The relevance of the scanning near-field optical microscope (SNOM), for near-field characterization, is often shaded by the appearance of artifacts, especially when geometrical characterization is intended. Artifacts are related to many features such as the feedback system or the scanning mode. For non-vacuum environmental conditions, artifact sources may be related to tip geometry and the pollutants attached, either on the tip or on the studied surfaces, altering the optical image. As an environmental element, water vapor could be treated as a source for artifacts, but could also be used as a tool for chemical characterization of hydrophilic patches. Spontaneous meniscus formation between hydrophilic surfaces, such as the tip and the sample, may guide light from the tip to the sample, enhancing the transmitted signal. This study focuses on the effects that water condensation at the nanoscale has on the signals achieved by SNOM, combining two computational methods (Monte Carlo and finite difference time domain) in order to deal with light propagation through heterogeneous media and water condensation.

We present an investigation of water menisci confined in closed geometries by studying the structural effects of their capillary forces on viruses during the final stage of desiccation. We used individual particles of the bacteriophage phi 29 and the minute virus of mice. In both cases the genomic DNA was ejected from the capsid. However, although the structural integrity of the minute virus of mice was essentially preserved, the phi 29 capsid underwent a wall-to-wall collapse. We provide evidence that the capillary forces of water confined inside the viruses are mainly responsible for these effects. Moreover, by performing theoretical simulations with a lattice gas model, we found that some structural differences between these 2 viruses may be crucial to explain the different ways in which they are affected by water menisci forces confined at the nanoscale.

By using a lattice-gas model, we report numerical simulation for the action of capillary forces of water confined at the nanoscale during desiccation of viruses. Results are compared with structural effects of desiccation measured by Atomic Force Microscopy on individual viruses of the bacteriophage ϕ 29 and the minute virus of mice (MVM).Structural integrity is found for theMVM, but not for the ϕ 29. Numerical simulations show that in the desiccation process, the meniscus shape formed inside the capsids strongly depends on the virus symmetry. This suggests that capillary forces could play a key role on the explanation of the different measured collapse processes (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)