Over the last years, the transport of charged species in bulk and at interfaces have been of great fundamental and technological interest. The corresponding importance of electrostatics and dynamics in biological systems, spanning a wide range of length scales, is a rapidly developing field of research. On molecular length scales, the ordering of water molecules plays an important role in mass transport of ions through confining geometries. Such structural ordering within hydration layers also interplays with the electrostatic interactions between charged (bio-) macromolecules [1-3]. Some of the combined effects of confinement-induced water structure and hydration of small charged species plays a role in, for example:

• the interfacial friction between planar and nanoscopic smectic layers lubricated by water, which is strongly affected by nano-confinement [4-6],
• a small protein is able to bridge colloids [7-9], where confinement has a strong effect on ion correlations and electro-osmotic flow [10-12],
• the confinement-induced structure of water affects membrane-desalination and osmotic power harvesting [2, 13,14], and elastic sheets show unusual structures under stress [15,16],
• there is a qualitative difference between the degree of ionic hydration for kosmotropic and chaotropic ions near the air-water interface [3, 17],
• and the crystallization temperature of fluids is strongly affected by nano-scale confinement [18], while the deswelling behaviour and accompanied transport properties of microgel particles depend on the release/capture of ions confined inside the microgel [19].

Therefore, the study of an interplay between water structure and electrostatics requires advanced molecular simulations.    

Charge regulation of macromolecules, either in equilibrium or induced by external fields, can affect their interactions to an extent that their phase behaviour is changed. The pH and salt concentration of the bacteriophage PP7 capsid, for example, regulate the chemical dissociation equilibrium of the amino acids, and thereby their electrostatic interactions [3, 20], which also plays a role in the formation of RNA pseudoknots [16] and virus transmission via droplets [21]. Electrostatic interactions between viruses also dependent on the spatial distribution of their genome charge [22]. The ionic-strength dependent osmotic pressure plays an important role in biological systems [23, 24], for which a fluorescens-based experimental technique has very recently been developed to measure the small osmotic differences at hand [25]. The variation/regulation of charges and the effect of complex charge distributions have not yet been thoroughly understood.

Apart from interactions between ions and charged macromolecules, electrostatic interactions between charged species with bio- and synthetic interfaces is of interest, both from the biological and technological perspective. The adhesiveness of bio-interfaces and cells is often regulated by the adsorption of proteins on the interfaces [26]. Interactions between entire cells, however, can also result from secretion and sensing of a chemical, which is referred to as quorum sensing for cell-to-cell communication [27]. The shape of artificially grown cell assemblies (organoids) is often dictated by the mechanical forces that the cell membranes exert onto each other [28]. We also discuss active particles, including non-Markovian data-driven modeling of single-cell motility [29] and the dynamics and structure in complex and crowded environments [30, 31].