Weak ac field-induced patterns in vertical deposition of colloids at various evaporation rates

Abstract

Abstract. Pattern formation in colloids by weak ac fields in vertical deposition-like configuration at different temperatures has been studied experimentally. At low evaporation (room temperature), the effect of the field leads to the evolution of a one-dimensional array of clusters along the contact line and columnar colloidal dried deposits are obtained at higher evaporation. We investigate the flow dynamics involved in this pattern formation. Homogeneous variation of the contact angle by electrowetting effect becomes unstable and breaks the translational symmetry at the meniscus. Electrokinetic forces together with capillary forces result in the accumulation of particles for pattern formation. The movement of electrically charged colloidal particles is controlled by weak ac electric field even at higher temperatures. We observe the effect of increasing initial particle concentration on the behavior of the clusters for various field frequencies. The average distance between clusters increase monotonically with an increase in the initial particle concentration. We also observe that the average width of columns increases according to the applied field strength.

Self-Assembly of Single-Sized and Binary Colloidal Particles at Air/Water Interface by Surface Confinement and Water Discharge

“…Several approaches have been developed throughout the years to obtain colloidal monolayers. Drop-casting is the easiest technique, but it suffers from severe limitations in terms of quality of the final monolayers due to the so-called coffee ring effect, i.e., the accumulation of particles at the edge of the drop and the resulting formation of rings of multilayers; in spite of variants to compensate for ring formation, control over the final patterns with this technique is still challenging especially for large areas. − Although better control on the final monolayer has been demonstrated with vertical convective assembly, the whole process requires complex apparatuses and careful calibration of several parameters, related both to the particles (e.g., diameter, volume fraction) and to the operating conditions (e.g., withdrawal speed, temperature, humidity), and is still prone to multilayer formation and stripe patterns. − Similar considerations hold true also for horizontal convective assembly or drag coating. − Electrostatic and electrokinetic phenomena have also been harnessed even in combination with the aforementioned methods for colloidal self-assembly, but they still require carefully designed assembly cells and proper electrode patterning and may also impose surface pretreatments which are not suitable for all substrates. − Spin coating, despite potentially suitable for self-assembly over large areas, requires complex protocols with multiple steps and accurate adjustment of process parameters, especially spinning speed and acceleration, essential to prevent major defects in the final colloidal crystal. − …”