DESIGN AND TECHNOLOGY CONCEPT FOR A NOVEL MICROPUMP COPING WITHOUTMOVING MECHANICAL COMPONENTS

L. Dittrich1, C. Endrody1 and M. Hoffmann1

1Micromechanical Systems Group, IMN MacroNano R©, Ilmenau University of Technology

Abstract — We present the design and atechnology concept for microstructures whichare the main component within a novel micropumpthat copes without moving mechanical parts.The interface of the liquid to be pumped itselfis periodically deflected into cavities containedin a microstructure by electrostatic actuation,generating the pump stroke at the same time.Emanating from established engineering conceptssuch as energy minimization, we demonstrate howthe initially indispensable Cassie-Baxter state of theliquid on the microstructure can be ensured andhow the stability of this state can be characterized.Based on the design, a technology concept for themanufacturing of micropumps is proposed, and wepresent the first manufacturing results.

Keywords: Micropump, Electrowetting onDielectrics (EWOD), Cassie, Baxter, Wenzel,Electrostatic Actuation, Superhydrophobic Surface

I – Introduction

Electrowetting on dielectrics (EWOD) is a powerfulmethod for manipulating liquids on free surfaces, andgrowing attention has been paid to its applications dur-ing the last decades, [1]. For example, electrowetting isthe standard transport method for droplets in labs-on-a-chip [1–3], it is utilized for dynamically tuning opticallenses [4, 5] or even in liquid displays [6] as well as forfluidic switches [7] and tunable optical filters [8].

Emanating from studies on water repellency in the1930s [9] and 1940s [10], it is well-known today that thecontact angle of water on a hydrophobic solid surfacecan be significantly increased by surface roughening,i.e. appropriately providing the surface with micro ornanostructures that are either periodically or randomlydistributed. Fig. 1 shows the two fundamental states aliquid can assume when being placed on topographicsurfaces.

Figure 1: Sessile droplet on structured surfaces in Cassie-Baxter state (left) [10] and in Wenzel state (right) [9]

Electrowetting on dielectrics is also a proven methodin order to change the state a liquid assumes on a

topographic surface [11, 12]. However, since an energybarrier between the two states impedes the self-actingreturn from Wenzel to Cassie-Baxter state, the attain-ment of the Wenzel state has to be prevented for pumpapplications. Emanating from the initial Cassie-Baxterstate, rather an electrostatic deflection of the liquid-gas interface (’virtual membrane’) is utilized in orderto generate the stroke of a novel micropump whichcopes without movable mechanical parts. A preferredflow direction arises from the combination of pumpingmicrostructures and accordingly the resultant periodicliquid flow with any type of (micro) valves. Fig. 2illustrates the working principle of the pump.

Figure 2: Working principle of the micropump without movingmechanical parts

II – Design of Superhydrophobic PumpingMicrostructures

Since the deflection of the liquid-gas interface is tobe utilized to generate the pump stroke, the Cassie-Baxter state must be the initial state of the liquid on themicrostructures and has to be maintained under elec-trostatic actuation. The stability of the Cassie-Baxterstate can be calculated making use of the total potentialenergy minization [13]. In the following, a sessile liquiddrop on a microstructured surface is considered neglect-ing gravity legitimated by a sufficiently small Bondnumber. Consequently, the drop assumes the shape ofa perfect spherical segment.

In order to ensure the Cassie-Baxter state as the initialstate, the surface energy of the liquid in the accordantconfiguration has to represent a global energy minimum[14, 15]:

ECB(R,θCB)< EW(R,θW) Vdr = constant (1)

ECB/W is the total surface energy in Cassie-Baxter andWenzel state, respectively, Vdr is the volume of a sessileliquid drop, R the droplet radius, and θCB/W the contactangle in Cassie-Baxter and Wenzel state, respectively.

Although the transition from Cassie-Baxter to Wen-zel state lacks of an universally valid model [14–17], anapproximate calculation for the energy barrier betweenthe two states is possible with the following simplifica-tions and assumptions [14, 15, 18]:

• The microstructures feature horizontal and verticalwalls only (no slopes, undercuts etc.).

• Contact angle hysteresis is neglected.

• The compression of the gas phase which is trappedin the microstructures is neglected.

• The liquid-gas interface is assumed to be horizon-tally flat during the transition and to have a contactangle of 90◦ to the (vertical) structure walls.

The transition occurs once the macroscopic contactangle under electrostatic actuation θEW is smaller thanthe microscopic contact angle θµ (similar to α in [19]).The stability of the Cassie-Baxter state is characterizedby the difference between a hypothetical semi-wettedstate with the highest possible surface energy Emax andthe surface energy in the Cassie-Baxter state:

EstabCB = Emax−ECB (2)

Emax corresponds to the (theoretical) configurationwhen the vertical structure walls are completely wettedwhereas the horizontal walls at the bottom are not yetwetted. Estab

CB can be considered as activation energywhich has to be externally supplied in order to initiatethe self-actuating transition from Cassie-Baxter to Wen-zel state.

The difference between the respective surface en-ergies in Cassie-Baxter (ECB) and Wenzel (EW) statecharacterizes the pumping structures as well:

∆ECW = EW−ECB (3)

The energy barrier arises from Eq. 2 and 3:

Ebarrier = EstabCB −∆ECW = Emax−EW (4)

Ebarrier denotes the local stability of the Wenzel state,i.e. the energy required to return back into the Cassie-Baxter state again.

A prismatic cavity matrix pattern with square cavityfootprint was chosen for the first tests (Fig. 3). Thestability number estab of the Cassie-Baxter state wasnumerically calculated with the parameters cavity widtha, depth h and wall thickness 2t (Fig. 4 and 5) fromestab = λ ·Estab

CB . The multiplier λ was chosen to be1

γlv ·πR2Y

in order to nondimensionalize the equation.

RY = 3

√√√√ Vdr

π ·(

23 −

3cosθY4 + cos3θY

12

) (5)

with the volume of the sessile drop Vdr, Young’s contactangle θY, surface tension γlv and the radius of the wettedarea in case of a smooth surface RY, [20].

Figure 3: Schematic representation of the structure matrix andits geometric parameters

Figure 4: Cassie-Baxter state stability dependency on thegeometric parameters a and t (in microns) with h = 20 µm.(θY = 115◦)

Figure 5: Cassie state stability dependency on the geometricparameters a and h (in microns) with t = 10 µm. (θY = 115◦)

The numerical calculations show that a smaller cavitywidth, a narrower structure wall and a bigger cavitydepth result in a greater stability of the Cassie-Baxterstate. From a theoretical point of view it can be con-cluded that a nanopore surface or its nanowire equiva-lent would be the most stable structure supporting theCassie-Baxter state [21]. At the same time, a wire-or pore-like structure necessitates higher electric fieldstrengths in order to achieve the desired deflection ofthe virtual membrane than structures having a smalleraspect ratio. All structures featuring an energy barrier

will support the Cassie-Baxter state. Too small barrierscan be vanquished by little mechanical impacts alreadywhereon the transition to Wenzel state occure.

Based on the design results, four different cavitymatrix layouts have been chosen for the fabrication ofpump prototypes (Table 1). Fig. 6 shows the diagramfor the comparison of the selected cavity layouts withh = 20 µm.

Table 1: Geometry parameters of four selected cavity matrixlayouts for water (γlv = 72.8 mN/m, θY = 115◦, Vdr = 7 µl)

a[µm] t[µm] EstabCB [nJ] ∆ECW[nJ] θW[◦]

G1 10 5 53.9 53.9 180G2 30 5 19.1 19.1 180G3 36 5 15.1 15.1 176G4 20 10 53.9 46.5 148

Theoretically, G1 and G2 exhibit no energy barrier.G3 features the lowest stability of the Cassie-Baxterstate and should thus possess the highest pumping effectat the lowest actuation voltage. In contrast to G1 andG2, the Wenzel state is a local energy minimum for G4.

Figure 6: Comparison of the selected cavity layouts, t = 5 µm

The Wenzel contact angle of 180◦ results in avanishing liquid-solid interface in our current model.This error clearly shows that the characterization of amicrostructured surface by macroscopic surface ratios(as done by Cassie, Baxter and Wenzel and adopted forthe presented approach) are not sufficiently complexto model the energy barrier near the upper limits ofsuperhydrophobicity (θ → 180◦). From a practicalpoint of view it has to be mentioned that the gasvolume which is trapped under each virtual membranewill significantly contribute to the stability of theCassie-Baxter state due to the rising pressure comingalong with membrane deflection. For this reason,closed cavities have been chosen for the pumpingmicrostructures.

III – Technology Concept

Based on the design, a process sequence for the fabri-cation of the described micropump was deduced. In thefollowing, the main process steps are described. Fig. 7

gives an overview over the main fabrication processesand the purpose of the respective functional layers.

Figure 7: Overview – technology concept for manufacturingthe micropump

1. Cover plate

C1: Indium tin oxide (ITO)-coated glass wafersare the starting point for the process sequenceof the cover plate. ITO serves as counterelectrode under the pumping structures.

C2: Silicon nitride is deposited in a chemicalvapour deposition (CVD) step as insulationof the counter electrode in order to preventfrom effects such as electrolysis which woulddamage the pumping structures by letting thevirtual membranes collapse into the cavities.

C3: The pumping microstructures are generatedby UV-lithography employing the negativephotopolymer SU-8.

C4: The contact angle of water on the SU-8 cavitymatrix is raised by dip-coating the structuresin a Teflon R© AF solution.

2. Bottom plate

B1: The fabrication of the bottom chips emanatesfrom blank silicon wafers.

B2: A metallization is sputtered onto the waferbackside in order to contact the wafer andthus the liquid in the pump chamber as wellas to ensure the later connectivity.

B3: The pump chamber (walls) are defined duringfrontside processing of an SU-8 layer whosethickness exceeds the one of the pumpingstructures.

B4: The connection between pump chamber andthe fluid ports which are mounted on thebackside in a later step is realized by a deep

reactive ion etch (DRIE) step during whichfluid vias are etched through the wafer.

3. Both the processed cover and bottom substrate arediced into single chips by a wafer saw.

4. After alignment, cover and bottom chips arebonded to each other based on the SU-8 definitionof the pump chamber on the silicon part.

5. In a final step, fluidic ports are mounted and theelectric wiring is carried out.

Fig. 8 shows a photograph which proves that wateron the fabricated microstructures assumes the Cassie-Baxter state. It can be clearly seen that the cavities (G4)are not filled by the advancing water volume (from theleft to the right due to electrowetting actuation) evenafter the contact line retracted again.

Figure 8: Photograph of water in Cassie-Baxter state onG4 pumping structures: the triple phase contact line of asessile water droplet, advancing from the left to the right byelectrowetting actuation

IV – Conclusion

It was demonstrated how the behaviour of a liquidon topographic microstructures can be modelled withthe objective of pump applications. Pumping structureshave been designed, manufactured, and their function-ality was proven. In further works, the influence ofliquid damping on the maximum actuation frequencyis to be indentified. The pumps will be characterizedregarding their maximum pressure and flowrate as wellas regarding the stability of the Cassie-Baxter state atcontinuous operation. In view of common technologytrends such as integration and miniaturization, theo-retical considerations will give information about theminiaturization potential of the new pumping principle.

The presented research is funded by the German Fed-eral Ministry of Education and Research under contract16SV5368.

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