Impact of Pinning of the Triple Contact Line on Electrowetting

ARTICLE pubs.acs.org/Langmuir

Impact of Pinning of the Triple Contact Line on Electrowetting Performance Rohini Gupta, Danica M. Sheth, Teno K. Boone, Arianne B. Sevilla, and Jo€elle Frechette* Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States

bS Supporting Information ABSTRACT: Pinning of the triple contact line adversely affects electrowetting on dielectric. Electrowetting response of substrates with contact angle hysteresis ranging from 1° to 30° has been characterized, and the results are interpreted within the framework of electromechanics corrected for pinning. The relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis is quantified. Our results demonstrate that a modified electrowetting equation, based on balance of forces (including the pinning forces) acting on the triple contact line and on the drop, describes the electrowetting response of substrates with significant contact angle hysteresis. Finally, the surface properties of PDMS Sylgard 184 were found to be influenced by the electric field.

’ INTRODUCTION Spreading (or actuation) of a conductive liquid drop on a dielectric in response to an electric field (Figure 1a) is referred to as electrowetting on dielectric (EWOD),1
3 and is employed for digital microfluidics,4,5 variable-focus lenses,6,7 and electronic displays.8,9 Recent needs for the design of low-power and efficient electrowetting systems have triggered studies to understand the mechanism and limitations of electrowetting. According to electromechanics, the fringe fields distributed over a distance comparable to the dielectric thickness away from the solid
liquid interface (indicated by the red dashed box in Figure 1b) lead to a nonuniform drop curvature. The electric field beyond the red dashed box (Figure 1b) is negligible, and the macroscopic drop shape away from the solid
liquid interface, therefore, corresponds to a spherical cap with an apparent (macroscopic) contact angle (θapp) whose variation with the applied potential is described by the electrowetting equation (eq 1). The electrowetting equation originates from the quasi-static macroscopic balance of forces (excluding the pinning forces) that act on the liquid drop in contact with a charged dielectric parallel to the plane of the surface. The electrostatic force at the triple contact line is zero, and the potential and charge density across the electric double layer is never sufficient to alter the solid
liquid interfacial energy. The balance of forces that act on the triple contact line, therefore, indicates that there is no change in the local (microscopic) contact angle (θlocal) with applied potential.10
14 cV 2 ð1Þ 2γlv In eq 1, γlv is the liquid
vapor interfacial energy, c is the capacitance per unit area of the dielectric stack, and V is the applied

potential. The local angle (θlocal) represents the apparent angle measured at zero applied potential (or the initial angle), and is equal to Young’s angle (θY) for substrates with negligible contact angle hysteresis. Because of limitations in the optical resolution of the imaging technique used to estimate contact angles, only the apparent angle and not the local angle are accessible in macroscopic electrowetting experiments. The nonuniform drop curvature has, however, been observed experimentally by extending the distance over which the fringe fields are distributed via the use of thicker dielectrics, combined with imaging at higher magnification.12 The performance of electrowetting systems is determined by the fluid
surface interactions. Contact angle hysteresis is a manifestation of the pinning of the triple contact line due to surface chemical and physical heterogeneities. The presence of significant contact angle hysteresis (or appreciable difference between advancing and receding angles) leads to a threshold potential below which liquid actuation is inhibited,4,15
22 and adversely affects the reversibility of electrowetting response (or difference between apparent angles for increasing and decreasing potentials).18,23,24 Pinning also manifests itself as a stick
slip motion of the triple contact line for sessile drops,25 and adversely affects the switching speed.22 Moreover, models for drop dynamics need to account for pinning to make reasonable predictions regarding the correct shape and time scale of drop motion, splitting, and merging.26
29 Electrowetting systems operating at the oil
water interface6,15,20,23,30
35 on low contact angle hysteresis substrates18,24,36
39 are, therefore, preferred to ensure reliability

cos θapp ¼ cos θlocal þ

r 2011 American Chemical Society

Received: August 23, 2011 Revised: October 1, 2011 Published: November 03, 2011 14923

dx.doi.org/10.1021/la203320g | Langmuir 2011, 27, 14923–14929

Langmuir

ARTICLE

Figure 1. Schematic of (a) the setup employed for macroscopic electrowetting measurements, and (b) the nonuniform drop curvature resulting from the forces (excluding pinning forces) that act on the triple contact line and on the drop.

