Variable optofluidic slit aperture

Abstract

The shape of liquid interfaces can be precisely controlled using electrowetting, an actuation mechanism which has been widely used for tunable optofluidic micro-optical components such as lenses or irises. We have expanded the considerable flexibility inherent in electrowetting actuation to realize a variable optofluidic slit, a tunable and reconfigurable two-dimensional aperture with no mechanically moving parts. This optofluidic slit is formed by precisely controlled movement of the liquid interfaces of two highly opaque ink droplets. The 1.5 mm long slit aperture, with controllably variable discrete widths down to 45 µm, may be scanned across a length of 1.5 mm with switching times between adjacent slit positions of less than 120 ms. In addition, for a fixed slit aperture position, the width may be tuned to a minimum of 3 µm with high uniformity and linearity over the entire slit length. This compact, purely fluidic device offers an electrically controlled aperture tuning range not achievable with extant mechanical alternatives of a similar size.

Keywords: electrowetting on dielectrics (EWOD); optofluidics; scanning slit aperture; tunable.

Figure 1

(a) Picture of the unfilled adjustable optofluidic slit demonstration chip with attached cover. The slit is formed in the central quadratic transparent area defined by a chromium layer on the bottom side of the chip. (b) Top view schematic of the ink-filled device.

Figure 2

(a) Illustration of the electrowetting actuation showing the digital scanning of the slit activation and deactivation of adjacent electrodes. (b) Schematic close-up of the electrowetting region showing the electrode gaps and a snapshot of the charge distribution during EWOD actuation.

Figure 3

Excerpt of the mask design including the most important design features. The inset shows where the sketched layout is located on the fabricated and diced chip.

Figure 4

Illustration of the fabrication process: standard cleanroom processes are used to deposit and pattern thin films and permanent dry film structures for alignment. Dispensing and cover assembly are completed on chip level.

Figure 5

Left side: image series showing the advancing and receding of one liquid front for analysis of the maximal switching times for 106 V_rms at 1 kHz. Right side: images of different switching states with the measured maximal switching times between the states at the same conditions.

Figure 6

Evolution of the advancing (triangles up) and receding (triangles down) liquid front position over time for 106 V_rms at 1 kHz, as observed for both sides of the slit and for the maximal actuation distance of 1.4 mm. The measured time constants for 10%–90% transition are indicated.

Figure 7

(a) Scanning slits down to a slit width of 45 µm formed by electrowetting on structured electrodes of widths down to 25 µm. (b) Images showing the dewetting of the semi-hydrophobic Cytop fingers that supports forward actuation.

Figure 8

(a) Analog tuning of the slit in a central position with the according evaluated microscope images. Reciprocal fit with rms voltage at 1 kHz. (b) Excerpt of the graph showing a clear trend for the slit widths at high voltages.

Figure 9

(a) Schematic and (b) photograph of the band-selector setup. The scanning slit is positioned in the first diffraction order of a grating illuminated by a collimated white light source. The transmitted spectrum is evaluated by a spectrometer via a fiber coupled homogenizer. (c) 16 bands of the full spectrum can be selected by different slit positions. The single bands sum up to the full spectrum.