Sepideh Razavi

In several environments, including wastewater treatment plants (WWTPs), plastics encounter surface-active molecules such as antibiotics and surfactants, essential components of consumer products, detergents, and drugs. In these conditions, synergistic and antagonistic effects could affect how surfactants and antibiotics compete for access to plastic surfaces. Experimental and computational results are presented here to quantify the adsorption on polystyrene (PS) of the antibiotic Novobiocin (NVB) in combination with adsorption of three surfactants─the anionic docusate sodium salt, AOT; the cationic cetyltrimethylammonium bromide, CTAB; and the nonionic hexaethylene glycol monododecyl ether, C12E6. The experimental results obtained with a quartz crystal microbalance with dissipation monitoring (QCM-D) suggest that, rather than removing NVB, the three surfactants yield an adlayer on the NVB film pre-adsorbed on PS. On the contrary, an aqueous solution of NVB partially removes pre-adsorbed surfactant films, and co-adsorption at the PS surface likely occurs. To rationalize these observations, molecular simulations were conducted at low concentrations of NVB and surfactants, somewhat similar to those found in aquatic environments. The computational results show that the interactions between NVB and PS are stronger than those between surfactants and PS. Combined, the results show that NVB and these surfactants are likely to co-adsorb on the polystyrene surface. Although generalization is premature, the observed synergism might facilitate the environmental dispersion of both antibiotics and microplastics, suggesting that advanced technologies are required to properly handle plastics, microplastics, and antibiotics in WWTPs, towards preventing the spread of antibiotic-resistant microorganisms.

A paradigm for the study of wrinkling in elastic sheet is the Lamé configuration, in which azimuthal wrinkles form in an annular sheet subjected to tensile loads at both edges. Since wrinkles are spatially extended, this instability provides a mechanism for stress transmission over long distances. A natural extension of this problem is wrinkling in sheets with multiple holes or broken symmetry. Here, we investigate tension-induced wrinkling in thin elastic sheets containing two circular holes by combining analytical modeling and experiments. The pre-buckled state is solved analytically using bipolar coordinates, enabling identification of the wrinkling threshold as a function of the distance between the two holes. Near-threshold wrinkling and interactions between wrinkles are analyzed, and we validate our theoretical predictions against experimental observations obtained through video imaging of spin-coated polystyrene sheets floating on liquid surfaces with controlled surface tension. Our results demonstrate that geometric symmetry breaking, such as the presence of a second hole, strongly influences wrinkle nucleation, orientation, and spatial extent. Beyond mechanics, these findings might provide a simple mechanism for cellular mechanosensing, where force transmission is amplified by mechanical instabilities.

Particle–laden fluid interfaces exhibit complex linear viscoelastic behavior resulting from collective particle dynamics confined to two dimensions.

Fingering is crucial when a low-viscosity fluid (e.g., water) displaces a high-viscosity fluid (e.g., oil). While this phenomenon has been investigated for bare oil-water systems, the impact of nanoparticle and surfactant presence at the interface on the fingering behavior, has not been studied in detail. The hypothesis is that the presence of interfacially active molecules in synergy with nanoparticles affects the development of fingering instabilities and that the wettability of the particles is important in the resulting behavior. To test this hypothesis, the effects of different types of nanoparticles in the presence of nonionic surfactants on the oil-water interface and in channel flow were probed via coarse-grained computations. Finger-like instabilities were characterized by the driving force for the flow, the three-phase contact angle, and the deformation of the interface. In the presence of surfactants, the instability developed at lower pressure drops compared with the bare oil-water case. In the presence of both surfactants and nanoparticles, a larger driving force was required for the instabilities to occur compared to either the bare oil-water system or the surfactant-only case. A critical contact angle dictated the detachment of the oil droplet from the channel wall. Our findings and discussion can be used for predicting displacement efficiency and interfacial instabilities that are common in industrial processes involving immiscible flows, such as in enhanced oil recovery, separation processes, and environmental remediation of the subsurface.

