3D capillary valves for versatile capillary patterning of channel walls

3D CAPILLARY VALVES FOR VERSATILE CAPILLARY

PATTERNING OF CHANNEL WALLS

V.A. Papadimitriou

_{, A.}

_{van den Berg}

_{, and J.C.T. Eijkel}

1

_{BIOS-Lab on a Chip Group, MESA+ Institute of Nanotechnology, MIRA Institute for}

Biomedical Technology and Technical Medicine, University of Twente, The Netherlands

ABSTRACT

We demonstrate passive capillary patterning of channel walls with a liquid in situ. Patterning is performed using a novel 3D capillary valve system combining three standard capillary stop valves. A range of different patterns is demonstrated in three channel walls. Capillary patterning was designed for precise in situ patterning of phase-change materials (e.g. UV curable hydrogels, gels, resins), but may find further applications such as local surface functionalization.

KEYWORDS: 3D capillary valves, Capillary patterning INTRODUCTION AND THEORY

In capillary stop valves, the capillary filling of a channel by a liquid will stop when it meets an abrupt expansion. Capillary stop valves have been extensively studied and used. A two dimensional functional model was proposed by Man et al. [1], explaining the behavior in terms of changes in the liquid-solid-gas interface energy of the system, which creates a local negative pressure (pressure barrier) forcing the liquid to stop. Later also 3-D models were proposed [2,3]. Capillary valves have found application in capillary pinning of hydrogels [4], as phase-guides for passive routing of liquids [5], and more.

The 3D capillary stop valves we propose here can be formed when a deep channel intersects another swallower channel (Figure 1a). When the liquid, that fills the deep channel via capillary forces, reaches the intersection, the top part of the liquid will meet an abrupt opening and it will be pinned in the vertical direction similar to the traditional 2D capillary stop valves (valve 1). The lower part of the liquid is able to flow freely across the swallow channel in the ditch on its bottom wall created by the deeper channel (valve 2). The liquid is getting gradually pinned in the horizontal direction as it fills the ditch (Figure 1c). When the liquid has crossed the entire shallow channel it will fully wet the deep channel and be pinned again vertically similar to the other side (valve 3) (Figure 1d). As final result, three channel walls are patterned with the liquid in this configuration.

Figure 1:a) Schematic of the 3D capillary valve. b) Dimensions of the valve. c) Partial filling of the valve. The top part of the liquid is pinned in the vertical direction and the lower part flows in the ditch while is getting pinned in the horizontal direction. d) Completely filled valve, the pinned edges are shown with dashed lines. The shape of the meniscus shifts from concave to convex due to pressure.

EXPERIMENTAL

In order to test our hypothesis, chips with varying horizontal valve length (Figure 1b) were prepared from SU8 masks in PDMS which after plasma treatment was bonded to standard microscope glass

978-0-9798064-9-0/µTAS 2016/$20©16CBMS-0001 1649 20th International Conference on Miniaturized Systems for Chemistry and Life Sciences 9-13 October, 2016, Dublin, Ireland

slides. The chips were treated with O2 plasma for 1min and heated to 60°C for 30min before every

filling trial to ensure the consistent hydrophilicity of the channel. The valves were tested by introducing a droplet of different vol. % mixtures of water and ethanol to the channel’s reservoir and allow the channel to be filled passively via capillary forces. The higher the percentage of ethanol in the solution, the lower the surface tension and the smaller the contact angle, which translates to lower valve pressure barrier.

RESULTS AND DISCUSSION

We were able to capillary-pattern patches, dead-end channels and rings as shown on Figure 2. A success rate of approximately 100% was obtained for a 75/25% (V/V) water/ethanol mixture. A higher amount of water caused an early stop of flow or incapability to fill the ditch. In contrast, a higher amount of ethanol resulted in valve bursting and flooding of the shallow channel. These results indicate the importance of the contact angle in the functioning of the method. Nevertheless, similar designs can be used with a wide variety of solutions with proper tuning of the dimensions of the system. Importantly, the fact that the ditch always has one side exposed to air makes the system insensitive to air trapping and formation of bubbles.

CONCLUSION AND OUTLOOK

New three dimensional valves have been demonstrated as a tool for passive in situ capillary patterning of various geometries on a channel wall. We have shown that, since the horizontal valve (ditch) is not the critical feature in the design, non-straight ditch geometries can be used in order to create various capillary patterns on the floor of the swallow channel. With smart designs that ensure that the pressure barrier of the pattern remains higher than pressure barrier of the vertical valve, we expect that still more complex patterns can be formed than shown here.

ACKNOWLEDGEMENTS

This work was supported and funded by Horizon 2020 Framework Programme of the European Union under the project H2020-PHC-634013 (PHOCNOSIS). We would like also to acknowledge Hai Le-The for providing the 3D CAD images.

Figure 2:Different capillary patterns in the bottom channel wall prior to filling (left) and after filling (right). REFERENCES

[1] P.F. Man, C.H. Mastrangelo, M.A. Burns, and D.T. Burke, “Microfabricated Capillary-Driven Stop Valve and Sample Injector,” Proceedings of Micro Electro Mechanical Systems 1998, IEEE, 45-50, 1998. [2] B. Hagmeyer, F. Zechnall, and M. Stelzle, “Towards plug and play filling of microfluidic devices by utilizing

networks of capillary stop valves,” Biomicrofluidics, 8, 05650, 2014.

[3] Tzong-Shyng Leu, and Pei-Yu Chang, “Pressure barrier of capillary stop valves in micro sample separators,” Sensors and Actuators A 115, 508–515, 2004

[4] B. Gumuscu, J.G. Bomer, A. van den Berg, and J.C.T. Eijkel, “Large scale patterning of hydrogel microar-rays using capillary pinning” Lab Chip., 15, 664-667, 2015.

[5] E. Yildirim, S.J. Trietsch, J. Joore, A. van den Berg, T. Hankemeier, and P. Vulto, “Phaseguides as tunable passive microvalves for liquid routing in complex microfluidic networks,” Lab Chip., 17, 3334-3340, 2014.

CONTACT

* V.A. Papadimitriou; phone: +31-633 621 802; v.papadimitriou@utwente.nl