Dynamics of underwater microparticle adhesion to soft hydrated surfaces: Modeling and analysis by time-dependent AFM force spectroscopy.

HYPOTHESIS: Understanding the attachment and detachment of microparticles and living cells to surfaces is crucial for developing antifouling strategies. Hydrogel coatings have shown promise in reducing fouling and particle adhesion due to their softness and high water content, yet the mechanisms involved are dynamic and complex, and relevant parameters are not easily accessible. AFM-based force spectroscopy (FS) experiments with colloidal probe particles is a direct way of evaluating adhesive and mechanical relaxational dynamics, yet their interpretation and modeling has been challenging. The present study proposes and examines several dynamic models, suitable for quantitative analysis of FS results with model probe particle on hydrogels surfaces. EXPERIMENTS: FS were performed using polyethylene glycol (PEG) hydrogels and polystyrene microspheres including particle attachement to the hydrogel surface (loading), holding the particle on the surface with a constant force for variable times (dwell) and pulling the particle away from the surface (unloading) FINDINGS: It was found that a viscoelastic extension of the classical JKR model with energy of adhesion unevenly distributed over the contact area and vanishing at its circumferences accurately described all FS experiments and yielded physically consistent viscoelastic and adhesive dynamic parameters, steadily changing with dwell time and applied force. The observed time evolution and force dependence were rationalized as combination of osmotic and osmo-mechnical relaxation in the contact region.

Spatial pattern and surface-specificity of particle and microorganism deposition and attachment: Modeling, analytic solution and experimental test.

HYPOTHESIS: Understanding microparticle and living cell deposition and attachment on surfaces from a flow is a long-standing surface-science problem, pivotal to developing antifouling strategies. Recent studies indicate a complex non-conservative and surface-specific nature of adhesion and mechanical contact forces that determine attachment kinetics. This requires new models and kinetic data, however, observed deposition rates (e.g., in parallel-plate flow chamber, PPFC) represent a superposition of attachment and bulk transport. Here, we propose to deduce attachment rates (as an appropriate rate constant) from spatial deposition profiles along PPFC and develop an analytical solution for the full problem, suitable for deposition data analysis and parameter fitting. EXPERIMENTS: Analytical solution, validated by numerical simulations, reveals relation between the deposition profile along PPFC and key model parameter B, the ratio of sedimentation and attachment rates. Its use is demonstrated on experimental data obtained in a PPFC for particles and bacteria on various surfaces. FINDINGS: Fitted B values highlight correlation with the particle/substrate nature and consistently explain the observed trends along PPFC, both decreasing and increasing. Thus derived attachment rates will serve as basis for future microscopic modelling that would relate attachment to appropriate surface and contact-mechanical characteristics of particles and substrate and flow conditions.