Why liquids can appear to solidify during squeeze-out - Even when they don't

HY Gao and MH Muser, JOURNAL OF COLLOID AND INTERFACE SCIENCE, 562, 273-278 (2020).

DOI: 10.1016/j.jcis.2019.10.097

When liquids are squeezed out between two solid surfaces, they often exhibit layering, load-bearing ability, and a much increased viscosity. The combination of these phenomena is frequently interpreted as confinement-induced solidification. Here we propose that such behavior may often better be rationalized as the non-zero wavevector response of a pressurized liquid: bulk liquids contain structure even beyond the nanoscale as evidenced by their (damped) sinusoidal density correlations. Under confinement, this structure enables liquids to sustain non-isotropic stresses and thereby to carry load over a time span that is long enough for molecules to rearrange in the confined zone. In response to the load, viscosity can increase locally, in which case liquid flow is suppressed. This interpretation is supported by molecular-dynamics simulations of a key commercial base-oil component (1-decene trimer), which is squeezed out between a ridge and a substrate. The layering of the oil reflects the density correlations of the bulk liquid. At the same time, the confined liquid can sustain von Mises stresses exceeding locally 100 MPa over sufficiently long times for molecules to diffuse within the confined zone. (C) 2019 Elsevier Inc. All rights reserved.

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