Janus particles as solid surfactants

Janus particles are asymmetric colloids with polar and apolar sides. Their amphiphilicity makes this new class of colloids exhibit behaviors that are similar to those of surfactant molecules. The major goal of our work is to address the following intellectual questions: are Janus particles efficient “solid surfactants” for the stabilization of multiphasic fluid mixtures such as emulsions and foams? We are inspired by how the chemical composition and shape of molecules influence the surfactant properties of molecular amphiphiles. In this talk, I will discuss our recent work on the thermodynamics of emulsion stabilization using Janus particles, the emulsion stabilization and phase inversion emulsification using shape-changing/amphiphilicity-reversing Janus particles. Because the attachment energy of Janus particles to fluid-fluid interfaces is significantly larger than that of homogenous particles, it is possible to generate Pickering emulsions that are thermodynamically stable when Janus particles are used as emulsifiers. I will also describe recently developed Janus particles that undergo significant changes in their shape and amphiphilicity in response to changes in the solution pH. We show that it is possible to stabilize different types of emulsions and also induce the phase inversion of emulsions using these stimuli-responsive Janus particles. New synthetic routes that enable the lage-scale production of Janus particles of various shape and chemistry will be presented

Toward Scalable Nanomanufacturing Using Capillarity

In this talk, I will describe new approaches for scalable manufacturing of nanocomposites with unique structures and properties by exploiting capillary interactions between solid particles and fluids. In the first part of this talk, I will describe our work on generating polymer nanocomposite films (PNCFs) with extremely high loadings of nanoparticles using capillary rise infiltration (CaRI). Owing to the high loadings of nanoparticles, these PNCFs have extraordinarily high hardness, modulus and scratch resistance. In CaRI, PNCFs are formed by thermally annealing a bilayer of polymer and nanoparticle, which induces imbibition of polymer into the nanoparticle layer. CaRI represents an interesting transport phenomenon in which the size of the fluid molecule (i.e., polymer) is comparable to the pore size in the nanoparticle layer. I will share our current understanding of the transport processes involved in CaRI. By adjusting the amount of polymers undergoing CaRI, we can also generate three-phase nanocomposites, which may have potential applications in the areas of membrane separations and energy storage and conversion. In the second part of this presentation, I will describe our recent efforts in creating bicontinuous interfacially jammed emulsions (BIJELs), which are a new class of soft materials with potential applications in reactive separation, membrane separation and catalysis. We have developed a new method to enable continuous generation of bijel microparticles, fibers and membranes using solvent-transfer-induced phase separation (STRIPS). Transport of molecules with opposite polarity as well as membrane separation of nanoparticles using STRIPS bijels will be demonstrated. Also, a new in situ technique to characterize the mechanical properties of these STRIPS bijel fiber as well as the formation of ultrafiltration membranes using STRIPS will be discussed

Elastic instability of polymer-shelled bubbles formed from air-in-oil-in-water compound bubbles

We study the stability of polymer-shelled bubbles with controlled dimensions generated from air-in-oil-in-water (A/O/W) compound bubbles. We show that the ratio of the shell thickness to bubble radius is critical in generating un-deformed polymer-shelled bubbles from A/O/W compound bubbles. In addition, the effects of shell stiffness and encapsulated gas on bubble stability are also investigated

Materials via interfacial assembly: strips bijels, nice, & awesome structures

Fluid interfaces are versatile sites for materials assembly; molecules and particles alike adsorb at interfaces to produce novel functional structures via processes suitable for scalable production. Here, three types of structures of interest to this community are described. First, hierarchical bijel structures are discussed, formed by Solvent Induced Phase Separation (STRIPS). Bijels, bicontinuous interfacially jammed emulsion gels, feature bicontinuous networks with oil-continuous and water-continuous channels. In STRIPS, asymmetric bijel structures are formed via phase separation of a ternary mixture of oil, water and co-solvent. Phase separation, induced by solvent extraction, is arrested to form a bicontinuous structure by the inter­facial attachment and jamming of nano­particles. These structures feature dense populations of nanoparticles on their interfaces that provide additional functionality, and are ideal for loading with hydrophilic or hydrophobic molecules for delivery. Furthermore STRIPS bijels can be crosslinked to a polymeric structure with high continuous tortuous channels suitable for diverse applications. Second, polyelectrolyte (PE) capsules formed by nanoscale interfacial complexation in emulsions (NICE) are described. Using microfluidics, single emulsions or water-oil-water emulsions are formed. PEs in the oil and aqueous phases complex at the interface to form a membrane. Important similarities and differences from related structures formed by layer-by-layer (LbL) assembly are discussed. In particular, interfacial complexation process is a one step process that is far less laborious than its LbL counterpart. Finally, capsule formation is explored via complexation at the fluid interface formed between droplets in an external phase from an aqueous two phase system. This avoids potentially deleterious oil phases. We study complexation of pairs of PEs, one in the drop phase, the other in the external phase. By balancing the fluxes, capsules are formed. Extensions to complex charged nanoparticles (NPs) with an oppositely charged PE lead to an unexpected structure: multiple emulsions form, with internal droplets that are free of adsorbed materials encased within an external PE/NP shell. These All Water Emulsion Bodies, or AWEsomes, are discussed as means to encapsulate entities ranging from molecules to microbes. References Haase, Martin F., Kathleen J. Stebe, and Daeyeon Lee. Continuous fabrication of hierarchical and asymmetric bijel microparticles, fibers, and membranes by solvent transfer‐induced phase separation (STRIPS). Advanced Materials 27, no. 44 (2015): 7065-7071. Kim, Miju, Seon Ju Yeo, Christopher B. Highley, Jason A. Burdick, Pil J. Yoo, Junsang Doh, and Daeyeon Lee. One-step generation of multifunctional polyelectrolyte microcapsules via nanoscale interfacial complexation in emulsion (NICE). ACS nano 9, no. 8 (2015): 8269-8278. Hann, Sarah D., Kathleen J. Stebe, and Daeyeon Lee. AWE-somes: All Water Emulsion Bodies with Permeable Shells and Selective Compartments. ACS applied materials & interfaces 9, no. 29 (2017): 25023-25028

Surface engineering using layer-by-layer assembly of pH-sensitive polymers and nanoparticles

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007.Includes bibliographical references (p. 184-204).Surface engineering of a variety of materials including colloidal particles and porous membranes has been achieved by using layer-by-layer assembly of pH-sensitive polymers and nanoparticles. In the first part of this thesis, hydrogen-bonded multilayer coatings comprising poly(acrylic acid) and polyacrylamide were used to functionalize spherical colloidal particles. Multilayer-modified colloids showed an excellent resistance to cell adhesion. Hydrogen-bonded multilayer coatings on microspheres also could be utilized as templates for in situ nanoparticle synthesis enabling the formation of nanoparticle-loaded hollow microcapsules. Silver nanoparticle-loaded multilayer coatings were created on magnetic microspheres to create antibacterial agents that can be manipulated using a magnetic field. In the second part, the surfaces of track-etched polycarbonate membranes were functionalized with multilayer coatings that undergo discontinuous swelling transition. Multilayers comprising poly(allylamine hydrochloride) and poly(styrene sulfonate) were deposited at a high pH condition (pH > 9.0). These multilayer-modified membranes exhibited hysteretic gating behavior that could be useful for the separation of pH-sensitive materials such as proteins.(cont.) The growth and swelling behavior of the multilayers in the cylindrical pores of TEPC membranes were also investigated. Heterostructured magnetic nanotubes could be created by further modifying the multilayer-coated TEPC membranes. These magnetic nanotubes were utilized for the separation and controlled release of anionic molecules including active pharmaceutical ingredients. In the last part of this thesis, all-nanoparticle thin film coatings were created by sequentially depositing oppositely charged nanoparticles. The fundamental investigation of all-nanoparticle multilayers revealed that a narrow processing window exists in which multilayers of oppositely charged nanoparticles can be assembled in a true layer-by-layer manner. It was also demonstrated that structure and properties of all-nanoparticle thin films could be varied by controlling the assembly conditions. All-nanoparticle thin film coatings consisting of titanium oxide and silica nanoparticles exhibited potentially useful antifogging, antireflection and self-cleaning properties.by Daeyeon Lee.Ph.D

Robust Scaling of Strength and Elastic Constants and Universal Cooperativity in Disordered Colloidal Micropillars

We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a bidisperse mixture of 3- and 6-μm polystyrene particles. Mechanical annealing of confined pillars enables variation of the packing fraction across the phase space of colloidal glasses. The measured normalized strengths and elastic moduli of the annealed freestanding micropillars span almost three orders of magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between ultimate strengths and elastic constants that is invariant to relative humidity, implying a critical strain of ∼0.01 that is strikingly similar to that observed in metallic glasses (MGs) [Johnson WL, Samwer K (2005) Phys Rev Lett 95:195501] and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow