D Daniel Florea

Long-range repulsion of colloids driven by ion exchange and diffusiophoresis

Significance The ability to displace particles or solutes relative to a background liquid is of central importance to technologies such as filtration/separation, chromatography, and water purification. Such behavior is observed in so-called exclusion zone formation, an effect where particles are pushed away from a surface over long distances of up to hundreds of micrometers. However, it is still unclear which physical mechanisms are responsible. Our work provides a detailed understanding of this exclusion zone formation, enabling a precise control of the behavior. This could be exploited, for instance, for sorting in microfluidic devices, in advanced antifouling coatings, or for elucidating biological processes where it is likely to play an important, yet unexplored role. Interactions between surfaces and particles in aqueous suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles has been observed over distances of up to hundreds of micrometers, in the absence of any additional external fields. Although a range of hypotheses have been suggested to account for such behavior, the physical mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed experiments on a well-defined experimental system, using a setup that minimizes the effects of gravity and convection. Our experiments clearly indicate that the observed long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. We develop a simple model that accounts for our data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.

Micromechanics of temperature sensitive microgels: dip in the Poisson ratio near the LCST

Microgels of poly-N-isopropylacrylamide (pNIPAM) exhibit a remarkable sensitivity to environmental conditions, most strikingly a pronounced deswelling that occurs close to the lower critical solution temperature (LCST) of the polymer at ≈32 °C. This transition has been widely studied and exploited in a range of applications. Along with changes in size, significant changes are also expected for the mechanical response of the particles. However, the full elastic properties of these particles as a function of temperature, T, have not yet been assessed at the single-particle level. Here we present measurements of the elastic properties of pNIPAM particles as a function of both temperature and cross-linking density using capillary micromechanics, a technique based on the pressure-dependent deformation of particles trapped in a tapered glass capillary. The shear elastic modulus G increased monotonously upon increasing temperature. In contrast, but in qualitative agreement with previous experiments on macroscopic pNIPAM hydrogels, we found that the compressive elastic modulus K of our microgels exhibits a dip close to the LCST. Remarkably, this dip is less sharp and deep than that observed in macroscopic hydrogels. The Poisson ratio of the particles also exhibits a pronounced dip close to the LCST, reaching unusually low minimum values of σ ≈ 0.15. To rationalize this behavior, we compared our experimental data to Flory–Rehner theory; the theory is able to qualitatively predict the general mechanical behavior observed, thus indicating that the observed dip in the Poisson ratio can be accounted for by simple thermodynamic arguments.

A new approach for calculating the true stress response from large amplitude oscillatory shear (LAOS) measurements using parallel plates

The parallel plates geometry is often deemed unsuitable for nonlinear viscoelasticity measurements because the strain field, and thus the nonlinear response, varies across the sample. Although cone–plate and Couette geometries are designed to circumvent this problem by ensuring a uniform strain field, it is not always easy to shape the material to the complex shapes that is required for these geometries. This has motivated the development of techniques to accurately determine the nonlinear stress response using the more convenient plate–plate geometry. Here, we introduce a new approach to obtain this true material response in large amplitude oscillatory shear (LAOS) experiments using the plate–plate geometry. By tracing the Fourier components of the torque response and their derivatives with respect to the maximum applied deformation, we accurately obtain the material’s true stress–strain response from parallel plate measurements. The approach does not require any assumptions about the material’s viscoelastic behavior. We test our approach experimentally on fibrin biopolymer gels, as well as numerically on a Giesekus model. We confirm in both cases that our approach captures the detailed shape of the true stress response in LAOS measurements. Moreover, we also show that our method is less sensitive to experimental noise present in the data than the previous standard method. Our approach for obtaining the true stress response from parallel plate measurements is directly applicable to measurements on a wide range of solid-like nonlinear materials, including biological networks, tissues, or hydrogels.

Towards the self-assembly of anisotropic colloids: Monodisperse oblate ellipsoids

We present a robust and straightforward method for producing colloidal particles of oblate ellipsoidal shape via thermo/mechanical stretching of elastomeric films with embedded spherical particles. Our method produces uniformly sized and shaped colloidal particles. The method can be used for producing biaxially stretched particles of different aspect ratios and volumes; moreover, the method has a higher yield and batch size than previously reported methods for producing non-spherical particles via film stretching. These particles are ideal model systems for studying the self-assembly and gel formation for systems with anisotropic shapes and interactions. We illustrate this by adding of a non-adsorbing polymer to the solvent, thereby inducing directional depletion interactions between the particles.

Compression and reswelling of microgel particles after an osmotic shock

We use dedicated microfluidic devices to expose soft hydrogel particles to a rapid change in the externally applied osmotic pressure and observe a surprising, nonmonotonic response: After an initial rapid compression, the particle slowly reswells to approximately its original size. We theoretically account for this behavior, enabling us to extract important material properties from a single microfluidic experiment, including the compressive modulus, the gel permeability, and the diffusivity of the osmolyte inside the gel. We expect our approach to be relevant to applications such as controlled release, chromatography, and responsive materials.

Interfacial water: unexplained phenomena

The common understanding in up-scaling of porous media mechanics is that pore scale phenomena are well understood. Navier-Stokes, Poiseuilles law, double-layer theory, thermal convection, all are classical theories coming to our rescue to describe the pore scale events in porous media in a strictly quantitative manner. So, most researchers do not feel the need to physically look at what is really happening. In this work we study a number of poorly understood phenomena that take place at that pore scale. These phenomena include formation of hundreds of micrometers wide exclusion zones next to hydrophilic surfaces, formation of colloidal crystals, micro-convective transport and unexpected pH and electric potential gradients across the exclusion zone. Investigating these phenomena is of great benefit for our understanding of porous media and could spark a wide range of applications in different fields. Until now, there is no comprehensive understanding of why these phenomena occur. There are several possible root causes for the formation of the exclusion zone being investigated. These include formation of liquid crystals of water molecules in the exclusion zone and Frohlich-like coherent oscillations. To develop the understanding about these phenomena, we have conducted experiments where we have used a variety of techniques to measure a number of parameters such as flow velocity and viscosity. These techniques include optical microscopy, optical tweezers, multi-particle tracking and interferometry. Examples of the results of these experiments are shown. - See more at: http://ascelibrary.org/doi/10.1061/9780784412992.246#sthash.ZVhhEikf.dpuf