Effect of Grafted Oligopeptides on Friction

Frictional and normal forces in aqueous solution at 25 °C were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of either glutamine, leucine, glutamic acid, lysine, or phenylalanine and one heteropolymer were each “grafted from” an oxidized silicon wafer using microwave-assisted solid-phase peptide synthesis. The peptide films were characterized using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Frictional force measurements showed that the oligopeptides increased the magnitude of friction compared to that on a bare hydrophilic silicon wafer but that the friction was a strong function of the nature of the monomer unit. Overall we find that the friction is lower for more hydrophilic films. For example, the most hydrophobic monomer, leucine, exhibited the highest friction whereas the hydrophilic monomer, polyglutamic acid, exhibited the lowest friction at zero load. When the two surfaces had opposite charges, there was a strong attraction, adhesion, and high friction between the surfaces. Friction for all polymers was lower in phosphate-buffered saline than in pure water, which was attributed to lubrication via hydrated salt ions

Flow of Water Adjacent to Smooth Hydrophobic Solids

The boundary flow condition for water near a hydrophobic plate was determined by measuring the force acting on a hydrophilic sphere as it was driven toward a hydrophobic plate in water. Comparison to the theoretical forces for different boundary conditions showed that the measured force was weaker than expected for the no-slip boundary condition and consistent with partial slip with a magnitude of tens of nanometers on the hydrophobic solid. The slip length increased steeply when the water contact angle on the solid increased above 90°, with the most hydrophobic plate (terminated with a perfluorinated alkane) having a contact angle of about 120° and a fitted slip length in the range 40–100 nm. The latter was fitted assuming that the hydrophilic solid retained zero slip length. Analysis of a probe driven by thermal oscillations produced similar results. The sharp increase in slip length when the water contact angle is above 90° has been previously predicted, but the magnitude is much greater than in previous measurements or simulations

Surface Topography Hinders Bacterial Surface Motility

We demonstrate that the surface motility of the bacterium, Pseudomonas aeruginosa, is hindered by a crystalline hemispherical topography with wavelength in the range of 2–8 μm. The motility was determined by the analysis of time-lapse microscopy images of cells in a flowing growth medium maintained at 37 °C. The net displacement of bacteria over 5 min is much lower on surfaces containing 2–8 μm hemispheres than on flat topography, but displacement on the 1 μm hemispheres is not lower. That is, there is a threshold between 1 and 2 μm for response to the topography. Cells on the 4 μm hemispheres were more likely to travel parallel to the local crystal axis than in other directions. Cells on the 8 μm topography were less likely to travel across the crowns of the hemispheres and were also more likely to make 30°–50° turns than on flat surfaces. These results show that surface topography can act as a significant barrier to surface motility and may therefore hinder surface exploration by bacteria. Because surface exploration can be a part of the process whereby bacteria form colonies and seek nutrients, these results help to elucidate the mechanism by which surface topography hinders biofilm formation

Phase State of Interfacial Nanobubbles

Interfacial nanobubbles represent an interesting state of matter: tiny bubbles decorating the interface between a solid and liquid. Yet, there is only sparse evidence supporting the idea that they are indeed gaseous. Here we present evidence that nanobubbles are composed of air rather than oil and that the solid surface underneath the nanobubbles is exposed to air rather than water. The differentiation between air and water was achieved by creating a sensor on the solid surface. The solid was coated in a hydrophobic monolayer containing dansyl fluorophores, which acts as a reporter for the environment around the fluorophore: at an excitation wavelength of 340 nm the emission maximum is 515 nm under water and 480 nm in dry or humid air. The difference in emission could thus be used to determine whether individual parts of the monolayer were in air or in water. Interfacial nanobubbles, created using the standard technique of ethanol exchange, were imaged with fluorescence microscopy, interference contrast microscopy, and phase contrast microscopy. Results show that the positions of nanobubbles shown by interference contrast microscopy or phase microscopy coincide with air pockets shown by fluorescence emission, thereby demonstrating that the interfacial nanobubbles are indeed bubbles. Differentiation between air and oils was achieved by absorption of a fluorescent dye, Nile blue. Whereas oils absorb Nile blue, the interfacial nanobubbles do not absorb the dye and therefore are not composed of oil

Scanning electron microscope (SEM) images of films fabricated from the 1, 2, 4, and 8 μm diameter silica spheres.

<p>The small bridges between the particles are evident in the images of the 4 and 8 μm particles. These bridges, together with similar bridges to the solid, stabilize the film so that it is unaffected by exposure to a stirred solution, rinsing etc.</p

Photographs of <i>C</i>. <i>albicans</i> grown in (RPMI) media for 24 hours, 37°C on test solids as indicated.

<p>Photographs of <i>C</i>. <i>albicans</i> grown in (RPMI) media for 24 hours, 37°C on test solids as indicated.</p

Atomic Force Microscope image of test surface consisting of 1 μm diameter silica spheres adhered to a PDMS solid.

<p>The entire sample is coated in a very thin layer of silica.</p

Colloidal Crystals Delay Formation of Early Stage Bacterial Biofilms

The objective of this work was to examine whether close-packed spheres of polystyrene (colloidal crystals) could be used to delay the development of biofilms. We examined early stage biofilm formation of <i>Pseudomonas aeruginosa</i> after 2 days on a flat sheet of polystyrene and on the same solid coated in polystyrene spheres of 450 or 1500 nm diameter. All materials were coated in fetal bovine serum to enable comparison of the effects of different surface curvature while maintaining constant surface chemistry. After 2 days, fluorescence imaging showed that the volume of bacterial colonies was much smaller on the 1500 nm colloidal crystals than on the flat film. In addition, electron microscopy showed that the area covered by structures containing more than one layer of bacteria was significantly reduced on both the 450 and 1500 nm colloidal crystals compared to the flat sheet. This provides proof of concept of biofilm inhibition of a pathogen by a simple nonchemical coating that may find future application in reducing the incidence of infections. Even though the density of adhered bacteria on 450 and 1500 nm was similar after 1 day, biofilm formation after 2 days was delayed more on the 1500 nm spheres than on the 450 nm spheres. We also observed that bacteria have preferred adsorption sites on the 1500 nm colloidal crystals and that cell bodies were often separated. This leads us to hypothesize that the greater spacing between favorable sites on the 1500 nm colloidal crystal hindered the early stage biofilm formation by separation of cell bodies