How to Achieve Reversible Electrowetting on Superhydrophobic

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How to achieve reversible electrowetting on superhydrophobic surfaces Michail E. Kavousanakis, Nikolaos T. Chamakos, Kosmas Ellinas, Angeliki Tserepi, Evangelos Gogolides, and Athanasios G. Papathanasiou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04371 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Langmuir

How to achieve reversible electrowetting on superhydrophobic surfaces† Michail E. Kavousanakis,

¶

Tserepi,

‡School

‡,§

Nikolaos T. Chamakos,

¶

Evangelos Gogolides,

‡,§

Kosmas Ellinas,

¶

Angeliki

and Athanasios G. Papathanasiou

∗,‡

of Chemical Engineering, National Technical University of Athens, 15780, Greece

¶Institute

of Nanoscience and Nanotechnology, NCSR Demokritos, Aghia Paraskevi, 15310, Greece.

§These

authors contributed equally to this work

E-mail: [email protected] Phone: +30 210 772 3234

Abstract Collapse (Cassie to Wenzel) wetting transitions impede the electrostatically induced reversible modication of wettability on superhydrophobic surfaces, unless a strong external actuation (e.g. substrate heating) is applied. Here we show that collapse transitions can be prevented (the droplet remains suspended on the solid roughness protrusions) when the electrostatic force, responsible for the wetting modication, is smoothly distributed along the droplet surface. The above argument is initially established theoretically and then veried experimentally. †

Electronic Supplementary Information (ESI) available

1

ACS Paragon Plus Environment

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Introduction Droplets on microscopically structured surfaces can exhibit fascinating properties, with an o

antagonistic impact, including wetting states featuring very high (> 150 ) or very low (< o

30 ) apparent contact angle values (ACAs), as well as states with utterly adverse adhesion properties ranging from non-sticky to highly adhesive/immobile wetting states.

1,2

In order to

induce transitions between such dierent wetting states, it is required to modify the intrinsic wettability of the solid material through an external stimuli (electric, thermal, radiation etc).

Among the dierent techniques, electrowetting (EW)

modication, rapid switchability and reliability, application standpoint (e.g. lab-on-a chip,

5,6

4

3

combines large contact angle

rendering it a unique technology from an

electronic paper,

7

8

energy harvesting ).

Despite its merits, electrowetting is only ecient when applied on smooth surfaces, where only a limited range of apparent contact angle values are attainable, due to the constraints imposed by the material wettability (not exceeding and the contact angle saturation phenomenon, certain value (∼

o

60

9

120o

in common hydrophobic materials)

which prevents them from dropping below a

). When electrowetting is applied on structured surfaces

10,11

it can be ef-

cient only unidirectionally, and in particular inducing transitions from super-hydrophobicity to hydrophilicity;

12

however, driving the reverse de-wetting transition is not feasible through

voltage reduction or even by completely switching o the voltage application, thus, strong external actuations (e.g., thermal shocks, bration

15

13

means of an opposing at surface,

14

droplet vi-

) These irreversibilities occur when a droplet undergoes a collapse transition from

a Cassie state (where air inclusions are trapped between the droplet and the solid structure) to a Wenzel wetting state (where the liquid is fully impaled by the solid roughness).

16

The

latter corresponds to a local minimum of the free energy landscape, suggesting that an energy barrier has to be surpassed for a reverse de-wetting transition.

17,18

In particular, Cassie

and Wenzel states correspond to (meta)stable steady states (local minima of the free energy landscape), which are separated by an intermediate unstable (experimentally intractable) steady state. Then, the energy barrier to induce a wetting or de-wetting transition between

2


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Langmuir

the two (meta)stable states is determined by subtracting their free energy from the unstable state (saddle) that separates them. Several attempts have been made to prevent collapse transitions (Cassie to Wenzel) on structured surfaces, induced by electrowetting.

In particular, Lapierre et al. o

a moderate contact angle reversibility interval (160

19

achieved

o

- 130 ) by using specially designed

substrates covered by two layers of packed silicon nanowires.

