A dual-trap optical tweezer approach to study emulsion droplet interactions
Authors:   , Griffiths Marjorie 
Published: November 2021

<p>Foods are a highly complex form of soft condensed matter. Their complexity arises from a number of interrelated factors including the natural heterogeneity of raw materials, intricate composition, and the subtle changes in molecular interactions and micro-structural arrangements dictated by food processing and storage. It is highly important to understand the forces dictating the food structure as the assembly and organisation of major structural entities (i.e biopolymers, droplets, bubbles, networks, and particles) are responsible for the foods stability, texture, flow properties and more inclusively their organoleptic properties.  The structural entities of foods exhibit numerous forms of self-organization and have significant structure complexity and dynamic behaviour on the mesoscopic length scales from 10 to 1000 nanometres. These dynamic weak interactions between the constituents define the organized state that ranges from simple spatial or temporal ordering to more intricate interactions making up the food microstructure. These interactions are often small in magnitude and are short ranged making them difficult to measure directly. Very few studies have been carried out on direct force measurements in foodstuffs.  The focus of this research was to develop a dual-trap optical tweezer method to directly measure interactions between micrometre colloidal particles and ultimately to design an apparatus where interactions between less homogeneous systems, such as emulsion droplets could directly measured as a function of separation. As the name suggests, optical tweezers provide the ability to control the position of particles using a focused laser beam. The general concept of this method is to immobilise two particles in two separate optical traps and step one particle closer to the other stationary particle in a controlled fashion. The droplet’s movement is then recorded using a high-speed camera that provides near-to-real-time images of the particle’s positions. The particle’s positions are determined by a 3-D tracking algorithm developed in-house which determines the position of both particles to a precision of sub-pixel accuracy. The force exerted on each droplet (by the other one) can be extracted as it is proportional to the trap strength (pN/μm) and the displacement of the particle from the centre of the optical trap (μm).  To demonstrate the optical tweezer method,the interactions between silica beads of a known size were measured as a function of bead separation. The measured force-distance curves agreed with the electrostatic component of the DLVO theory. Once the method was established it was applied at increasing salt concentrations (decreasing Debye lengths). Interestingly, a salt concentration was found beyond which the experimental data no longer agreed with the predictions of DVLO theory. Above 100 μM sodium chloride the Debye length was reduced to less than the Brownian fluctuations of the particles in the traps, which then dominated the apparent repulsion by restricting their particle trajectories, masking the actual nature of the electrostatic interactions. This resulted in force curves which fitted the exponential function, however, the fitted decay constant bore no resemblance to the actual Debye length. A diffusion experiment was designed to demonstrate the ability to measure interactions in multiple environments using the same pair of beads (at low salt concentrations where Debye lengths are faithfully recovered). The evolution of force-displacement curves was measured as the local salt concentration changed owing to the diffusion of salt from the interface and the results obtained were shown to agree with predictions based on a standard diffusion formalism.  Applying the dual-trap optical tweezers method, successfully demonstrated with silica beads, to less homogeneous systems such as emulsion droplets presented challenges which showcased that emulsion design was critical as certain criteria had to be met in order to facilitate undertaking the tweezer experiments. These criteria include particle size (1-3 μm ), low polydispersity, and a reasonable refractive index mismatch between the droplet and continuous phase. In keeping with food systems a protein stabilised oil-in-water emulsion was chosen. Two popular emulsifiers, sodium caseinate and β-lactoglobulin, were investigated at different ionic strength, pH and homogenisation pressures and phase volumes. The emulsion chosen for direct force measurements was a sodium caseinate emulsion when prepared in a 100 mM phosphate buffer at pH 7.0, 60 wt. % soya bean oil and 0.04 wt.% protein which provided an adequate droplet size with minimal polydispersity.  Interactions between pairs of sodium caseinate emulsion droplets were measured. Unlike for silica beads, the individual droplet size needed to be measured to deter- mine the surface-to-surface separation of droplet pairs. The droplet’s diameter was determined by measuring the restricted diffusion of the droplet in a weak optical trap and fitting the short time mean squared displacement behaviour to a Brownian motion simulation. It was found that the droplet size can be determined in this fashion to within 50 nm.  Moving forward, the interactions between pairs of emulsion droplets were measured in water using the same method gleaned from the silica bead interaction study. The experimental data fitted well to the electrostatic force described by the DLVO theory with reasonable ζ-potentials extracted. To further demonstrate this dual optical tweezer method, interactions between the same pair of droplets were measured at increasing NaCl concentrations by means of diffusion. The expected trend has found to agree from calculations of increased local salt concentration based on a diffusion equation. At salt concentrations above 100 μm significant deviations in the force-curves were observed that may signal salt induced changes of the droplet’s interface or be attributed to the small magnitude of the force being within the noise. This warrants further investigation.  In conclusion, the dual-trap optical tweezers have shown incredible potential to become a robust method to measure the interactions between droplets. This method has some clear advantages over current methods including that force, and spatial resolution is superior, sample preparation is straightforward, forces are measured in 3-dimensions, and the droplets are free in solution during measurement, not wetted on surfaces. Accordingly, dual-trap optical tweezer methodology has provided the ability to measure interactions to a precision that has not yet been achieved by any other method for the study of emulsion systems, which in itself is a major achievement. This method is another tool in the toolbox of a colloid chemist, food scientist and physicists to probe interactions in soft materials.</p>