The wetting of solid surfaces by fluids has been studied for more than two centuries. However, it was only in recent years that investigations of the first milliseconds of spontaneous drop spreading on solid surfaces started. For non-deformable surfaces, this fast dynamic wetting process is known to be dominated by inertia and controlled by surface wettability. In this work we studied spontaneous spreading of liquids on soft, viscoelastic rubber films with shear moduli |G| between 0.2 and 510 kPa and thickness d between 30 and 160 mm. We found that the early stage of fast wetting of soft surfaces is also dominated by inertia and that the wetting dynamics follows a power law which mainly depends on wettability, but not on softness. This finding allows us to apply fast dynamic wetting measurements for inferring the equilibrium contact angle q eq on soft surfaces. On such surfaces static contact angle measurements with sessile drops would not yield univocal results and Young's equation is not directly applicable. On the other hand, the duration of the fast inertial wetting is controlled by surface softness. This is an indication of a viscoelastic dissipation process occurring during wetting that starts after some characteristic time dependent on the surface tension of the liquid, on the viscosity of the surface, and on the speed of wetting.

The early spreading of a liquid drop on a solid surface driven by inertial, capillary, and electrostatic forces is of fundamental interest, since most commonly used surfaces are (naturally) charged. We studied the effect of applying an electric potential between a drop and a surface on the early spreading of aqueous electrolyte drops. We found that spreading dynamics not only depended on the potential, but also on the electrolyte concentration. Based on molecular dynamics simulations of the ion distribution in spreading nanodrops under an applied potential, we propose a simple model to explain the relation between applied potential, electrolyte concentration, and early spreading dynamics.

3D cell and organoid cultures, which allow in vitro studies of organogenesis and carcinogenesis, rely on the mechanical support of viscoelastic matrices. However, commonly used matrix materials lack rational design and control over key cell-instructive properties. Herein, we report a class of fully synthetic hydrogels based on novel DNA libraries that self-assemble with ultra-high molecular weight polymers, forming a dynamic DNA-crosslinked matrix (DyNAtrix). DyNAtrix enables, for the first time, computationally predictable, systematic, and independent control over critical viscoelasticity parameters by merely changing DNA sequence information without affecting the compositional features of the system. This approach enables: (1) thermodynamic and kinetic control over network formation; (2) adjustable heat-activation for the homogeneous embedding of mammalian cells; and (3) dynamic tuning of stress relaxation times over three orders of magnitude, recapitulating the mechanical characteristics of living tissues. DyNAtrix is self-healing, printable, exhibits high stability, cyto- and hemocompatibility, and controllable degradation. DyNAtrix-based 3D cultures of human mesenchymal stromal cells, pluripotent stem cells, canine kidney cysts, and human placental organoids exhibit high viability (on par or superior to reference matrices), proliferation, and morphogenesis over several days to weeks. DyNAtrix thus represents a programmable and versatile precision matrix, paving the way for advanced approaches to biomechanics, biophysics, and tissue engineering.

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