that of the electrowetting, considering an aqueous droplet placed on a conductive substrate as seen in Fig. 1.

a, For the proposed electrodewetting, a conductive substrate (Rs) is covered with a native oxide (tunnel oxide50), which is conductive (Rox). An external electric source (Vext or Iext) lets the current flow through the liquid (Il) and forms a voltage drop inside the liquid (Vl), which drives the ionic surfactant by electrophoresis. b, For electrowetting, a conductive substrate (Rs) is covered with an insulating dielectric material and a hydrophobic topcoat, which provide capacitance (Cd) and strong hydrophobicity. An external voltage source (Vext) establishes a voltage drop across the dielectric (Vd) but little voltage drop and no current across the liquid. Rl and Cl represent the resistance and capacitance of the droplet, respectively.

Extended Data Fig. 3 Experiments to support the proposed mechanism of electrodewetting.

a, Confocal microscopic images show that a fluorescent ionic surfactant (R18) is concentrated near the air–liquid interface of the droplet before actuation (red arrow), and becomes concentrated near the solid–liquid interface during the electrodewetting actuation (red arrow), corroborating that surfactant is driven to the substrate by electrodewetting. b, Steam condensation images reveal the wettable state of the substrate after blowing away (in the direction of the blue arrow) a water droplet containing DTAB. Unlike the unactuated droplet (left), the electrodewetted droplet leaves a dewettable area (right), corroborating that electrodewetting deposits surfactant on the surface. c, A water droplet with R18 was actuated to wet (by reverse electrodewetting), dewet (by electrodewetting), and wet (by reverse electrodewetting) the surface successively, and then the droplet was blown away to reveal a surfactant population map on the surface. Starting with an autophobed droplet, reverse electrodewetting (step 1, black arrow) cleans up the high-concentration R18 before electrodewetting (step 2, green arrow) deposits normal-concentration R18 (orange). Another reverse electrodewetting (step 3, red arrow) cleans up the normal-concentration R18 (orange) deposited by the previous electrodewetting, making the surface inside the droplet largely surfactant-free (blue). The fluorescence intensity on a fresh silicon substrate (that is, no surfactant) has a similar blue colour. This experiment corroborates that the deposited surfactant is removed by reverse electrodewetting actuation.

Extended Data Fig. 4 Effect of surfactant concentration and actuation voltage (Fig. 2) shown separately for each surfactant to include error bars or all data.

a-d, Contact-angle increase and dewetting time vs. surfactant concentration (left graph) and contact-angle increase and current flow vs. actuation voltage (right graph) for DTAB, TTAB, CTAB and SDS, respectively. Each symbol and error bar show an average and standard deviation of nine measurements (using about 180 images) made with three new droplets at three different locations across a wafer. Under the natural (unactuated) state, the contact angle was found to increase with surfactant concentration for all four surfactants. However, under the electrodewetted state, the contact angle was found to increase with surfactant concentration at low concentrations and decrease at high concentrations, with a maximal value in between.

Extended Data Fig. 5 Droplet evaporation prevention setup.

A glass cup was flipped upside down into a water tank to create an air pocket containing a wire, a silicon wafer and a droplet. Two varnished wires were passed through the water to connect the wire and wafer to a power source placed outside the water tank. A relay served as a switch to toggle the polarity of the current source. The silicon wafer and glass cup were mounted on stands and the water was adjusted to be higher outside the air pocket than inside. This setup slowed down the evaporation effectively, extending the droplet evaporation time, and thus the maximum testing time, from only a few minutes to 6 h, while allowing the replacement of the silicon chip and test droplet to be quick and easy.

Extended Data Fig. 6 Fabrication process of the ionic-surfactant-mediated electrodewetting device used to demonstrate the digital microfluidic operations (Fig. 3).

Not drawn to scale. The thin-down step was added only because SOI wafer with thin-enough top silicon layer was not available at the time of fabrication.

Extended Data Fig. 7 Electric actuation of a droplet atop two adjacent electrodes explained with an imaginary top wire, assuming a cationic surfactant.

a, When a droplet is actuated on the electrodewetting microfluidic device (Supplementary Video 4), it sits across a 0 V electrode and a 5 V electrode. For simplicity, we assume the droplet is symmetric and imagine an equipotential line of 2.5 V at the centre of the droplet. b, The case of a is electrically equivalent to having a 2.5 V wire in the droplet along the equipotential line. c, The case of b is electrically equivalent to having a 0 V wire and having a −2.5 V electrode and a 2.5 V electrode. We note that the left half of the droplet, where an electric field is formed from the wire (0 V) to the left electrode (−2.5 V), relates to Fig. 1b (that is, dewetting), and the right half of the droplet, where an electric field is formed from the right electrode (2.5 V) to the wire (0), relates to Fig. 1a (that is, wetting). Combining the left half (dewetting) and right half (wetting), the net effect is forcing the droplet to the right. We note that the red arrows indicate the overall direction of the electric field between electrodes and do not imply electric field intensity.

Extended Data Table 1 Electrowetting, EWOD and ionic-surfactant-mediated electrodewetting

Supplementary information

Video 1

A 3 μl droplet of 0.2 mM DTAB aqueous solution on bare silicon was actuated with around ±3 V. Because an aqueous solution with higher pH would induce more negative charges on the native oxide of silicon, autophobing by the cationic surfactant was dominant for a basic solution (pH = 11.2), masking the electrodewetting effect. For a neutral solution (pH = 6.5), autophobing was significant but not dominant, allowing the electrodewetting effect to add additional dewetting. For an acidic solution (pH = 2.3), no autophobing was found, allowing the electrodewetting actuation to fully control the wetting behavior.

Video 2

An aqueous droplet (pH ~ 2) containing 0.2 mM DTAB on bare silicon was actuated using highly excessive voltage of ~10 V with over 3 mA of current in this stress test, compared with the usual ~3 V and ~3 μA. The electrodewetting actuations continued to operate successfully even during the severe electrolysis. This robustness is a major advantage over electrowetting and EWOD devices, for which even slight electrolysis causes actuation degradation or device failure.

Video 3

To slow down droplet evaporation for the long-term test, the wire-droplet system was placed in an evaporation prevention setup described in Extended Data Fig. 5. The dewetting-rewetting cycles were performed by applying 3 μA and 2.5-3.0 V of alternating polarities. The 0.5 Hz switching was continued for > 6 hours and ended only because the droplet ultimately evaporated.

Video 4

The demonstration was obtained with an open-surface configuration (i.e., no cover plate) in air (i.e., no filler oil) on a silicon device. The electrode pads underneath the droplet were sequentially connected to 5 V or ground (equivalent to +2.5 V or -2.5 V for sessile drop) by a programmed electronic control.

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