The lubrication behavior of the hydrated biopolymers that constitute tissues in organisms differs from that outlined by the classical Stribeck curve, and studying hydrogel lubrication is a key pathway to understand the complexity of biolubrication. Here, we have investigated the frictional characteristics of polyacrylamide (PAAm) hydrogels with various acrylamide concentrations, exhibiting Young's moduli (E) that range from 1 to 40 kPa, as a function of applied normal load and sliding velocities by colloid probe lateral force microscopy. The speed-dependence of the friction force shows an initial decrease in friction with increasing velocity, while, above a transition velocity V*, friction increases with speed. This study reveals two different boundary lubrication mechanisms characterized by distinct scaling laws. An unprecedented and comprehensive study of the lateral force loops reveals intermittent friction or stick-slip above and below V*, with characteristics that depend on the hydrogel network, applied load, and sliding velocity. Our work thus provides insight into the closely tied parameters governing hydrogel lubrication mechanisms, and stick-slip friction.

The precipitation of calcium carbonate in hydrogel-like environments is used by certain living organisms to build functional mineral–organic composite structures. Here, we investigate a pathway for calcium carbonate mineralization in agarose hydrogels with a wide range of polymer networks. The experimental investigation demonstrates that the formation of amorphous calcium carbonate (ACC) throughout the agarose hydrogels is a diffusion-limited process, and therefore, it is affected by the supersaturation of the solution and by the hydrogel network. In contrast, the inclusion of the polymer into the calcite crystals and their morphology as well as the rate of crystal growth are controlled by the amorphous precursor, and thereby, they are quite unaffected by the initial supersaturation. The nucleation rate of calcite in agarose is sufficiently high to hinder ion diffusion limiting the calcite growth rate, so that a uniform mineralization takes place in the hydrogel, in the absence of concentration gradients. This work demonstrates that the precipitation of ACC affords a tight control of calcium carbonate mineralization in the hydrogel over a wide range of calcium carbonate concentrations and hydrogel microstructures. The results of this work not only reveal an important mechanism underlying (bio)­mineralization, but they can also inspire a new avenue to craft biomimetic materials with a high degree of precision.

The biomineralization of calcium carbonate is masterfully directed by organic macromolecules present in many organisms. In this biomimetic study, absorbance measurements accompanied by microscopy and infrared spectroscopy are used to evaluate the superposed effect of organic additives and surface chemistries on the kinetics of surface-directed nucleation and growth of calcium carbonate. Polyelectrolyte films with carboxylic and/or amine groups serve as the organic template for mineralization, and amino acids and monosaccharides as the selected solution additives. A grain-boundary kinetics model describes surface precipitation of calcium carbonate to provide a mechanistic insight into the precipitation pathway via two parameters, the near-surface supersaturation and the crystal number density. While an organic matrix rich in ternary amines strongly promotes vaterite nucleation, the selected carboxylic-enriched polyelectrolyte film significantly stabilizes ACC in the near-surface region, while it equally promotes vaterite and calcite nucleation. The combined effect of organic additive and surface template determines the near-surface supersaturation. Soluble additives can also be directly involved in surface nucleation if they strongly interact with the organic interface. Our mechanistic approach reveals two different precipitation pathways that result from the synergy between surface template and organic additives.

Ab initio simulations of large hydrated calcium carbonate clusters are challenging due to the existence of multiple local energy minima. Extensive conformational searches around hydrated calcium carbonate clusters (CaCO3·nH2O for n = 1-18) were performed to find low-energy hydration structures using an efficient combination of Monte Carlo searches, density-functional tight binding (DFTB+) method, and density-functional theory (DFT) at the B3LYP level, or Møller-Plesset perturbation theory at the MP2 level. This multilevel optimization yields several low-energy structures for hydrated calcium carbonate. Structural and energetics analysis of the hydration of these clusters revealed a first hydration shell composed of 12 water molecules. Bond-length and charge densities were also determined for different cluster sizes. The solvation of calcium carbonate in bulk water was investigated by placing the explicitly solvated CaCO3·nH2O clusters in a polarizable continuum model (PCM). The findings of this study provide new insights into the energetics and structure of hydrated calcium carbonate and contribute to the understanding of mechanisms where calcium carbonate formation or dissolution is of relevance.

Recognizing the significance of surface interactions for ion rejection and membrane fouling in nanofiltration, we revise the theories of DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) and non-DLVO forces in the context of polyamide active layers. Using an atomic force microscope, surface forces between polyamide active layers and a micrometer-large and smooth silica colloid were measured in electrolyte solutions of representative monovalent and divalent ions. While the analysis of DLVO forces, accounting for surface roughness, provides how surface charge of the active layer changes with electrolyte concentration, scrutiny of non-DLVO hydration forces gives molecular insight into the composition of the membrane-solution interface. Importantly, we report an expansion of the diffuse layer at high ionic strength, consistent with the recent development of the electrical double layer theory, but in contrast to the widely accepted phenomenon of aggregation in the secondary minimum. Further, the enhanced repulsion acting on modified membranes via polyelectrolyte adsorption can be quantitatively predicted by DLVO and non-DLVO forces. This work serves to solve past misunderstandings about the interaction forces acting on nanofiltration membranes, and it provides guidance for future work on the relation between surface properties and rejection mechanisms and fouling.