Manipulating the Dynamic Adaptivity of a Fluid Interface to Maintain the Multipotency of Mesenchymal Stromal Cells.

The native extracellular matrix is highly dynamic with continuous mutual feedback between cells being responsible for many important cell function regulators. However, establishing bidirectional interaction between complex adaptive microenvironments and cells remains elusive. Herein an adaptive biomaterial based on lysozyme monolayers self-assembled at a perfluorocarbon FC40-water interface is reported. The dynamic adaptivity of interfacially assembled protein nanosheets is modulated independently of bulk mechanical properties by covalent crosslinking. This provides a scenario to establish bidirectional interactions of cells with liquid interfaces of varying dynamic adaptivity. This is found that growth and multipotency of human mesenchymal stromal cells (hMSCs) are enhanced at the highly adaptive fluid interface. The multipotency retention of hMSCs is mediated by low cell contractility and metabolomic activity involving the continuous mutual feedback between the cells and materials. Consequently, an understanding of the cells' response to dynamic adaptivity has substantial implications for regenerative medicine and tissue engineering.

Coalescence of a Ferrofluid Drop at Its Bulk Surface with or without a Magnetic Field.

The coalescence of a ferrofluid drop at its bulk surface, with or without a magnetic field, was investigated experimentally by a high-speed camera. Shape deformations of both the pendant ferrofluid drop and the bulk surface in the axial direction were observed during the approaching process even in the absence of a magnetic field. The angle of the upper pendant peak at the first contact decreases with the magnetic flux density, while the lower ferrofluid peak displays an opposite trend. The coalescing width of the ferrofluid drop follows a power-law relationship. The exponent of 0.64 under medium and high magnetic fields as well as the case without magnetic field confirms the inertial regime of drop coalescence. Under the low magnetic field, the significant exponent increasing from 0.59 to 3.02 at about 4 ms is in coincidence with the sudden change to a smooth coalescing section according to the visualized images. A high-speed microparticle image velocimetry (micro-PIV) technique was employed with a transparent model fluid to reveal the flow fields during the drop coalescence instead of opaque ferrofluids.

Initial coalescence of a drop at a planar liquid surface.

The initial coalescence of a pendant drop at bulk liquid was jointly investigated by an ultrahigh-speed DC electrical device, a high-speed camera, and a fast micro-Particle Image Velocimetry (micro-PIV). Extended to highly viscous non-Newtonian liquids, the variation of the coalescing width vs time confirms the distinct regimes reported for drop-drop configuration: linear in the inertially limited viscous regime; square root in the inertial regime; possibly a transient viscous regime in between with a logarithmic correction. The measured flow fields during coalescence reveal the transformation of surface energy to kinetic energy, so that the highly located inertia could play a dominant role in relation to the viscous force.

Self-Sustained Coalescence-Breakup Cycles of Ferrodrops under a Magnetic Field.

The self-sustained coalescence-breakup cycles of ferrodrops were investigated for the first time by a high-speed camera under various magnetic fields. Under an axial magnetic field, the upper ferrodrop would deform into a conic shape before coalescing with the bottom ferropeak. Within 0.2 ms after coalescence, the minimum width of the expanding neck obeys a power-law relationship with time, while the exponents increase with the magnetic field and deviate with a decreasing trend in the later coalescence. The cone angle of the upper ferrodrop before coalescence gradually decreases while it increases before breakup with the magnetic field. A critical magnetic field around 35 mT was reported, above which the ferrofluid column undergoes the periodic phenomenon of coalescence and breakup. The frequency for the whole coalescence-breakup cycle increases exponentially with the applied magnetic field. A simplified force balance allows capturing the periodic mechanism involved in this driven harmonic oscillator.