Dense and tall structures

seem to help preventing Cassie to Wenzel transitions and increase the reversibility range. However, such structures feature poor mechanical properties rendering them unsuitable for various applications. McHale et al.

20

have demonstrated the use of liquid marbles, where

hydrophobic silica particles and grains of lycopodium are used as a conformable skin on the water droplet in order to achieve reversible electrowetting.

However, reversibility is

only guaranteed only over a limited range of applied voltages (up to approximately 200 V) and a contact angle modication not higher than

∼

o

10 .

At higher voltages collapse

transitions were encountered, and if the voltage was increased too high, the marble burst. A fully reversible wettability modication has been also demonstrated by Vrancken et al.

21

on

grooved solid substrates. Such solid topography (parallel grooves) can minimize the energy barriers for de-wetting transitions, however, it can not exhibit large apparent contact angles. The maximum attainable angle is

∼ 130

o

at zero voltage, which is comparable to that on a at

o

hydrophobic surface (120 on a PTFE at surface). Later, Barberoglou et al.

22

demonstrated

an electrowetting induced reversible contact angle modication on structured surfaces, which are produced by laser irradiation. wider in this case (140

o

Although the reversibility range appears to be slightly

o

- 108 ), it is obscure whether the droplet at the nal (reverted)

state exhibits high mobility, which is the second requirement for superhydrophobicity. The wettability modication cannot be termed reversible if the above criterion is not met. Finally, the largest contact angle reversibility up to now (from 140 et al.

23

o

o

to 78 ) has been reported by Im

by using specially designed convex micro-structures. There, an unusual dependence

of the apparent contact angle on the applied voltage has been presented and the reversibility

3


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Page 4 of 23

of the contact angle modication has not been clearly shown; pinning of the contact line has been observed only on advancing (by increasing the voltage), but not on receding which is strange. Summarizing, all the above studies are mainly focusing on optimizing the solid structure characteristics (e.g the curvature of the structures, the intermediate distance) for achieving electrowetting induced reversible wetting transitions. In this work, however, guided by detailed computations presented in our recent work,

24

we theoretically establish, and experimentally verify that a fully reversible wettability tuning on superhydrophobic surfaces can be possible when the thickness of the dielectric layer is suciently large, provided that the surface texture is such that it shows low contact angle hysteresis i.e. a droplet does not easily pin on the surface. In particular, Chamakos et al.

24

have shown that the local mean curvature of the droplet prole at the contact line exhibits strong dependence on the applied voltage when the solid dielectric layer is relatively thin (< 10 um). Dielectrics of such a thickness, or even much smaller, are typically used in electrowetting experiments. Surprisingly enough, it was also found that when using a thicker dielectric layer (> 50 um), the local mean curvature of the droplet prole at the contact line remains virtually unaected as the applied voltage increases.

This behavior is attributed

to the fact that the electrostatic pressure, acting on the droplet surface with a local negative contribution to the total pressure, is essentially active at a length scale proportional to the thickness of the dielectric layer. When the dielectric layer is thin, the electrostatic pressure contribution is concentrated at the vicinity of the contact line, leading to a signicant increase of the local mean curvature of the droplet (which is the same curvature as the local curvature). On the other hand, in the case of a thick dielectric layer, the electrostatic pressure contribution is smoothly distributed, resembling droplet compression experiments where a pressing force is applied to the entire droplet trapped between two parallel plates

25

(the pressure distribution here is uniform along liquid free surface). Specially designed structured surfaces can easily sustain high liquid pressures, either dynamic (generated by droplet impact) or static, resisting to wetting transitions. However they cannot resist to the elec-

4


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Langmuir

trostatic pressure generated by electrowetting (see for example

12,13

), since the electrostatic

stress is fairly localized, resulting considerable increase of the liquid surface curvature. High liquid curvature facilitates the liquid impalement by the solid roughness protrusions, thus causes the Cassie to Wenzel transition. The localized increase of the liquid surface curvature can be prevented by smoothly distributing the electrostatic force along the droplet surface using a thick dielectric layer.

