No abstract

By integrating mesoscale models for hydrodynamics and micromechanics, we examine the fluid-driven motion of microcapsules on compliant surfaces. The capsules, modeled as fluid-filled elastic shells, represent polymeric microcapsules or biological cells. We examine the motion of these capsules on flat, mechanically uniform surfaces, mechanically patterned surfaces that contain alternating regions of hard and soft domains, and topologically patterned surfaces, i.e., a corrugated substrate and a substrate that encompasses a regular array of compliant posts. We isolate conditions where the mechanically and topographically patterned surfaces can transmit "stop" and "go" instructions, causing the capsules to halt at specific locations on the substrate, and with an increase in the imposed flow velocity, to resume moving. In the case of the corrugated surfaces, varying the size of the asperities permits significant control over the translational velocity of a capsule for a given shear rate. For surfaces containing regular arrays of compliant posts, the substrates also affect the capsules' gait, causing them to "crawl", "walk", or "jump". The latter behavior could promote the intermixing of reactants that are encapsulated within the microcapsules. These topographically patterned surfaces can also be utilized to sort capsules accordingly their size. Such control over capsule dynamics can enable the fabrication of arrays of mobile microreactors and facilitate various biological assays.

Simple expressions are given for the Newtonian viscosity N ( ) as well as the viscoelastic behavior of the viscosity ( , ) of neutral monodisperse hard-sphere colloidal suspensions as a function of volume fraction and frequency over the entire fluid range, i.e., for volume fractions 0Ͻ Ͻ0.55. These expressions are based on an approximate theory that considers the viscosity as composed as the sum of two relevant physical processes: ( , )ϭ ϱ ( )ϩ cd ( , ), where ϱ ( )ϭ 0 ( ) is the infinite frequency ͑or very short time͒ viscosity, with 0 the solvent viscosity, ( ) the equilibrium hard-sphere radial distribution function at contact, and cd ( , ) the contribution due to the diffusion of the colloidal particles out of cages formed by their neighbors, on the Péclet time scale P , the dominant physical process in concentrated colloidal suspensions. The Newtonian viscosity N ( )ϭ ( , ϭ0) agrees very well with the extensive experiments of van der Werff et al., ͓Phys. Rev. A 39, 795 ͑1989͒; J. Rheol. 33, 421 ͑1989͔͒ and others. Also, the asymptotic behavior for large is of the form ϱ ( )ϩ 0 A( )( P ) Ϫ1/2 , in agreement with these experiments, but the theoretical coefficient A( ) differs by a constant factor 2/ ( ) from the exact coefficient, computed from the Green-Kubo formula for ( , ). This still enables us to predict for practical purposes the viscoelastic behavior of monodisperse spherical colloidal suspensions for all volume fractions by a simple time rescaling.

We integrate the lattice Boltzmann model (LBM) and lattice spring model (LSM) to capture the coupling between a compliant bounding surface and the hydrodynamic response of an enclosed fluid. We focus on an elastic, spherical shell filled with a Newtonian fluid where no-slip boundary conditions induce the interaction. We calculate the "breathing mode" oscillations for this system and find good agreement with analytical solutions. Furthermore, we simulate the impact of the fluid-filled, elastic shell on a hard wall and on an adhesive surface. Understanding the dynamics of fluid-filled shells, especially near adhesive surfaces, can be particularly important in the design of microcapsules for pharmaceutical and other technological applications. Our studies reveal that the binding of these capsules to specific surfaces can be sensitive to the physical properties of both the outer shell and the enclosed fluid. The integrated LBM-LSM methodology opens up the possibility of accurately and efficiently capturing the dynamic coupling between fluid flow and a compliant bounding surface in a broad variety of systems.

For both biological cells and synthetic microcapsules, mechanical stiffness is a key parameter since it can reveal the presence of disease in the former case and the quality of the fabricated product in the latter case. To date, however, assessing the mechanical properties of such micron-scale particles in an efficient, cost-effective means remains a critical challenge. By developing a three-dimensional computational model of fluid-filled, elastic spheres rolling on substrates patterned with diagonal stripes, we demonstrate a useful method for separating cells or microcapsules by their compliance. In particular, we examine the fluid-driven motion of these capsules over a hard adhesive surface that contains soft stripes or a weakly adhesive surface that contains "sticky" stripes. As a result of their inherently different interactions with the heterogeneous substrate, particles with dissimilar stiffness are dispersed to distinct lateral locations on the surface. Since mechanically and chemically patterned surfaces can be readily fabricated through soft lithography and can easily be incorporated into microfluidic devices, our results point to a facile method for carrying out continuous "on the fly" separation processes.