Elucidating the Mechanism of Lubrication for Sliding Droplets:

Hydrodynamics, Surface Forces,

and the Role of Surfactants and Polymers





Project operates in collaboration between:

Project Context and Objectives

Oily liquid droplets suspended in water represent a large number of natural and industrial products and materials. The properties of these colloidal systems, termed emulsions, rely on many physicochemical characteristics, such as oil/water ratio, relative viscosity of the oil and water phases, the presence of polymers and surfactants, and solution properties like pH and ionic strength. These characteristics can be manipulated to produce the desired properties relevant for the target application. Many of the applications of emulsions are dependent on the interaction of the oily droplets with solid surfaces, whether those surfaces are metals, metal oxides, skin, or hair. Of the many areas of formulation for which emulsions are used, their role as lubricants is central to food products and personal care products, in addition to engineering applications such as metalworking fluids. Flow properties, emulsion stability, oil film formation, and texture perception are all important features of the interaction between oil droplets and solid surfaces in these applications.

The project was designed with three objectives in mind: (i) altering hydrodynamics of sliding droplets through manipulation of surface forces; (ii) studying the influence of adsorbed surfactant and polymer layers on droplet attachment and sliding; (iii) bridging the gap to biological systems to look at cell attachment and sliding on surfaces. By combining these three objectives, the desire was to go from physicochemical modification of surface forces and sliding for simple droplet systems, to practical application of the new knowledge for cell flow and adhesion within microfluidic devices. The methodology to be used included atomic force microscopy of droplet-surface interactions (lateral and normal), spectroscopy of oil-water interfaces, and microfluidic channel studies of droplet flow and wetting film properties.

Description of Work

A necessary precursor to the majority of studies planned was also included into the project plan. This involved the study of droplet attachment to solid surfaces, performed for free-rising and captive droplets in solutions of varying composition. The attachment and spreading of droplets on solid surfaces, and the dependence of these processes of solution conditions and the presence/absence of polymers and surfactants, is required to be studied before one can attempt measurements of sliding (i.e. not attaching) droplets. Two molecules know to stabilize droplets/emulsions were used: a stimulus responsive peptide surfactant (AM1), and a hydrophobically-modified dextrin (octenyl succinate anyhydride dextrin, trade name: Capsul).

The adsorption of the molecules to the oil and solid interfaces was studied using dynamic surface tension and quartz crystal microbalance, respectively. In addition adsorption to these interfaces was characterized as a function of solution pH (for AM1) and solution ionic strength (for Capsul). The interaction of the droplets with solid surfaces was interrogated using high speed video microscopy and image processing, which allowed for the extraction of the time for liquid film rupture between the droplet and the surface, and for the dynamic dewetting (spreading of the oil droplet) for the droplet-surface attachment. These processes were found to be highly sensitive to the presence of surfactant or polymer at the solid surface, and at the oil droplet surface, and also strongly dependent on the pH and ionic strength of the solution through which the droplet interacted with the solid surface. Attempts were made to relate the droplet spreading to existing theories of dewetting (i.e. molecular kinetic theory).

For systems seen to be stable with respect to attachment and spreading, initial experiments on droplet flow in microchannels were performed (as planned in objectives (i) and (ii)). This involved the design and fabrication of custom microfluidic devices for the creation and surface modification of droplets, in addition to the controlled flow and monitoring of droplet velocity. These experiments were initiated toward the end of the first reporting period, and will continue into the second reporting period. Planned experiment with the atomic force microscope (objectives (i) and (ii)) did not take place in the first reporting period due to the need to complete the abovementioned studies prior to the commencement of normal and lateral force measurement for droplet systems. These experiments will now take place in the second reporting period.