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RAPID VACUUM-DRIVEN ASSEMBLY OF DISPERSED MICROSPHERES ON THE SURFACE OF (NON-) PROFILED PERFORATED DEVICES
Abstract
Here, we propose a universal technique to firmly capture dispersed microparticles on any desired two-dimensional array of through-pores on a surface. The 10 μm silica or polystyrene particles are reversibly captured, making them accessible for direct manipulation and inspection or subsequent transfer to other surfaces. To obtain perfect arrays with a pitch of 1.25 μm, perforated devices with profiled surfaces were required. Additionally, the method has proven successful for both types of particles, either dispersed in water or ethanol. The assembly technique may serve as a platform for manufacturing hierarchical materials, e.g., ordered chromatography packings, or performing cellular assays.
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Analytical techniques are essential in detecting components within complex samples retrieved from various applications, e.g., rapidly detecting pathogens to prevent an outbreak. High Performance Liquid Chromatography (HPLC), is an analytical technique, in which a sample is pumped through a packed bed of microspheres (the column). Within this column, the components of the injected sample interact differently with the beads, hence leaving the column at different intervals.
Improving the efficiency of HPLC systems might benefit the clinical diagnostics of diseases, e.g., cancer. One approach to achieve this is by changing the packing of microspheres in the column from a random to an ordered state. We propose to accomplish this goal by a layer-by-layer assembly of these beads. This strategy is pursued by exploring the concept of a vacuum-driven force capturing a monolayer of precisely positioned beads on a micromachined device. As simple as it sounds, it suffices to say that it has been a challenge to assemble the particle monolayer. The main difficulties originated from the aggregation of these beads and their uncontrollable supply under dry conditions.
We have applied various techniques, such as rubbing, high voltage power supply systems, and shakers, to break the large cluster of microspheres, prior to offering them to the experimental setup. Furthermore, we have studied the interaction forces of silica or polystyrene beads on several surfaces to understand the mechanism of why they would stick on surfaces. It was observed that after rubbing, the microspheres unexpectedly had a preference to stick on a Teflon-like material. This result was explained by the tribocharging mechanism, which is the same mechanism responsible for the charging of a balloon while rubbing it on your hair. A key part of the project involved the design and fabrication of devices using micromachining technology. These devices were deployed in several domains of the project: to break the clusters, to control the supply of single particles with a filter, and to capture the particles with the vacuum force. Our studies revealed that droplets carrying the beads enhance the supplement as well as the quality of the obtained particle assembly. Moreover, funnel-like structures on which the microspheres are captured on the device, have proven to enhance the quality of the closely packed assemblies significantly.
On the cusp of a miniaturized device era, a number of promising methods have been developed to attain large-scale assemblies of micro- and nanoparticles. In this study, a novel method is proposed to firmly capture dispersed microparticles of nominal sizes of 10 μm on a two-dimensional array (1.0 × 1.0 mm²) of through-pores on a surface. This is obtained by dispensing a droplet of the particle dispersion on the pores, which drains by applying a vacuum-driven force at the backside of the pores. The assembled particles are captured on the surface in a reversible way, making them available for direct manipulation and inspection, or subsequent transfer of the particles to a second surface. The relevant process parameters dispersant concentration, dispersant type, particle properties, and pitch distance d, are optimized to obtain (near-)perfect ordered particle arrays. Furthermore, to significantly improve the quality of the particle assembly, washing steps are added to remove excess particles from the surface. Silica or polystyrene (PS) particle assemblies with an error ratio (ER) as low as 0.2% are obtained, demonstrating the universality of the proposed method. For the smallest pitch, d = 1.25 μm, even with optimal process parameters, higher ER-values (=1.1%) are obtained.
This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions