Curvature gradient driven assembly of trapped and reconfigurable structures

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Non-technical AbstratThe ability to organize microscale particles into well-defined structures lies at the heart of our ability to design new soft, reconfigurable materials. Often, external electrostatic or magnetic fields are used to guide particles into positions where they can interact and form structures. This work studies fields that have not been widely appreciated or used in the past. Particles on fluid interfaces deform and increase the area of the interface around them. The product of surface tension and this area increase is an energy field that depends on the curvature of the fluid interface, so particles move along curvature gradients. Through this simple but remarkable fact, the geometry of the interface itself can be used to direct assembly. Here, these fields are studied to identify new ways to form structures difficult to form by conventional means to make new materials whose properties are explored. Graduate and undergraduate students are trained in the course of performing the proposed research, including students in the Louis Stokes Alliance for Minority Participation program, the Advancing Women in Engineering Program and the UPENN MRSEC REU program. New knowledge developed will be incorporated in a graduate course on interfacial phenomena.Technical AbstractThis research seeks to establish new strategies for directed assembly of micron and sub-micron scale particles to go well beyond the usual close packed assemblies. Particle trapped at fluid interfaces interact and migrate along interface curvature gradients via capillarity. These energies drive formation of complex structures strongly correlated with the interface curvature field, influenced by particle-particle interactions. Since soft matter is inherently deformable, such interactions are a natural route to form reconfigurable, tunable assemblies. Different classes of structures are studied using optical microscopy to observe structures, lithographically defined vessels to mold fluid interfaces and magnetic and other probes to perturb the structures and to guide their reconfiguration. Kinetically trapped structures are studied to form colloidal monolayer membranes with voids, dense regions and oriented structures which respond to changes in interface shape. Equilibrated structures are studied to form structures aligned along principle axes of the interfaces and to study their reconfiguration upon interface perturbation. The (dynamics of) structure formation is observed by optical microscopy for particle shapes and sizes selected for the scale of capillary interactions that they excite and their ability to form oriented structures with associated anisotropies in the structural response to perturbation. For both limits, particle positions/ orientations are compared to and correlated with the interface curvature. Observations are compared to appropriate prediction based on, for example, Stokesian Dynamics simulations for trapped structures and Monte Carlo simulations for equilibrated structures.