Scalable self- assembly of nanoparticles for industrial applications

Recent technological advances have opened up new fields that apply ordered structures of the nano and micro- scales made by assembly of colloidal particles. By applying this fabrication technique, one can achieve intrinsic structures using top-down approaches, such as soft lithography and pre-patterning of substrates. Alternatively, one can employ bottom-up self-organization processes that involved reduced production steps hence have of late become a hot research area. Several of these bottom-up techniques have already been developed, unfortunately, the scalability of some of their aspects to industrial applicable setup have proven to be problematic. To be specific, the aspect relating to the mechanism of mixed colloids size segregation in drying menisci is still hazy and need further investigation. More so, were it possible to study the size segregation with scalable experimental techniques, then it would metamorphose into a future technique of manufacturing complex structures by facilitating the design of the process responsible for binary colloids deposition.

To this end, Texas State University researchers (Sayantan Das, El-Shazly Duraia, Orlin Velev ( North Carolina State University), Maedeh Amiri) led by professor Gary Beall conducted a study with the objective of adapting the convective self-assembly technique, well known for producing highly ordered monolayer structures, to create novel surface patterns using the binary mixture of colloids. They intended to demonstrate that different patterns could be formed, based on the size ratio (Small/Large) of the particles, and particles volume ratio. Their work is currently published in the research journal, Applied Surface Science.

The research method employed commenced with the systematic examination of the effects of size ratio of the particles, volume fraction, and surface tension of the solvents. These examinations enabled the formation of various surface patterns and the understanding of the mechanism behind size-based segregation. Next, the researchers prepared the setup and kept it in a humidity controlled box. Then they undertook a qualitative analysis of the developed film using optical microscopy and videography. Lastly, a quantitative analysis procedure was undertaken where scanning electron microscopy was utilized.

The authors observed that certain binary particle mixtures resulted in spontaneous size based segregation. Moreover, in some cases, they noted that the particle separation occurred along the direction of the meniscus contact line. Consequently, their study revealed that some parameters, namely: particle volume fraction, size differences, surface tension, and the curvature of the meniscus played a crucial factor in the segregation process as well as in determining the width of each of the stripes.

In conclusion, the S. Das et al study presented the development and demonstration of a technique and mechanism for fabricating periodic size-segregated stripe patterns using binary colloidal films. Generally, it was seen that based on both empirical and numerical analysis, a mechanism for size-based segregation of particles via directed self-assembly could be developed. Altogether, this work has presented the periodic size segregated pattern formation by the mere design of industrially scalable process useful for various applications.