24,26

Then the liquid impalement is prevented even for high applied voltages since the curvature remains practically unchanged. Here we propose that a key requirement for achieving contact angle modication reversibility in EW is the high enough thickness of the dielectric, securing that the solid surface, in terms of chemistry and texture, preserves its superhydrophobicity when high enough hydrostatic pressures are applied (EW resembles a droplet compression experiment in this case). In other words, a robust surface in terms of superhydrophobicity can perform reversible EW if it is dielectric and is thick enough. The above argument is next conrmed by performing detailed electrowetting computations and nally experimentally veried by demonstrating reversible wettability modication on superhydrophobic surfaces.

Computational Method Theoretical establishment Surfaces with topographic patterns can accommodate plenty of droplet shapes. The highly nonlinear dependence of the apparent wettability and adhesion properties on the intrinsic material wettability has been computationally demonstrated in.

18,27

In particular, by solving

the Young-Laplace (YL) equation augmented with a Derjaguin pressure term,

pLS ,

under a

proper parametrization, we enabled the ecient calculation of equilibrium of entire droplet shapes with an a priori unknown number of three phase contact lines (TPLs).

5


The aug-

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mented YL equation reads:

R0 LS p + C = K, γLV where,

(1)

C , is the dimensionless local mean curvature, K , is a reference pressure which remains

constant along the droplet surface,

γLV

is the liquid/vapor interfacial tension and

characteristic length used for non-dimensionalization.

The Derjaguin pressure,

R0 ,

pLS ,

is a

mod-

els the solid/liquid interactions and is composed of van der Waals, electrostatic and steric forces;

28

its isotherm has a Lennard-Jones form, with strong repulsion exerted by the solid

to the liquid in close proximity and attraction at intermediate distances. One mathematical formulation capturing the isotherm features described above, is the following:

18

l2 + δl0 − δ 2 R0 LS p (δ) = wLS 0 exp (−δ/l0 ) . γLV δ 2 l0

(2)

Here, the intrinsic wettability of the solid surface is quantied through the parameter which can be correlated with the Young's contact angle,

√ 3 − 5 cos(θY ) = wLS exp 2 A modied Young's angle,

θM Y ,

√

θY

5+1 2

through:

18

! .

(3)

is introduced in order to account for the eect of nanoscale

roughness on top of the microscale geometric features of a surface under study. rameter,

δ,

wLS ,

The pa-

is the signed distance of the liquid surface from the solid boundary, and can be

computed rigorously by solving the Eikonal equation.

27

This enables the application of the

augmented Young Laplace equation for any kind of solid surface geometry. The parameter l0 regulates the minimum distance, tions, l0

=

δmin

between the solid and liquid phase. In our computa-

−3 2×10 √ , which sets the dimensionless minimum distance to 1+ 5

δmin = 1 × 10−3 .

Then,

using standard numerical techniques for the solution of the augmented Young-Laplace equation (e.g., the nite element method), along with special parameter continuation techniques,

6


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Langmuir

a complete mapping of the solution space can be constructed. This provides information on the connection between the intrinsic wettability of the material and the apparent wettability of the patterned surface. Briey, in,

18

we found distinct intervals of contact angle values (material wettability

regimes) where only Cassie, or only Wenzel states can be accommodated by the structures. Interestingly enough, we also found regimes where all types of states i.e.

Cassie, Wenzel

or even mixed, commonly called partially impregnated, states can satisfy the equations and therefore they can be observed in experiments.

We computed their relative stability (by

comparing their surface energy) and we found that transitions from Wenzel to Cassie type states can only be spontaneous if the material wettability is very low i.e. when the Young's CA becomes greater that 135 degrees. At this material wettability regime a patterned surface can always sustain a Cassie state droplet even if the droplet spreads or recedes, due to an external forcing by an external pressure, mechanical or electrical. The key is to preserve the Young's angle above a certain value. In the case of applying a mechanical pressure the Young's CA is not modied. In the case of electrowetting, however, Young's contact angle practically changes when the dielectric thickness is lower than approximately 10 um and the applied voltage is suciently high.

