Microchannels with Self-Pumping Walls

Abstract

When asymmetric Janus micromotors are immobilized on a surface, they act as chemically powered micropumps, turning chemical energy from the fluid into a bulk flow. However, such pumps have previously produced only localized recirculating flows, which cannot be used to pump fluid in one direction. Here, we demonstrate that an array of three-dimensional, photochemically active Au/TiO₂ Janus pillars can pump water. Upon UV illumination, a water-splitting reaction rapidly creates a directional bulk flow above the active surface. By lining a 2D microchannel with such active surfaces, various flow profiles are created within the channels. Analytical and numerical models of a channel with active surfaces predict flow profiles that agree very well with the experimental results. The light-driven active surfaces provide a way to wirelessly pump fluids at small scales and could be used for real-time, localized flow control in complex microfluidic networks.

Keywords: Janus structured surfaces; active surfaces; chemical micropump; microfluidics; photocatalysts.

Figure 1

Left: A Janus pillar illuminated with UV light catalyzes a water splitting reaction, which gives rise to osmotic flow (blue arrows) around the pillar. Center: Within an ensemble of oriented pillars, a cooperative effect of these local osmotic flows leads to an unidirectional macroscopic flow (red arrow) along the alignment (TiO₂ → Au direction) axis of the pillars. Right: Within a channel, the active surface provides an active traction and drives bulk flow.

Figure 2

Fabrication of active surfaces by shadow deposition onto photolithographically patterned substrates. (a) Glancing angle deposition of Au, followed by deposition under normal incidence of (b) TiO₂ and (c) SiO₂. (d) Lift-off of the photoresist results in the TiO₂–Au micropillars as shown in the SEM image in e.

Figure 3

Pumping speed as a function of the light intensity and the spacing between micropillars. The surface with micropillars is covered by a 300 μm thick water film containing tracer particles. The tracer particles are imaged in a plane 1.5 μm above the micropillars. (a) Tracer particles undergo Brownian motion when the UV light is off, as opposed to (b) when the illumination is on and the tracer particles reveal directional flow along the channel. The small oscillations in the particle trajectories are caused by hydrodynamic interactions with the pillars and lead to a slight underestimate of the true slip velocity at the wall. The scale bar indicates 5 μm. The white dashed arrow at the left bottom indicates the flow direction. (c) The pumping speed increases linearly with the UV light intensity (photocatalytic activity). The dotted line is a linear fit to the data. (d) Pumping speed is also seen to depend on the spacing s between the micropillars. The maximum flow speed is observed for a spacing of approximately 2 μm.

Figure 4

Pumping is seen with 3D pillars but not with 2D disks. (a) Schematic of the two-dimensional Janus microdisks (top) and corresponding SEM image at bottom (scale bar = 1.5 μm). (b) Schematic (top) and SEM image (bottom, scale bar = 1.5 μm) of the three-dimensional Janus micropillar array. (c) Flow rate measurements above the two different surfaces reveal no pumping and pumping for the geometries shown, respectively, in panels a and b. Pumping speeds were measured using tracer particles at a height of 1.5 μm above the disks and pillars, respectively.

Figure 5

Flow profile in a 2D microfluidic channel can be engineered with self-pumping walls. Different flow profiles are achieved with (a) a single active surface, (b) two symmetric active surfaces, and (c) two antisymmetric active surfaces. We note that because of experimental variability between identically fabricated samples, the channels with two active surfaces do not always have identical slip velocities (see, e.g., panel c). Blue arrows in the upper schematics indicate the pumping direction of the top and bottom surfaces, respectively. In all three cases, experimental measurements agree with results of analytical and numerical modeling.