Applications of stimuli-responsive functional colloids

Colloidal dispersions of particles have an exceptionally long history across a wide range of formulated products. More recently, the use of environmentally-responsive polymers as a key component of the colloidal particles used in various formulations, whether in the core, on the surface or both, has opened up an additional range of novel applications that we review here. We initially briefly introduce the different types of particles and give examples of the range of responsive polymers used for their preparation. Promptly, we focus our attention on describing a broad range of applications for these systems ranging from oil recovery, emulsifiers and sensors to drug delivery

Dispersion polymerization in non-polar solvent: Evolution toward emerging applications

Currently, there is a resurgence of interest in the preparation of monodisperse, size-controlled latex particles in non-polar solvents by dispersion polymerization. This technique has great potential for manufacturing designed latex particles for emerging applications such as the use of latex particles in electrophoretic displays, where one of the numerous requirements is that the particle systems be suspended in low dielectric constant, non-polar solvents. This article reviews the academic literature around the typical monomers used in non-polar dispersion polymerization. It briefly introduces the origin of the technique and the initial seminal work carried out in this area. It also describes how such particles have been used in the past as model colloids for academic purposes and provides recent examples where dispersion polymerization is used to create novel functional particles. Subsequently, the article provides a thorough knowledge basis for each monomer used in non-polar dispersion polymerization, with a focus on the evolution of the technique, including progress in controlling the final particle characteristics and in designing novel effective stabilizers. Finally, a brief review on the use of the technique to prepare well-controlled latex particles in supercritical fluids is presented

Effect of film thickness and particle size on cracking stresses in drying latex films

The stress at which latex films crack during drying was investigated using beam bending. Two systems were investigated: (i) poly(methyl methacrylate/butyl acrylate) particles cast as thin films to examine the effect of film thickness on cracking film stress and (ii) polystyrene particles dried as drops to investigate the effect of particle size. Results indicated an inverse relationship between film thickness and film stress, whilst film stress was shown to be independent of the original particle size. These outcomes were in good agreement with Tirumkudulu and Russel’s theoretical analysis [M.S. Tirumkudulu and W.B. Russel, Langmuir 21 (2005) 4938], albeit the measured stress values were almost twice the theoretical estimation

Colloidal suspension rheology and inkjet printing

This work reports the first systematic survey of colloidal suspension jetting [1], as opposed to dripping liquids containing particles [2], and it complements a previous survey of the jetting of complex fluids [3]. Colloidal suspensions of stabilised polystyrene particles in water/ethylene glycol were formulated for maximum stable loadings (vol%) and low poly-dispersity index (PDI), for a range of spherical particle sizes (80 nm to 850 nm). Each preparation batch was characterised using squeeze mode rheometry [4] and filament stretching devices [5, 6] while being independently assessed using drop-on-demand (DoD) inkjet printing from MicroFab nozzles with either 30 μm or 80 μm diameter. Nozzle blocking was reduced for the jetting tests by maintaining a 100 Hz printing frequency throughout waiting periods. Additional experiments used a transparent containment chamber around the 30 μm nozzle exit to examine jetting behaviours that might be caused by the humidity level. Jetting for each batch (characterised by colloidal particle size, vol%, nozzle size, etc.) was considered successful if high speed videos used for measurements of drop speed and determination of the jet break-off time from nozzle meniscus were reliably and consistently achieved at several drive voltages. Jetted drop speeds for all the colloid suspensions tested showed a linear dependence on drive voltage above a threshold voltage as previously reported for Newtonian and weakly elastic drop speeds [7]. Mapping of successful DoD jetting as a function of colloidal particle size (nm) and vol% for 80 μm (30 μm) nozzle diameter reached 37 vol% (30 vol%) without any evidence for any spherical 80-850 nm (300-850 nm) particle size effect on jetting. The rheology of these colloidal suspensions, obtained independently from jetting, exhibits rather Newtonian behaviour with a range of viscosities within a factor of 2. Likewise, the filament stretching experiments that are sensitive to non-linear effects such as relaxation time [5, 6] could not discriminate between solvent and suspensions. Beyond issues with blocking (and stability), colloidal suspensions were jetted easily, in line with expectations based on the measured rheology and low nonlinear effects

Jetting of complex fluids

Recent results from a number of UK academic inkjet research studies advance the understanding of complex fluid jetting behavior and may be of interest to the wider digital fabrication community for the enhancement of inkjet printing applications