24,26

Then once the Cassie to Wenzel transition sets in, by applying a voltage in an electrowetting system the dewetting transition is not spontaneous, when the voltage is removed, and requires considerable energy supply. Experimental

29

and theoretical

24

studies suggested that the transition sets in when the

outer three phase contact line depins from the edge of the asperity and slides down the pillar wall. When an electric eld is the cause, Cassie to Wenzel transitions are initiated when the local radius of curvature of the liquid meniscus, formed between the substrate protrusions, drops below a certain value (the distance between the solid protrusions).

As shown in,

24

the local curvature of the droplet surface at the contact line can signicantly depend on the thickness of the dielectric. In particular, it was demonstrated that the local curvature is more

7


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Page 8 of 23

sensitive to the applied voltage for relatively thin dielectrics, whereas on thick dielectrics the local curvature remains practically invariant, i.e. independent of the applied voltage. This nding drives us to the conclusion that, Cassie to Wenzel transitions through electrostatic actuation can be prevented if the dielectric thickness is suciently large, so that it can preserve the local curvature of the droplet surface between the surface protrusions at a value below the critical one (which initiates a collapse transition). Our claim is rst validated computationally by solving Eq.1, augmented with an electric stress:

27

Ne E 2 R0 LS p +C =K + , γLV 2 Ne

is the electric Bond number:

Ne =

o V 2 , with, γLV R0

o ,

(4)

and,

V,

and the applied voltage, respectively. The electric eld strength,

the vacuum permittivity

E,

is calculated along the

droplet surface by solving Gauss' law of electricity:

∇ · (r ∇u) = 0,

(5)

for both the ambient phase and the dielectric material, where tivity,

u, is dimensionless electric potential and E ≡ |∇u|.

ambient (air) phase and,

r = d ,

r

is the corresponding permit-

We note that,

r = air = 1, in the

in the dielectric material. The electric stress term depends

on the electric eld strength, which is computed by solving the equations of electrostatics for the ambient and the dielectric, in a domain which is bounded by the unknown droplet shape. This feature, in addition to the highly nonlinear Young Laplace equation (4), renders the problem nonlinear and free boundary which is solved iteratively through a matrix-free computational scheme preserving the computational cost at a low level.

8


30

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Langmuir

Experiments In order to support our computational ndings, we perform electrowetting experiments for water sessile droplets on superhydrophobic surfaces in air ambient. Among various types of o

structured surfaces that exhibit superhydrophobicity (i.e apparent CA > 150 ) we selected surfaces with high impalement resistance (in terms of critical hydrostatic or hydrodynamic pressure for preserving Cassie states) and low contact angle hysteresis. Low CA hysteresis is important for reversibility even in the Cassie state, as our computations suggest. High impalement resistance is important in order to preserve the Cassie state when high voltage is applied, in order to achieve large CA modication. And since it is expected that electrowetting easily induces Cassie to Wenzel transitions, we selected an exceptionally robust surface type, in terms of impalement resistance and low CA hysteresis and we made substrates with dierent dielectric thicknesses. Such surface type consists of submicron (quasi-)ordered pillar arrays, fabricated by means of colloidal lithography and plasma etching (see Fig. 1(a) for a scanning electron micrograph of the topography).

These structures exhibit extreme

resistance against droplet impalement, reaching up to 30 atm for water based mixtures.

31

The fabricated dielectric thicknesses range between 1.4 um to 75 um. The dielectrics underneath the surface structures are stacks consisting of tetraethoxysilane (TEOS), poly(methyl methacrylate) (PMMA), or SU8 photoresists. In particular we fabricated the following: a 'thin' stack featuring 400 nm tetraethoxysilane (TEOS) and a 1 um poly(methyl methacrylate) (PMMA) layer and three stacks featuring 12 um, 57 um, and 75 um thick layers of SU8 respectively.

Finally in order to show that even a surface structure with a moder-

ate resistance to impalement, but with low contact angle hysteresis, can perform reversible electrowetting if the dielectric thickness is large enough, we prepared a random dual scale micro-nannostructured topography on 188 um thick cyclic olen copolymer (COP) lm

32

(see Fig. 1(b)). Details of the fabrication method are included in the Supplementary Material section. The apparent contact angle of a water droplet (at zero voltage) is 158 the submicron (quasi-)ordered surface and 156

9

o

o

for

for the random micro-nanostructured COP


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surface, respectively. The contact angle hysteresis is ent contact angle,

θ

app

Page 10 of 23

∼5

o

for both surfaces.

