Note: A microfluidic chip setup for capillarity-assisted particle assembly

Abstract

We developed a microfluidic chip setup for capillarity-assisted particle assembly (CAPA). A capillary bridge is formed between the aperture of a silicon chip and the assembly template. The bridge is fed with particle suspension through a microfluidic channel on the chip top side. With this setup, we can control the particle assembly location and tune the suspension composition during particle assembly. In this note, we describe the chip setup, the CAPA process using the microfluidic chip, and results of complex particle assemblies, such as composite particle arrays and particle gradients, that could not be obtained using a conventional CAPA setup.

Insights into mechanisms of capillary assembly

“…Moreover, a sufficiently high local concentration of objects close to the trap is a 25 prerequisite to obtain a high assembly yield. Permanent trapping of the assembled objects, e.g., by binding to specific locations, is often also needed.…”

“…For the first time, confocal microscopy is used to visualize particle deposition into the traps, and we also provide a simple model that accounts for our observations and that can be used to gain useful insights into directing the assembly of controlled particle patterns simply by tuning the trap depth. In this respect, we show how controlling the trap geometry allows the fabrication of bi- 25 functional dimeric structures.…”

“…A glass piece (100 mm × 125 mm, 200 µm thick, Schott/DESAG, Mainz, Germany) was used as a backplane to support the PDMS. Two Teflon foils (100 µm thick) were used as 25 spacers around the patterned area. Prior to use, both the master and the glass were cleaned by oxygen plasma (Tepla 200 plasma system, 200 W, 1 mbar, 60 s) and then silanized by 1H, 1H, 2H, 2H -perfluorodecyltrichlorosilane and vinyltrichlorosilane (ABCR GmbH & Co KG, Germany), respectively.…”

“…Self-assembly, that is the spontaneous organization of the nanoparticles into such complex structures as the sole result of their properties and interactions, is the assembly route that has by far received the most attention. It has the advantages of being parallel, scalable and facile, and it has been successfully used to obtain 25 complex nanoparticle alloys, 3 directional clusters using DNA 4 or polymer 5 linkers, and simple colloidal molecules. 6 Often however, the results of self-assembly depend very sensitively on the assembly conditions and on the details of the particle properties and interactions, so that robust material fabrication, with high yield, reproducibility and the accuracy in particle organization and placement needed for 30 some applications (e.g., micro-electronics, sensing and plasmonics) cannot be achieved.…”

“…The same principle works also for objects ranging from the micrometre to the nanometre scale. So far, patterns have been assembled at single-particle resolution with particles down to 50 nm in 25 diameter. 17 However, not only particles can be assembled, also large molecules can be trapped by capillary assembly.…”