Metamaterials engineered based on transformation optics have facilitated inaccessible manipulation of various physical phenomena. However, such metamaterials have not been introduced for flowing viscous matter. Here we propose a hydrodynamic metamaterial cloak that can conceal an object in two-dimensional creeping flow by guiding viscous forces. Coordinate transformation of fluidic space is implemented to calculate a tensoric viscosity based on a form invariance of Navier-Stokes equations. The hydrodynamic cloak with the viscosity tensor is numerically simulated to verify a fictitious fluidic empty space created in it. The corresponding metamaterial microstructure is systemically designed and fabricated in a microfluidic device. The experimental results reveal that a solid object amid the flow can be hydrodynamically hidden without entailing a disturbance in flow fields and experiencing a drag.

We introduce a multiplex particle focusing phenomenon that arises from the hydrodynamic interaction between the viscoelastic force and the Dean drag force in a microfluidic device. In a confined microchannel, the first normal stress difference of viscoelastic fluids results in a lateral migration of suspended particles. Such a viscoelastic force was harnessed to focus different sized particles in the middle of a microchannel, and spiral channel geometry was also considered in order to take advantage of the counteracting force, Dean drag force that induces particle migration in the outward direction. For theoretical understanding, we performed a numerical analysis of viscoelastic fluids in the spiral microfluidic channel. From these results, a concept of the ‘Dean-coupled Elasto-inertial Focusing band (DEF)’ was proposed. This study provides in-depth physical insight into the multiplex focusing of particles that can open a new venue for microfluidic particle dynamics for a concrete high throughput platform at microscale.

Metamaterials are rationally designed periodic micro- or nanostructures that do not exist in nature. Recently, metamaterials for hydrodynamic phenomena have attracted immense attention from scientists, demonstrating magical fluid manipulation. Here we report a passively operating hydrodynamic metamaterial rotator that allows control of the flow direction without application of external forces and fluidic properties. Viscosity tensors to model the rotators for virtually twisting fluidic coordinate systems are derived on the basis of transformation hydrodynamics and are subjected to numerical simulations to predict the pressure and velocity fields. A microstructured rotating metamaterial is designed and produced in a microfluidic device. The hydrodynamic rotation of flowing water is experimentally confirmed, demonstrating the artificial manipulation of the flow direction.

Cells and tissues undergo complex physical processes during cryopreservation. Understanding the underlying physical phenomena is critical to improve current cryopreservation methods and to develop new techniques. Here, we describe multi-scale approaches for modelling cell and tissue cryopreservation including heat transfer at macroscale level, crystallization, cell volume change and mass transport across cell membranes at microscale level. These multi-scale approaches allow us to study cell and tissue cryopreservation.