The appar-

, was measured using a real time image processing software that is

developed in house and is based on drop shape analysis technique (a)

Figure 1:

31,32

9

.

(b)

(a) SEM image of the solid surface topography displaying submicron (quasi-

)ordered pillar arrays fabricated on the 1.4 um (400 nm TEOS + 1 um PMMA), the 12 um, the 57 um and the 75 um (SU8) dielectric layers. The pillars typical height, width and spacing (pitch) are 600 nm, 125 nm and 300 nm, respectively. (b) SEM image of the random micro-nanotextures fabricated on the 188 um COP lm.

Results and Discussion Computational Findings Our computational predictions here, concern equilibrium of droplets sitting on multi-striped dielectric surfaces of dierent thicknesses. We chose orthogonal stripes with rounded edges because this is a simple and representative geometry that can be used in order to highlight the eects of the dependence of the liquid surface curvature on the dielectric thickness. Trapezoidal or inverse trapezoidal geometries (i.e with overhangs) could be also used, like the ones used in Kavousanakis et al.

18

without aecting the qualitative features of our results.

The optimization of the surface geometry for improved impalement resistance is out of the scope of the current work. Striped surfaces (2-dimensional geometries) instead of pillared surfaces (3-dimensional geometries) are considered since the computational requirements for 3-d geometries could become prohibitive. Notice that in our computations we compute the entire droplet shape along with the coupled eld distribution assuming only translational

10


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symmetry in the direction normal to the

xy -plane.

In particular:

1. A thin dielectric case, where the dielectric material covers the trenched surface with a uniform thickness of

5

um.

2. A thick dielectric case, where the surface protrusions are on top of a dielectric slab of

100

um. In this case the surface protrusions are also considered as dielectric with the

same dielectric strength, i.e., the thickness of the dielectric ranges from um, where

h

100

to

100 + h

is the protrusions height.

The height and width of the protrusions are between protrusions is

18.6 um wide.

31

um and

18.6

um, respectively, and the gap

The dielectric in both cases is SiO2 with

ed = 3.8.

top hydrophobic coating we consider a material with a modied Young's angle of which corresponds to

wLS = 3.089

As a o

θM Y = 140

,

(see Eq.3).

Such a high contact angle can be achieved by secondary roughness scale (at the nanoscale). The chosen liquid is water in air ambient and the corresponding interfacial tension,

0.072

-1

N m . The characteristic length in our computations is

a droplet with volume 1

R0 = 0.62mm

γLV =

(if we consider

µL).

In Figs. 2(a) and (c), we present the dependence of the equilibrium apparent contact angle (calculated by tting a circle to the water/air interface above the solid surface asperities) on the dimensionless electrowetting number, dened as

η=

o d o d V 2 , where is the capaci2dγLV d

tance per unit area. Considering that a specic electrowetting number results, according to Lippmann's equation, in the same "electrowetting eect" for both a thin and a thick dielectric layer (with high capacitance and low voltage at the thin case and low capacitance and high voltage at the thick case, respectively), these two cases can be conveniently compared in Figs. 2(a) and (c). The main feature of this diagram is the solution multiplicity which is interpreted as wetting states multiplicity, e.g. for a given,

η , a considerable number of states

(solutions of the governing equations) is computed. Continuous lines correspond to stable states, whereas dashed ones correspond to unstable states.

11


Computed Cassie type states

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Figure 2: (a) Apparent contact angle vs electrowetting number,

η,

for a water droplet on a

pillared surface with 5 um dielectric thickness. Dashed lines (blue and red) depict unstable equilibrium states. The dashed gray line is drawn as a guide to the eye, showing the ACA variation by increasing the applied voltage. (b) Blow up of the bifurcation diagram showing the co-existence of Cassie, unstable partially penetrating and Wenzel states. (c) Apparent contact angle dependence on the electrowetting number for the thick dielectric case. grey shaded area illustrates the region of fully reversible transitions.

