For this experiment, we used Insight Chip’s advanced Nano Channel Chip. This chip stands out with several key features that allow for the detailed study of fluids or particles at an atomic scale using TEM. Its ultrathin liquid layers, down to 20 nm, are supported by Silicon Nitride membranes that bulge, typically less than 10 nm, due to the channel widths of the chip being very narrow, 1-2 μm.
The chip’s channel patterns vary depending on the type of flow experiments needed, including particle trapping, controlled diffusion, and liquid mixing. With 4 inlets/outlets creating a 2-bypass-channel configuration, the chip enables quick fluid exchange and stable continuous flow.
The chip is designed for TEM but also works well with SEM, optical microscopes, and even synchrotrons.
Here, we focus on testing the chip’s sensitivity and flow control by manipulating the flow of 100 nm gold particles in water. This involves delicate pressure management, utilizing the principle of hydrostatic pressure. By adjusting the height difference between the system’s inlet and outlet, we can finely control the pressure within the nano channels, demonstrating the chip’s precision and responsiveness.
To showcase the sensitivity of hydrostatic-pressure particle control, we simply aim to demonstrate the following:
- Forward and backward movement of particles, corresponding to the two reservoirs of the chips’ two inlets taking turns being higher than the other.
- Demonstrate the ability to stop particles with high precision by placing the two reservoirs at exact same height.
The pressure induced by 1 cm height advantage of one reservoir over another, corresponds to 1 mbar pressure applied to the one inlet inside the chip.
- 3x Flow tubes with connectors
- 1x Insight Chips SEM/OM holder
- 1x Insight Chips chip; B25W5 U-channel design
- Olympus BX51 optical microscope with a 50x and 20x objective lenses
- Solution from Sigma-Aldrich having 100 nm Auparticle concentration of 3.8E+9 particles/mL is used. For this experiment, the solution is 16 times up-concentrated and mixed with surfactant (Triton-X 100).
The highly concentrated 100 nm Au particle solution is made by centrifuging stock-concentrated samples in Eppendorf tubes. After centrifuging the clear liquid is removed from the tube and only the up-concentrated sediment remains. This up-concentrated solution has a particle concentration of over 10E+10 particles per milliliter.
The up-concentrated solution is mixed with Triton-X 100 at about 0.1 w.%.
The chips come in a variety of liquid thicknesses ranging from just under 10 nm, up to microns. It is important to select an appropriate liquid thickness for the particles used, in this case, 100 nm gold particles. A chip with ∼300 nm liquid thickness was found suitable as this would provide plenty of room for the particles and, without significant confinement of the particles’ free movement with respect to the flow.
Additionally, to enable finer tuning of the flow-rates, a design featuring so-called ”u-channels” was chosen. These channels connect only to 1 of the 2 bypass channels in the chip and therefore the pressure gradient across the nano channel is dictated only by the pressure applied to one bypass channel. The other bypass channel could be used for another particle, controlled with different pressures, but for this experiment, only one half of the chip is inspected, according to one bypass-channel being manipulated with pressure.
The chip used is shown in a 20x optical microscope image in Figure 2.
The chip is prepared simply by puncturing the membranes sealing each inlet in the chip with the gentle poke of a needle. These membranes are simply 25 nm SiN and are there to ensure the chip is clean and hydrophilic until used.
The solution is sonicated for around 10 minutes to break down any conglomerated particles.
The chip is placed in an SEM/OM compatible holder by putting its inlets facing down on top of four O-rings and sealing with a lid on top. Flow tubes are connected to the holder, fulfilling a flow circuit through the chip and its nano ”u-channels”.
The configuration is depicted in Figure 3.
The flow tubes are uniformly filled with the particle solution, wetting all internal surfaces of the tubes, holder, and chip.
Manually, the height difference of the tubes is changed, and the particle motion in the nano channels is recorded.
- Forward Flow: Decrease the height of the out-put reservoir relative to the input-inducing a positive flow-rate.
- Backwards Flow: Raise output reservoir above the input.
- Stopping the flow: Bring the input and output reservoir to the same level.
The movement 100 nm Au particles at three states(forward, backward, and stopping) under varying hydrostatic pressure gradients was successfully observed and quantified. Figure 4 illustrates different frames from the video recording, showing the particles in forward flow, backward flow, and stationary states. The frames were processed using ImageJ to improve clarity by removing background noise and highlighting particle trajectories.
The full video can be viewed here:
Figure 5 shows the measured velocity of the highlighted particle as a function of time divided into periods of different applied height differences. At a maximum height difference of 60 cm, the particle velocity peaked at 6.63 μm/s, which closely aligns with the theoretical velocity of 7.6 ± 1.0 μm/s.
The yellow line shows the theoretical particle velocity based on the pressure applied and calculated hydraulic resistance in the bypass and nano channels of the chip according to the relationship: ∆P = Rh · Q
Our observation is that this basic relationship can be used for nano channels and that flow control in the channels is extremely sensitive to small applied pressures.
The minor deviations in the velocity measurements could stem from
- slight sticking to channel walls, despite surfactant usage
- small inconsistencies in the manual height adjustment of reservoirs
- manual tracking of particles could result in slight misinterpretation of particle center.
These results validate the chip’s capability for precise particle control through its high sensitivity, demonstrating its suitability for applications requiring fine flow adjustments, such as microfluidic particle sort-ing or targeted delivery of nano particles.
The design of the presented Nano Channel Chip in this application note is but one of many different designs available from Insight Chips. Each chip is designed and made in the state-of-the-art clean room at the Technical University of Denmark (DTU). As each chip is made using maskless lithography, Insight Chips offers many different designs on the go. These allow for different applications and features of nano channels, suitable for trapping particles, mixing two liquids together in the field of view, or simply flowing a liquid as demonstrated here.
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