and reversibility. To expand the range of fluids and substrates for reliable electrowetting systems, it is important to gain a better understanding of how pinning influences the electrowetting performance. Experiments where the triple contact line is forced to advance or recede via the use of either tilted plane or drop expansion and contraction have revealed that the magnitude of contact angle hysteresis in electrowetting systems is independent of the applied potential,40
42 and can be reduced significantly via the use of alternating applied potential.41,43 This independence has been explained via electromechanics by incorporating the pinning forces in the macroscopic balance of forces that act on the drop.41 These experiments, however, did not address the presence (or magnitude) of the threshold potential for actuation or the degree of irreversibility of the electrowetting response. Models that take into account pinning have been proposed to predict the threshold potential for actuation of translating drops.18,20 However, these models are not applicable for sessile drops. Moreover, neither of these models has been directly interpreted to predict the electrowetting response beyond the threshold potential or to quantify the impact of pinning on the reversibility of electrowetting. Therefore, systematic experiments and their analysis aimed at quantifying the correlation between pinning and electrowetting performance are needed. In this Article, we establish the quantitative relationship between the contact angle hysteresis, the threshold potential for liquid actuation, and the electrowetting hysteresis for sessile drops. We have characterized the electrowetting response of substrates (115 samples) with contact angle hysteresis ranging from 1° to 30° using the modified electrowetting equation, which is based on the balance of forces (including the pinning forces) that act on the triple contact line and on the drop. Our results reveal that the electrowetting hysteresis and the contact angle hysteresis are equal in magnitude. We also observe that the electrowetting response deviates from the modified electrowetting equation in two transition regions: a threshold region prior to the onset of drop spreading and a transition region as the drop begins to recede with a decrease in applied potential. We show that these transition regions correspond to a change in the local angle from an initial angle to the advancing angle necessary for the triple contact line to advance, and from the advancing angle to the receding angle necessary for the triple contact line to recede, respectively. The magnitude of the threshold potential for actuation observed experimentally during this study is consistent with that estimated using the modified electrowetting equation. Finally, we observe that for PDMS Sylgard 184, the contact angle hysteresis is larger than the electrowetting hysteresis.

Table 1. Advancing Angle (θadv), Receding Angle (θrec), and Contact Angle Hysteresis for Water on the Hydrophobic Dielectrics Used hydrophobic dielectrics

contact angle

advancing

receding

hysteresis

angle

angle 84
102°

PDMS Sylgard 184

11
31°

108
116°

PTFE

14
23°

111
116°

93
98°

2
10° 3°

110
115° 107°

100
108° 104°

e1°

105
111°

104
110°

Cytop on PDMS PDMS 7-9600 films soaked in silicone oil

’ MATERIALS AND METHODS Hydrophobic Dielectrics. Polymer films (thickness: 6
12 μm) were used to systematically vary contact angle hysteresis (see Table 1). For spin-coatable polymers (polydimethylsiloxane and Cytop), a clean p-doped silicon (100) wafer (WRS materials) coated with 20 nm of chromium (99.9% purity, Kurt J. Lesker) followed by 200 nm of silver (99.999% purity, Alfa Aesar) acted as both support and electrode. Commercially available 10 μm thick polytetrafluoroethylene (PTFE) films (Goodfellow Corporation, εr = 2.1) were used as is, and the electrical contact was made via 200 nm of silver thermally evaporated directly on the rear of the films. Two varieties of polydimethylsiloxane (PDMS) were used: PDMS Sylgard 184 (two-part elastomer containing base and curing agent in ratio 10:1 by weight, Dow Corning, εr = 2.65) and soft elastomer PDMS 7-9600 (two-part elastomer containing base and curing agent in ratio 1:1 by weight, Dow Corning mixed with copolymer HMS-082, Gelest Inc.). The polymer mixtures were degassed for 15 min prior to spin-coating (slow spreading step of 500 rpm for 30 s and fast smoothing step of 5000 rpm for 60 s), followed by curing for 4 h at 80 °C in air for Sylgard 184 and 1 h at 140 °C in air for 7-9600. It has been reported that the presence of silica filler particles in PDMS (e.g., Sylgard 184) leads to contact angle hysteresis ranging from 20° to 40° as compared to