Complex fluids are ubiquitous in biology, geophysics, and industry [1]. These materials are challenging to characterize and predict [1–4], particularly when they incorporate multiple interfaces, as in colloidal suspensions [4], foams [5–7], or nanoporous membranes [8–10]. Many of these interfaces are micro- or nano-scale and evolve over short times, which can obscure them to observation and pose challenges to experimentalists [2–5, 11, 12]. This opens exciting opportunities for a strong partnership between the development of novel theoretical, computational, and experimental techniques.

Probing interfaces presents unique challenges compared to probing complex fluids in the bulk. The interfacial structure and constitutive behavior then depend on the composition of two fluids as well as the interfacial configuration [13, 14]. Translating this increased complexity to a computational framework involves developing reliable models describing molecular interactions near fluid-fluid or fluid-solid interfaces [15–17], as well as models for continuum stresses [18]. Molecular modeling is necessary to reveal the physics of chemically-complex structures [17], but is computationally expensive, and it can be challenging to identify the relevant physics to include [19]. Yet the interface also provides unique opportunities for control: in liquid crystals, for example, interfacial stresses can be transmitted through the bulk, leading to novel pattern formation [20] and optical materials exploiting interfacial control [21]. Finally, interfaces are prone to instabilities, which can make flows unpredictable, but opens opportunities to exploit unstable growth for spontaneous patterning.

This workshop aims to foster exchanges around the following broad questions:

• How do molecular phenomena determine the structural properties and interfacial dynamics of complex fluid interfaces?
• How do we approach a rigorous, robust, and predictive upscaling between non-continuum computational approaches (e.g. MD, coarse-grained models), which are computationally costly, and large-scale systems? Can we extract universal quantities or concepts from MD to be used in a continuum model? Are these potential quantities intrinsic properties or do they depend on the flow configuration and hence require an ad hoc calibration for each flow situation?
• How can emerging experimental and computational techniques inform our understanding of interfacial instabilities in complex fluids? Can we account for instabilities arising from molecular and meso-scales in a macroscopic stability analysis?
• Is it possible to incorporate microscopic effects into macroscopic models which 'go beyond' the conventional Navier-Stokes-Fourier paradigm? For example, can effective viscosities adequately account for molecular effects, or can noise terms incorporate thermal fluctuations? Can these models be captured by extending existing computational approaches, or do they require entirely new frameworks?