Wael Salalha

Investigation of fluidic assembly of nanowires using a droplet inside microchannels

Nanowires are common building blocks for the bottom-up assembly of electronic and photonic devices. A significant challenge is to introduce a single nanowire into an oriented assembly in order to express its unique anisotropic properties or to fabricate a nanodevice. In this work we focused on the development of a micrometer length scale approach, based on a fluidic method for alignment and assembling of nanowires. The alignment is achieved by manipulating a droplet composed of a dilute nanowire suspension by creating thermocapillary motion inside a microchannel. Our purpose is to explore the nanowires’ alignment mechanism in the middle region between the droplet’s front and rear menisci, and their interaction with the free surface and the contact lines. Experimental results show that nanowires which are found in the middle region of the droplet are generally aligned with the flow direction. Nanowires which reach the front meniscus move together with the displacing fluid which undergoes a “rolling” type m...

Alignment and self-assembly of elongated micronsize rods in several flow fields

Microfluidic alignment has attracted attention as a possible tool for the orientation of micronsize rods suspended in a solvent and the deposition of these rods in ordered arrays on solid surfaces. In the present work the following situations are realized experimentally: (i) a flow through an abruptly converging (or diverging) joint between wide and narrow straight channels which entrains and aligns (or misaligns) microrods and (ii) a tiny droplet of the order of a hundred microns that contains microrods is displaced due to Marangoni convection forces in a channel subjected to a temperature gradient. The flow near the advancing contact line orients the random suspension of rods and guides them towards the channel bottom where they are deposited in an ordered array. In parallel with this experimental study, the processes of microrod reorientation and alignment in the above-mentioned flows and in a similar sinklike flow into an issuing jet were studied using solutions of the Fokker-Planck equation for the o...

Microscale fibre alignment by a three-dimensional sessile drop on a wettable pad

Fluidic assembly provides solutions for assembling particles with sizes from nanometres to centimetres. Fluidic techniques based on patterned shapes of monolayers and capillary forces are widely used to assemble microfabrication devices. Usually, for self-assembly, the precondition is that the components must be mobile in a fluidic environment. In the present work, a shape-directed fluidic self-assembly of rod-like microstructures, such as an optical fibre on a wettable pad is demonstrated experimentally with submicrometre positioning precision. A model of the process is proposed, which accounts for the following two stages of the orientation of a fibre submerged in a sessile drop: (i) the drop melting and spreading over a wettable pad; (ii) fibre reorientation related to the surface-tension-driven shrinkage of the drop surface area. At the end of stage (ii), the fibre is oriented along the pad. The experimental results for the optical-fibre assembly by a solder joint have been compared to the modelling results, and a reasonable agreement has been found. The major outcome of the experiments and modelling is that surface tension forces on the fibre piercing a drop align the fibre rather than the flow owing to the spreading of the drop over the horizontal pad, i.e. stage (ii) mostly contributes to the alignment.

Entrainment of a film on a surface from the meniscus of a liquid wedge during coating

The shape evolution of an entrained film from the meniscus of a liquid wedge is studied, both experimentally and theoretically. The liquid wedge is formed by a droplet of liquid injected between a substrate and a tilted plate. When the substrate moves relative to the tilted plate with a constant velocity, a film of a constant slope is entrained on it, while another film remains on the tilted plate. The numerical and analytical investigation of the process provides the dependence of the length and slope of the entrained film after the end of drawing process, as well as the maximum thickness of the film on the tilted plate, on the capillary number. The length of the entrained film was found to be minimal for infinitely large capillary numbers when the surface tension effects are negligibly small. Experimental data confirm the predicted characteristic geometry of the film for capillary numbers up to 0.75.