Capillary Based Sealing

This chapter provides a practical illustration of how the favourable scaling of surface tension can provide new engineering solutions at the microscale. The problem studied here is the fabrication of dynamic seals for fluidic microactuators . To date, fluidic microactuators are uncommon in microsystems because they are difficult to seal. However, they offer great potential for applications requiring high actuation force and strokes. The sealing problems is originating from the fact that standard seals such as rubber O-rings are unsuitable for microdevices, since they generate either high leakage or friction. Because of the advantageous scaling of surface tension at microscale, the authors were inspired to replace the classic rubber “O-rings” by liquid seal rings that are maintained by surface tension. This technology provides an elegant solution for this sealing problem since it eliminates friction and leakage problems. In this chapter, we will first provide a strong physical analysis of this problem based on capillary laws. This exercise shows how the physical phenomena described in Chap. 1 such as contact angle hysteresis, interfacial energy, and Young’s law are needed to understand this problem. Next we continue by pointing out how the selection of the sealing liquid strongly influences the performance of these capillary based devices. It is important to note that at the microscale the amount of material required to fabricate certain devices can be very low. Therefore, certain products that are prohibitively expensive in normal applications can still offer solutions in microsystems. We will present for instance devices based on liquid gallium and low melting point eutectic alloys. Further, we will describe how these capillary seals are fabricated and implemented in devices. We will discuss in detail all the fabrication aspects that are relevant for these systems, including coatings that are needed to optimally profit from the surface tension of the seals. Finally, measurements performed on prototypes are discussed, and we will provide insight in differences between analytical predictions and results obtained with experimental prototypes. We believe that this chapter not only provides an elegant example of how microsystems can profit from surface tension, but also provides a good example of the design of surface tension based systems, which is particularly helpful for engineers that are developing new surface tension based microdevices.