Molecular Pressure Sensors

Abstract

Flow properties of molecularly thin films are at the foundation of many practical applications such as lithography, microfluidics, coatings, and lubrication. [1][2][3][4][5][6][7][8][9][10] Further advances in these fields depend on understanding the mechanisms that control the kinetics of flow. [11][12][13][14] However, one of the problems in flowing monolayers is the independent characterization of the driving and frictional forces that are intimately coupled through the molecular interactions between the fluid and the substrate. In this regard, the visualization of compressible macromolecules during flow provides an exceptional opportunity to study these forces.[15] Here we report on the monitoring of brush-like macromolecules as they change their shape in response to variations in the film pressure during flow. After appropriate calibration, these molecular sensors can be used to gauge both the pressure gradient and the friction coefficient at the substrate. We anticipate the utilization of such miniature sensors for probing flow properties on nanometer length scales. The design of the pressure-responsive macromolecules is based on brush-like polymer architectures comprised of a flexible backbone surrounded by a dense shell of side chains (Fig. 1a). The characteristic property of these macromolecules is their ability to change shape upon lateral compression on a substrate. [16][17][18] If the film pressure increases, the number of side chains adsorbed to the surface decreases allowing the backbone to coil. This causes the macromolecules to become more compact and occupy less area on the substrate. Therefore, the area per molecule can be used as a pressure sensitive parameter to gauge the variations of film pressure within flowing monolayers. Molecular brushes (Fig. 1a) with the same degree of polymerization of a poly(2-hydroxyethyl methacrylate) backbone (n = 570 ± 50) and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains (n = 35 ± 5 and n = 51 ± 5) were synthesized by atom transfer radical polymerization. [19,20] At room temperature, the materials are fluid melts that spontaneously spread when placed on higher surface energy substrates such as mica and graphite.[21] Small drops of molecular brushes (volume ∼ 1 nl, radius ∼ 100 lm) were deposited on a substrate inside an environmental chamber under controlled temperature (T = 25°C) and relative humidity (RH = 30-99 %). Like many other fluids, the drops first spread by generating a molecularly thin precursor film moving ahead of the macroscopic drop (Fig. 1b). [22] Using an atomic force microscope (Multimode, Nanoscope 3A, Veeco Metrology Group), we monitored the spreading process over a broad range of length scales ranging from the motion of the film front all the way down to the movements of the individual molecules within the film.[15,23] Figure 1c shows the time dependence of the film length L= R -R 0 observed on mica at a high relative humidity (RH = 99 %), where R 0 = 63 lm is the initial drop radius and R is the total radiu...

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