Directed Assembly by Capillarity

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Award: 1066284 PI: Stebe We will perform fundamental studies to establish quantitative guiding principles for anisotropic capillary assembly, i.e. to relate particle shape, contact angle, and interface shape, to the range and strength of interactions and to the assemblies which form. To do so, we will study capillary assembly in experiment, numerics and theory. We have established an intellectual basis to study interactions between any anisotropic particles at interfaces which we are in the process of applying to cylindrically shaped particles with uniform contact angle. We choose cylinders as the first shape to study because they have intriguing properties when they are at interfaces- they have high interfacial area density near their planar end faces which cements end-to-end assembly. We now understand far field interactions between cylindrical particles at interfaces in detail. However, when the particles come close together, the strong deformations near the ends overlap, forming a strong interaction between particles termed a "capillary bond", whose strength we will quantify numerically. We will then check the validity of the calculation experimentally- this will establish the pair interactions between cylinders at interfaces at all separations. We will then address three assembly aims, selected to pave the way to fields which exploit oriented particle assemblies. We will: I. Establish in experiment whether, as scaling suggests, capillary interactions are important for sub-micron to nanoscale particles, important for making annealable structures and for extending the technological impact of capillary assembly. II. Characterize the strength of capillary assemblies using magnetic tweezers, important in applications that exploit particle-stabilized emulsions, colloidosomes, and other hierarchical assemblies. III. Develop capillary recognition elements, which would preferentially assemble only with mating particles like puzzle pieces on an interface. This requires that we develop and understand capillary repulsion to impart selectivity to particle interactions. These aims were selected to seed new materials-related research to exploit anisotropic particle assemblies. For example, ordered oriented particle monolayers could be exploited to epitaxially grow new 3-D structures; oriented, assembled anisotropic particles could allow new classes of metamaterials to be developed; capillary recognition elements could promote registry of particles with other features within devices, and could serve as sensors or read outs.