Fabricating colloidal particles with photolithography and their interactions at an air-water interface

A technique for fabricating nonspherical colloidal particles using photolithography has been developed. The particles are plate shaped and their profile within the plane of the plate is defined by a lithography mask and so can be any form desired. The thickness of the particles can also be controlled by varying the amount of material in the particle, and also by using the stresses induced during the evaporation of materials to distort the particles out of the plane. The particle-particle interactions can be tailored and made anisotropic by coating different faces of the particles with different chemicals or by making them of different materials. This technique is used to produce curved disks that are hydrophobic on their convex face and hydrophilic on their concave face. These particles are studied at an air-water interface, where the majority Lie with their hydrophobic face uppermost. The curvature of the particles distorts the water surface in a manner that can be described by a series expansion. The symmetry of this function is used to explain the interactions of the particles and the resulting ordered flocculated structures observed. Such anisotropic forces in two dimensions have not been studied previously on a colloidal length scale to our knowledge and extend the field of control of particles at interfaces.