Thermally enhanced vertical deposition of colloids has provided a relatively fast way of obtaining monolayers or multilayers of colloidal particles. In order to obtain structured morphologies (i.e. as vertical stripes) it is needed to use previously patterned substrates. In this work, we give important clues about how it is possible to deposit structured colloids on plain (non-treated and non-patterned) substrates. We account for experimental evidence on the formation of a colloidal cluster array at a three-phase contact line. We study the influence of low frequency external alternating electric fields on a diluted colloidal dispersion opened to the air. We observe that the clusters are separated by a well-defined length and that, in our experimental conditions, they survive between 5 and 15 min.

On applying the electric field, the electrophoretic effect supplies particles to the meniscus much more efficiently than the Brownian motion does. Then, the surface forces trap particles in the meniscus. Also, ac electrowetting generates a flow near the contact line (due to the increase and decrease in the contact angle) that may become unstable, leading to a transverse modulation which would be responsible for the cluster formation. Regarding other interesting results of this system, please look at the paper ([M. Pichumani et al. Phys. Rev. E 83 (2011), 047301]) or at the arxiv preprint.

These new results could be of technological relevance in building tailored colloidal structures in nonpatterned substrates.

• • M. Pichumani et al. Phys. Rev. E 83 (2011), 047301.

  • M. Pichumani, Ph.D. thesis. Universidad de Navarra (2012)

This work was partly supported by the Spanish Government (ref. FIS2008-01126) and by Departamento de Educación (Gobierno de Navarra). M. Pichumani and M. Giuliani acknowledge partial financial support from the "Asociación de Amigos de la Universidad de Navarra".

Snapshots and movie of the contact line region (front view) at 1.8 Vpp and 0.8 Hz. The frames are captured at intervals of 1 min:

Sketch of the region near the contact line. Left: lateral view. Effect due to electrowetting has been enhanced for better view. Right: front view. Local flows are indicated by red arrows, and the corresponding transverse modulation in the contact angle is shown by the color gradient near the contact line. :