Quantifying the wettability of clay surfaces and how it changes in the presence of gas mixtures is crucial for designing geo-energy applications such as underground hydrogen storage and carbon capture and sequestration. While computational studies exist for the wettability of atomically perfect mineral substrates, actual minerals possess heterogeneities. This study employs molecular dynamics simulations to examine the impact of surface defects on the wettability of kaolinite surfaces exposed to hydrogen, methane, and carbon dioxide. The results show that siloxane surfaces become more hydrophilic as defect densities increase and that the gases can strongly affect wettability. Carbon dioxide, in particular, shows stronger adsorption on heterogeneous surfaces than hydrogen and methane. As a consequence, carbon dioxide can strongly affect wettability. Additionally, our results show that higher salt concentrations reduce water contact angle, which is important because salt is likely present in the subsurface. A machine learning classification algorithm is applied to interpret the results and develop predictive capabilities. Our findings highlight the importance of surface defects on wettability, which is essential for designing geological repositories for geo-energy applications ranging from enhanced gas recovery to carbon sequestration and intermittent hydrogen storage.

Quantifying thermodynamics, structural, and dynamical properties of brine confined in clay pores is critical for a variety of geo-energy applications, including underground hydrogen storage (UHS) and carbon capture and sequestration (CCS). Atomistic molecular dynamics simulations are applied here to study aqueous NaCl brines within 10-Å kaolinite slit pores. NaCl concentrations are chosen at 5, 10, 12.5, and 15 wt. %, all below the solubility limit and high enough to provide statistically relevant information. The distribution of the ions within the nanopores is found not to be homogeneous. Explicitly, Na+ cations, preferentially attracted to the siloxane surface, accumulate in regions with low water density, whereas Cl− anions, attracted to the gibbsite surface of kaolinite, are found within the hydration layers. Confinement affects the properties of ions, with ion pairing being more pronounced within the pore than in bulk aqueous solutions at similar temperatures, pressures, and compositions. Conversely, the ions affect the properties of confined water. For example, the lifetime of water–water hydrogen bonds in confinement is shortened within the hydration shells; increasing salinity from 5 to 12.5 wt. % reduces the likelihood of water density fluctuations near the kaolinite surfaces, although when the NaCl concentration rises from 12.5 to 15 wt. %, Cl− anions enhance the likelihood of density fluctuations for the hydration layer near the gibbsite surface. The simulated molecular trajectories are studied further to extract diffusion coefficients. While confinement in the kaolinite nanopore reduces the mobility of all species, non-monotonic trends are observed as a function of salt concentration. The trends seem associated with the likelihood of ion pairing. Furthermore, the diffusion coefficients for the cations are predicted to be higher than those for the anions, which is contrary to what is typically observed in bulk brines. Because density fluctuations are correlated with properties such as the solubility of gases in confined water, our observations may have important implications for geo-energy applications such as UHS and CCS.

Bovine serum albumin nanoparticles synthesized via a modified desolvation method exhibit superior long-term stability in aqueous dispersion. Glucose investigated as a “green” alternative crosslinker.

Hydrogen's contribution to a sustainable energy transformation requires intermittent storage technologies, e.g., underground hydrogen storage (UHS). Toward designing UHS sites, atomistic molecular dynamics (MD) simulations are used here to quantify thermodynamic and transport properties for confined aqueous H₂. Slit-shaped pores of width 10 and 20 Å are carved out of kaolinite. Within these pores, water yields pronounced hydration layers. Molecular H₂ distributes along these hydration layers, yielding solubilities up to ∼25 times those in the bulk. Hydrogen accumulates near the siloxane surface, where water density fluctuates significantly. On the contrary, a dense hydration layer forms on the gibbsite surface, which is, for the most part, depleted of H₂. Although confinement reduces water mobility, the diffusion of aqueous H₂ increases as the kaolinite pore width decreases, also a consequence of water density fluctuations. These results relate to H₂ permeability in underground hydrogen storage sites.