The

Dashed lines depict

unstable equilibrium wetting states. (d) Droplet free surface and electric potential distribution (dimensionless) at the gap formed by the surface pillars, for the two dierent dielectric thicknesses and the same electrowetting number (η

= 0.884).

The radius of curvature,

Rc ,

for the thin dielectric case is approximately 4 times lower compared to the thick dielectric one.

12


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are marked with blue; Wenzel states are marked with red. It is worth mentioning that for the thin dielectric case (see Fig. 2(a)), even a small value of the electrowetting number is enough for realizing Wenzel states; for example, for

η = 0.25 we compute

an extended range o

of apparent wettabilities, ranging from super-hydrophobic Cassie states (ACA> 150 ) to hyo

drophilic Cassie or Wenzel states (ACA< 90 ). Starting from a super-hydrophobic droplet (e.g. point C in Fig. 2(a)) and gradually increasing,

η,

the ACA decreases as the droplet

spreads over an increasing number of surface protrusions. The advancing of the droplet contact line from protrusion to protrusion, as the droplet spreads, is associated with instabilities at specic turning points in the diagram. An important wetting transition described in the diagram of Fig. 2(a) is the Cassie to Wenzel wetting transition; what signals this transition is an instability at the turning point, marked with CW, for

η = 0.884.

A blow up of the

diagram is shown in Fig. 2(b). At the turning point, CW, the radius of curvature, the droplet free surface becomes suciently small, (see Fig.

Rc ,

of

2(d)) initiating the collapse

transition where the liquid enters the surface protrusions. The resulting ACA modication depends on various parameters that aect the droplet dynamics like liquid viscosity, surface oscillations due to noise etc, which are not accounted here. What is predicted in our diagram is the possible congurations that the droplet can attain in equilibrium. In the case of the critical,

η = 0.884,

plenty of Wenzel states can be observed with ACA from 120

o

o

to 90 .

Such a behavior is usual in experiments where one can observe various Wenzel states, even with relatively high ACA. Another important aspect of the diagram in Fig.

2(a), is that the solution families

corresponding to Cassie or Wenzel states i.e. blue or red zig-zag like curves cross the zero-

η

vertical axis without featuring turning points at their left part.

conguration denotes innite hysteresis when decreasing,

η.

Such a solution space

Practically this means that for

the thin dielectric case, electrowetting can induce spreading but not receding i.e. de-wetting even for the Cassie states. Such a solution space structure implies pinning on the tops of the structures in the Cassie state. This pinning, due to innite hysteresis, is calculated also

13


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Page 14 of 23

for the thick dielectric case (see the non gray shaded part of Fig. 2(c)). In order to achieve receding i.e.

reversibility, the existence of turning points on the left part of the solution

curves is required, and in particular in positive values of

η.

This requirement is fullled

in the thick dielectric case. In Fig. 2(c) one can observe two important features: a) only Cassie states are found and, b), there is a distinct region, shadowed with grey, where wetting modication is reversible. The corresponding range of reversibility is between 121

o

o

to 78

(corresponding to the points P2 and P1, respectively). The range of reversibility, obtained by the structure of the solution space shown in Figs. 2(a) and (c), depends on the thickness of the dielectric and the surface topography.

A thorough parametric study of nding the

critical thickness in order to obtain a certain range of reversibility, as a function of geometry surface features is out of the scope of this manuscript.

The exclusive existence of Cassie

states can be also attributed to the invariability of the curvature (or the radius of curvature, Rc) of the liquid surface at the contact with the solid surface, as Fig. 2(d) shows. It is worth comparing the curvature radii of the thick and the thin dielectric respectively for the same electrowetting number,

η = 0.884.

One can see that Rc for the thick dielectric case (picture

shown on the right in Fig. 2(d)) is almost 4 times larger than the one of the thin dielectric (picture shown on the left in Fig. 2(d)); notice that the the apparent contact angle for the thick dielectric is lower.

The electric potential distribution,

u,

at the gap formed by the

surface pillars, which is depicted in Fig. 2(d), also exhibits signicant dierences between the two dielectric cases.

