One-dimensional conductive particle assembly holds promise for a variety of practical applications; in particular, for a new generation of electronic devices. Synthesis of such chains with programmable shapes outside a liquid environment has proven difficult.
These 1D patterns can be assembled either from particle groups or from individual particles. The latter can also be employed in mechanical contexts; for example, as flexible artificial flagella or cilia. Compared to other methodologies such as lithography, cluster-assisted assembly, and colloidal polymerization, field-directed assembly in electro- or magneto-rheological fluids provides a simple, efficient, and controllable approach for particle chain formation.
This method suffers from two major limitations that hinder its application in electronic device manufacturing. First, the assembly typically takes place in bulk liquid, which limits the control over positioning of the chains. Second, maintaining the formed structures normally requires a continuous energy supply; once the external field is turned off, the structures disintegrate. Avoidance of this limitation requires special functionalization of the particle surfaces, such as grafting of DNA or polymeric crosslinkers, and designing charge groups.
A route was developed to simply “pull” flexible granular and colloidal chains out of a dispersion by combining field-directed assembly and capillary effects; a needle-shaped electrode is brought to the liquid surface, and an electric field is applied through it. Through dielectrophoresis (DEP), these conductive particles are attracted along the field gradient; that is, towards the tip of the electrode. When the electrode rises, it pulls the particles out of the liquid to form a particle chain. Along the chain, any two adjacent particles are connected by a sphere-sphere liquid bridge. The combined capillary effects of these bridges make it possible to maintain a stable particle chain even after the external field is turned off.
These chains, which could contain hundreds of thousands of spheres reaching up to 30 centimeters in length, are automatically stabilized by liquid bridges formed between adjacent particles, without the need for continuous energy input or special particle functionalization. They can further be deposited onto any surface, and form desired conductive patterns, potentially applicable to the manufacturing of simple electronic circuits.