Jason N. Connor

Hydrodynamic forces involving deformable interfaces at nanometer separations

A model is developed to describe the dynamic forces acting between two deformable drops, or between one drop and a solid surface, when they are in relative axisymmetric motion at separations of ≲100nm in a Newtonian liquid. Forces arise from hydrodynamic pressure in the draining liquid film that separates the interfaces and from disjoining pressure due to repulsive or attractive surface forces. Predictions of the model are successfully compared with recent experimental measurements of the force between two micrometer-scale surfactant stabilized decane drops in water in an atomic force microscope [S. L. Carnie, D. Y. C. Chan, C. Lewis, R. Manica, and R. R. Dagastine, Langmuir 21, 2912 (2005); R. R. Dagastine, R. Manica, S. L. Carnie, D. Y. C. Chan, G. W. Stevens, and F. Grieser, Science 313, 210 (2006)] and with subnanometer resolution measurements of time-dependent deformations of a millimeter-scale mercury drop approaching a flat mica surface in a modified surface force apparatus [J. N. Connor and R. G. Horn, Faraday Discuss. 123, 193 (2003); R. G. Horn, M. Asadullah, and J. N. Connor, Langmuir 22, 2610 (2006)]. Special limits of the model applicable to small and moderate deformation regimes are also studied to elucidate the key physical ingredients that contribute to the characteristic behavior of dynamic collisions involving fluid interfaces.A model is developed to describe the dynamic forces acting between two deformable drops, or between one drop and a solid surface, when they are in relative axisymmetric motion at separations of ≲100nm in a Newtonian liquid. Forces arise from hydrodynamic pressure in the draining liquid film that separates the interfaces and from disjoining pressure due to repulsive or attractive surface forces. Predictions of the model are successfully compared with recent experimental measurements of the force between two micrometer-scale surfactant stabilized decane drops in water in an atomic force microscope [S. L. Carnie, D. Y. C. Chan, C. Lewis, R. Manica, and R. R. Dagastine, Langmuir 21, 2912 (2005); R. R. Dagastine, R. Manica, S. L. Carnie, D. Y. C. Chan, G. W. Stevens, and F. Grieser, Science 313, 210 (2006)] and with subnanometer resolution measurements of time-dependent deformations of a millimeter-scale mercury drop approaching a flat mica surface in a modified surface force apparatus [J. N. Connor and R. G. ...

The influence of surface forces on thin film drainage between a fluid drop and a flat solid

An experiment is described in which a mica surface is driven towards a mercury drop immersed in aqueous electrolyte. Under appropriate conditions, hydrodynamic pressure in the aqueous film creates a classical dimple in the mercury drop. The use of optical interferometry and video recording to monitor the shape of the drop and the thickness of the aqueous film with sub-nanometre resolution yields a high density of precise data showing the formation and evolution of the dimple as the film drains. Variation of electrical potential applied to the mercury phase allows control of the surface forces acting between the drop and the mica surface, so that the effect of surface forces on the film drainage process is highlighted. It is found that the film thickness at the centre of the dimple and the lateral extent of the dimple are not significantly affected by surface forces. On the other hand, the minimum film thickness at the edge of the dimple is sensitive even to weak surface forces. Since this minimum film thickness is a major determinant of the film drainage rate, it is shown that surface forces have an important effect on the overall drainage process.

RootGraph: a graphic optimization tool for automated image analysis of plant roots

Highlight The method presented analyses root scans automatically, distinguishes primary from lateral roots, and quantifies a broad range of traits for individual primary roots and their associated lateral roots.

The effect of surface and hydrodynamic forces on the shape of a fluid drop approaching a solid surface

This paper describes an experiment designed to measure surface and hydrodynamic forces between a mercury drop and a flat mica surface immersed in an aqueous medium. An optical interference technique allows measurement of the shape of the mercury drop as well as its distance from the mica, for various conditions of applied potential, applied pressure, and solution conditions. This enables a detailed exploration of the surface forces, particularly double-layer forces, between mercury and mica. A theoretical analysis of drop shape under the influence of surface forces shows that deformation of the drop is a sensitive indicator of the forces, as well as being a very important factor in establishing the overall interaction between the solid and the fluid.

Elasticity of liquid marbles.

Liquid marbles are liquid droplets covered densely with small particles. They exhibit hydrophobic properties even on hydrophilic surfaces and this behaviour is closely related to the Cassie wetting state and the phenomenon of superhydrophobicity. Typical liquid marbles are of millimetre size but their properties are analogous to smaller capsules and droplets of Pickering emulsions. We study water marbles covered with an uneven multilayer of polyethylene particles. Their elastic properties were assessed under quasi-static conditions. The liquid marbles are highly elastic and can sustain a reversible deformation of up to 30%. The spring constant is of the same order of magnitude as that for bare water droplets. Therefore the elasticity of the liquid marble is provided mainly by the liquid menisci between the particles. Upon further compression, the spring constant increases up to the point of breakage. This increase may be due to capillary attraction acting across the emerging cracks in the particle coating. The stress-strain curve for liquid marbles is similar to that obtained with liquid-filled microcapsules. A mechanical scaling description proposed for capsules is qualitatively applicable for liquid marbles. The exact mechanical role of the multilayer particle network remains elusive.

Extending the surface force apparatus capabilities by using white light interferometry in reflection

An important factor in the success of the surface force apparatus (SFA) in measuring interactions between surfaces over nanometer separations has been the optical interference technique used to measure the surface separation. Until recently, this technique has only been used when both of the materials are transparent. As a result, thin sheets of mica have been the material of choice. We describe a simple method to extend the capabilities of the SFA so that a wide variety of material surfaces can be studied while retaining an optical measurement technique. The key to this technique is to modify the optics so that reflected, rather than transmitted, light is used to produce the interference pattern. Now, only one material is required to be thin and transparent while the other can be any material providing it is at least partially reflective. To succeed with this technique, it is necessary to maximize the visibility of the interference fringes. This is achieved by optimizing the thickness of a partially reflective coating (often silver) deposited on the back side of the transparent material.

Soft matter: from shapes to forces on the nanoscale

Soft matter deforms in response to imposed external forces. Here we demonstrate how dynamic surface forces are linked to far-field deformations. This offers a new paradigm for determining forces between soft particles in colloidal systems. The particular example we use to illustrate this concept is that of a fluid drop interacting with a solid wall through hydrodynamic drainage flow coupled with repulsive or attractive dissimilar electrical double layer interactions. The force can be deduced from a simple analysis of the drop surface geometry outside the interaction zone.

High-throughput 3D reconstruction of plant shoots for phenotyping

In this paper we report on development and analysis of a high throughput 3D reconstruction set-up for reconstructing cereal plants grown in pots for their phenotypic analysis. We are motivated with the idea of accurate and high-throughput reconstruction of shoots. We have developed a turntable setup for high-throughput and high accuracy 3D reconstruction of plant shoots, where the plants stays stationary and the camera moves on a circular path to take multi-view digital images. The leaves of cereal plants like wheat, barley, corn, etc. are very delicate and their movement during imaging causes errors in the volumetric reconstruction of the shoots. The synchronization of the turntable and camera has been designed by us. A novel and easy to use camera calibration for single axis motion is designed and implemented. Visual hull algorithm has been used for 3D reconstruction. We analyse results of reconstruction for three different modes of image acquisition against ground truth with respect to time taken for imaging and the number of images used. In results we obtain phenotypic quality 3D volumetric reconstruction with an acquisition time of less than a minute per potted plant.