Rapid vacuum-driven monolayer assembly of microparticles on the surface of perforated microfluidic devices

Wafer-Scale Particle Assembly in Connected and Isolated Micromachined Pockets via PDMS Rubbing

The present contribution reports on a study aiming to find the most suitable rubbing method for filling arrays of separated and interconnected micromachined pockets with individual microspheres on rigid, uncoated silicon substrates without breaking the particles or damaging the substrate. The explored dry rubbing methods generally yielded unsatisfactory results, marked by very large percentages of empty pockets and misplaced particles. On the other hand, the combination of wet rubbing with a patterned rubbing tool provided excellent results (typically <1% of empty pockets and <5% of misplaced particles). The wet method also did not leave any damage marks on the silicon substrate or the particles. When the pockets were aligned in linear grooves, markedly the best results were obtained when the ridge pattern of the rubbing tool was moved under a 45° angle with respect to the direction of the grooves. The method was tested for both silica and polystyrene particles. The proposed assembly method can be used in the production of medical devices, antireflective coatings, and microfluidic devices with applications in chemical analysis and/or catalysis.

Manipulating the morphology of colloidal particles via ion beam irradiation: A route to anisotropic shaping

Ion beam irradiation of spherical colloidal particles is a viable route to induce particle deformation, especially to get anisotropic shapes. Even though less common in comparison with dry etching techniques, different types of morphological changes can be attained depending on the process parameters (angle of incidence, energy, fluence of the ion beam, type of ion, temperature) and on particle material and initial particle arrangement (crystalline or disordered, made up of isolated or closely-packed particles). The technique can be harnessed to get anisotropic deformation of spherical colloidal particles into an ellipsoidal shape, but also to tailor the interstices between closely-packed colloidal particles, to get particle necking and coalescence as well as particle rearrangement. As such, particle deformation based on ion irradiation can find diverse applications from synthesis of ellipsoidal particles to modified templates for colloidal lithography. In this review, we examine in detail the principles and models of colloidal particle shaping via ion beam irradiation, the influence of process parameters on particle morphology and the applications of irradiated particles.

Self-organization of agitated microspheres on various substrates

The vibration dynamics of relatively large granular grains is extensively treated in the literature, but comparable studies on the self-assembly of smaller agitated beads are lacking. In this work, we investigate how the particle properties and the properties of the underlying substrate surface affect the dynamics and self-organization of horizontally agitated monodisperse microspheres with diameters between 3 and 10 μm. Upon agitation, the agglomerated hydrophilic silica particles locally leave traces of particle monolayers as they move across the flat uncoated and fluorocarbon-coated silicon substrates. However, on the micromachined silicon tray with relatively large surface roughness, the agitated silica agglomerates form segregated bands reminiscent of earlier studies on granular suspensions or Faraday heaps. On the other hand, the less agglomerated hydrophobic polystyrene particles form densely occupied monolayer arrangements regardless of the underlying substrate. We explain the observations by considering the relevant adhesion and friction forces between particles and underlying substrates as well as those among the particles themselves. Interestingly, for both types of microspheres, large areas of the fluorocarbon-coated substrates are covered with densely occupied particle monolayers. By qualitatively examining the morphology of the self-organized particle monolayers using the Voronoi approach, it is understood that these monolayers are highly disordered, i.e., multiple symmetries coexist in the self-organized monolayers. However, more structured symmetries are identified in the monolayers of the agitated polystyrene microspheres on all the substrates, albeit not all precisely positioned on a hexagonal lattice. On the other hand, both the silica and polystyrene monolayers on the bare silicon substrates transition into less disordered structures as time progresses. Using Kelvin probe force microscopy measurements, we show that due to the tribocharging phenomenon, the formation of particle monolayers is promoted on the fluorocarbon surface, i.e., a local electrostatic attraction exists between the particle and the substrate.