Note that, our computational methodology is not based on any

simplications regarding the actual shape of the droplet and the eld distribution.

Experimental Results Electrowetting experiments were performed on each surface using 7.5 ul water droplets and the results are presented in g. 3, where the dependence of the apparent CA on the electrowetting number,

η=

cV 2 , is shown. An eective Lippmann equation is tted (solid line 2γLV

in Fig. 3) as a guide to the eye (see also gure legend for details of tting):

14


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cV 2 , for a water droplet on 2γLV structured surfaces featuring dierent dielectric thicknesses. The red points (400 nm TEOS Figure 3: Apparent contact angle vs electrowetting number,

η=

+ 1 um PMMA, 12 and 57 um SU8) correspond to irreversible whereas the blue points (75 um SU8 and 188 um COP) to reversible modication of wettability. The capacitance per 0 d , is determined by tting an eective Lippmann curve (Eq. 6) to the unit area, c = d -5 -2 experimental measurements. In particular c = 2 × 10 F m for the thin dielectric substrate

c = 1.9 × 10 c = 3.1 × 10 F m for

-6

(400 nm TEOS + 1 um PMMA), m

-2

for the 57 um SU8,

-7

-2

F m

-2

c = 3.5 × 10 c = 7.8 × 10 F m

-7

for the 12 um SU8,

the 75 um SU8 and

-8

-2

F

for

the 188 um COP lm, respectively. A schematic of the experimental setup is presented in the inset of the gure.

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cosθ where

θ

app

0

app

= cosθ

Page 16 of 23

0 app

+ η,

(6)

is the static CA at zero voltage. The experiments are performed as follows: A

Figure 4: (a) Droplet snapshots showing an electrowetting experiment on a thin dielectric layer (400 nm TEOS and a 1 um PMMA). An irreversible wetting transition is observed in

η

this case (the ACA does not change by removing the applied voltage, 65.3 V or in (a)(iii)). (b) Snapshots demonstrating the contact angle reversibility (from 119

o

= 0.59, o

to 156 ),

when the voltage is switched o from 484 V (η = 0.44), on a 75 um thick SU8 substrate. A video clip of the later experiment is included in the supplementary material (S1).

DC voltage is applied in a step wise manner: in each step the desired voltage is increased linearly in time (order of a couple of seconds) and the resulting equilibrium CA is measured; then the voltage is switched o. so on.

In the next step, the voltage attains a higher value and

In each sample we proceeded by increasing the applied voltage until we observe a

CA modication from the previous step i.e.

the experiment is terminated when no CA

modication is observed. The red markers, in Fig. 3, correspond to experiments where the droplet spreads, when the voltage increases, but does not recede when the voltage is switched o from any value (i.e. no reversibility is observed). Blue markers correspond to experiments where the droplet reversibly retracts back to the initial (zero voltage) CA, when the voltage is switched o, respecting, though, the CA hysteresis; i.e. a small dierence from the initial CA is attributed to the inherent CA hysteresis of the surface. We measured that the CA o

can be modied almost 50 , though, without any sign of reversibility, for dielectrics having thickness below 57 um, except when we used very low voltages corresponding to

16


η < 2 × 10

-2

.

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Droplet snapshots showing the irreversible ACA modication, for the thinest dielectric layer (400 nm TEOS + 1 um PMMA), are demonstrated in Fig. 4(a). Interestingly enough, in the case of thicker dielectrics (75 um SU8 and 188 um COP) we observed reversibility with a considerable CA modication depending on the surface topography. maximum reversible modication of CA, observed in our experiments, is

In particular, the

∼ 37

o

o

, from 156

to

o

119 , in the case of the 75 um SU8 (see the corresponding droplet snapshots in Fig. 4(b)). We also note that in the nal (reverted) state, the droplet rolls-o easily by tilting the substrate only by

∼

o

5 , thus the droplet mobility has not been aected.

The COP surface i.e the one with arbitrary roughness but with large thickness (188 um) performed a reversible CA modication of

∼ 20

o

, namely from 157 to 137 degrees. It is worth

mentioning, however, that unlike the thiner dielectric layers (< 57 um), it is not the droplet impalement or even the surface pinning, that limits the wettability modication in this case. In particular, at a certain voltage threshold (η = 0.16 for the COP substrate), the droplet starts to randomly oscillate and nally ejects from the upper electrode. The above limitation cannot be attributed to the saturation phenomenon 60

o

3

since the latter usually appears at

∼

for a water droplet in air ambient. A possible explanation is that the electric stresses

along the macroscopic three-phase contact line are asymmetric (the electric charge is not evenly distributed) due to ever present chemical or morphological heterogeneities of the solid structure, thus leading to a droplet bulk motion on the superhydrophobic surface. The above justies the narrower range of wettability modication (from 157

o

o

to 138 ) in the

case of the random micro-nanotextured substrate (COP lm), as a result of the the larger structure heterogeneity. Such a behavior does not appear in the thiner dielectric cases, due to the strong pinning of the droplet on the solid structure (liquid impalement). In this case o

the droplet does not move even if the surface is tilted by 90 . The studied topograes, one with quasi-ordered pillared structures having extreme impalement resistance and one with random dual scale roughness with moderate impalement resistance, both having low CA hysteresis, showed considerable EW reversibility when the

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Page 18 of 23

dielectrics are made thick enough. Thus we suggest that the dielectric thickness is important in realizing reversible EW on surface with geometric patterns. Overall, we report that the thickness of the dielectric layer, which controls the action range of the electrostatic pressure,

24

is a key factor for fabricating surfaces with fully controllable wettability. This nding,

along with our theoretical analysis, explains the reversibility that Lapierre et al. in electrowetting experiments.

Lapierre et al.

19

reported

used very tall silicon nanowires (almost 40

um in height) which eectively provides a composite (air-silicon) thick dielectric, a suitable choice for reversibility, though the importance of the thickness was not highlighted therein.

Conclusions The nding that contact angle reversibility, in electrowetting, is closely correlated with the thickness of the solid dielectric opens new possibilities in manipulating micro-droplets, since the cost associated with the texture fabrication in miniaturized devices will be signicantly reduced. An important requirement concerning the surface structure is to secure high impalement resistance and low contact angle hysteresis. We note that reversibility comes at a cost of high applied voltage, due to the thick dielectrics required. One might achieve smooth force distribution at the TPL (in order to prevent Cassie to Wenzel transition), without necessarily using thick dielectrics, probably using composite materials. Notice, however, that the high required voltage is not prohibitive in applications, such as Lab on Chip systems utilizing dielectrophoresis. We believe that the potential applications of the real-time tuning of the wettability on superhydrophobic surfaces, using the electrowetting eect on a thick dielectric, are numerous despite the high voltage required and remain to be explored. Future work focuses on investigating the dependence of the critical dielectric thickness, above which reversibility is observed, for various solid structure geometries, respecting however the mechanical robustness of the structures. Small scale asperities, like nanowires with high aspect ratio, for example, seem to be advantageous for impalement

18


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resistance and thus for electrowetting reversibility but their mechanical strength is poor. On the other hand, strong structures lack impalement resistance for the sake of mechanical stability. Future work focuses on evaluating the wetting and de-wetting transition energy barriers

18

when performing electrowetting on various solid structure topographies. By using

such a tool, accompanied with dynamic simulations, we can design highly ecient surfaces with controllable liquid-solid adhesion properties.

Acknowledgement This work has been funded by the European Research Council under the Europeans Community's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [240710]. The authors acknowledge additional funding from the Hellenic National Strategic Reference Framework (NSRF) 20070-2013 under grant agreement no. MIS380835.

Supporting Information Available The following les are available free of charge.

• EW_supplementary.docx:

Surface preparation for the electrowetting experiments, Fab-

rication of submicron (quasi-)ordered pillar arrays, Random micro-nanotextured COP surfaces, Hydrophobization of the surface.

• S1_Reversible_modification_of_wettability.avi: contact angle reversibility (from 119

o

Video clip demonstrating the

o

to 156 ), when the voltage is switched o from

484 V on a 75 um thick SU8 substrate.

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How to Achieve Reversible Electrowetting on Superhydrophobic

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