This Particles on Curved Interfaces: Geometry, Mechanics, and Self Assembly
The adsorption of solid like particles on soft interfaces is ubiquitous in man-made systems as well as in the biological world. The versatility of these phenomena range from Pickering emulsions drops that are covered by a dense colloidal suspension, to proteins that reside on cellular membranes. Understanding the interactions between particles and interfaces and the mutual forces between particles themselves is crucial for developing effective self assembly methods and for controlling the stability of particle laden emulsions. A variety of physical mechanisms are believed to affect these interactions, from surface tension and contact line pinning to electrostatics and van-der-Waals forces. If particles are adsorbed on a curved interface, their mechanics may also be significantly influenced by the surface geometry. Such geometry induced effects have not been addressed in detail, and the purpose of this proposal is to commence their systematic study. Focusing on simple yet nontrivial geometries, we propose experimental and theoretical studies aimed at illuminating universal aspects of the mechanics of particles on curved surfaces.
Intellectual Merit:
(i) We will study the conditions under which the binding energy of solid particles to curved surfaces depends only on the local interfacial curvatures near the adsorbed particle.
(ii) We will study the geometry induced interactions among a few particles on a curved interface. We will develop a set of experiments to test the curvature dependent interactions. In particular, we will explore whether such forces can underlie the puzzling long range attraction that often exists between adsorbed particles.
(iii) We will characterize the behavior of adsorbed particles under nonequilibrium surface flows. We will address flow geometries that allow comparison between geometryinduced forces and the viscous stresses associated with nonequilibrium flow.
(iv) We will address basic problems related to the behavior of a dense suspension of adsorbed particles. In particular, we will explore whether the presence of many adsorbed particles can affect the stability of curved interfaces, and what is the nature of interactions between adsorbed particles in dense suspension.
Broad Impact:
(i) The proposed research will provide training for a graduate student and one or more undergraduate students in a variety of experimental and analytic methods, and the opportunity to combine both perspectives within the same project.
(ii) Both PI's are leading a number of educational projects, intended for graduate and undergraduate students in UMass Amherst and elsewhere. A new component of the proposal is the development of outreach programs for regional high school students and teachers, which will impart awareness, excitement, and active exposure to ongoing research in soft matter physics. An emphasis will be given to participation of students and teachers from low income and underrepresented communities.
(iii) Understanding the basic principles underlying geometry induced forces will yield transformative results, that could be used to exploring new approaches to directed assembly of particles, and to controlled, particle stabilized (Pickering) emulsification.
(iv) Finally, the proposal seeks to explore the most basic geometry induced effects by focusing on simple (axially symmetric) interfaces and solid (spherical) particles. The results of the proposed studies will spur a fruitful line of research that will address the manifestations of geometry induced interactions in more complicated systems, characterized by flexible particles (e.g., proteins), other types of surfaces (e.g., with bending modulus), and complicated surface geometries and particle shapes.
Our work addresses basic questions about the behavior of small particles and interfaces between fluids. Oil droplets or air bubbles suspended in water provide an everyday example of our research topic. Such materials (called emulsions or foams) are the basis of technologies including separating components of ore in mining, dispersing flavorful or nutritious oils in foods, constructing ingestible capsules for delivery of medicines, or cleaning up crude-oil spills at sea. In all of these cases, performance requires that the droplets be prevented from fusing or coalescing, which in turn requires coating their surfaces with either molecules (surfactants) or microscopic solid particles. Understanding under what conditions these small particles bind on fluid interfaces and how they arrange themselves on the surface of a droplet are the focus of our research activities. In particular, we have used a combination of laboratory experiments and calculations from theory to understand how the shape of an interface changes when a spherical particles binds to it. We use spheres as the simplest particle shape, which allows us to identify the essential aspects of this problem. We focus on interface shape for two reasons. First, when a particle binds, it deforms the shape, which leads to interactions with other particles. By a process that resembles the tendency of two people standing on a trampoline to fall toward one another, two or more particles on an interface can move toward or away from one another because of how they deform the interface shape. Similarly, particles might be directed on demand, by means of interface shape, toward particular regions of a droplet. We have develop theories that allow us to predict the deformations, which we have been testing by experiments. In addition, our experiments uncovered an unanticipated result, which is that the angle between the interface and the particle changes in a way that itself depends on the interface shape; despite its potential importance, this possibility had been omitted from prior work in this field. Overall, our results pave the way toward a general understanding of how to direct the motion and assembly of particles on fluid interfaces. They may show how to make more useful materials for encapsulating medicine, new formulations in foods, or cleaner methods to clean up oil spills in our oceans. In addition to creating new knowledge, our project provided support and training for young scientists. Original research with guidance from the principal investigators is an excellent training experience for college students and for PhD students and prepares them to contribute through research with private companies, national labs, or universities. In the area of public knowledge, colloids and emulsions provide an ideal opportunity to teach middle-school students both basic science and about the role of basic research in our daily lives. Our activities in this area have focused on seminars with K-12 teachers to discuss and develop activities that fit with the curriculum, followed by our in-class teaching of middle-school science classes. These are key steps toward optimizing these lessons and preparing them for broader implementation.
