The rheology of aqueous solutions of ethyl hydroxy-ethyl cellulose (EHEC) and its hydrophobically modified analogue (hmEHEC): extensional flow response in capillary break-up, jetting (ROJER) and in a cross-slot extensional rheometer

Abstract

Cellulose derivatives containing associating hydrophobic groups along their hydrophilic backbone are used as rheology modifiers in the formulation of water-based spray paints, medicinal sprays, cosmetics and printable inks. Jetting and spraying applications of these materials involve progressive thinning and break-up of a fluid column or sheet into drops. Strong extensional kinematics develop in the thinning fluid neck. In viscous Newtonian fluids, inertial and viscous stresses oppose the surface tension-driven instability. In aqueous solutions of polymers such as Ethyl Hydroxy-Ethyl Cellulose (EHEC), chain elongation provides additional elastic stresses that can delay the capillary-driven pinch-off, influencing the sprayability or jettability of the complex fluid. In this study, we quantify the transient response of thinning filaments of cellulose ether solutions to extensional flows in a Capillary Break-up Extensional Rheometer (CaBER) and in a forced jet undergoing break-up using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER). We also characterize the steady state molecular deformations using measurements of the flow-induced birefringence and excess pressure drop in an extensional stagnation point flow using a Cross-Slot Extensional Rheometer (CSER). We show that under the high extension rates encountered in jetting and spraying, the semi-dilute solutions of hydrophobically modified ethyl hydroxy-ethyl cellulose (hmEHEC) exhibit extensional thinning, while the unmodified bare chains of EHEC display an increase in extensional viscosity, up to a plateau value. For both EHEC and hmEHEC dispersions, the low extensibility of the cellulose derivatives limits the Trouton ratio observed at the highest extension rates attained (close to 10(5) s(-1)) to around 10-20. The reduction in extensional viscosity with increasing extension rate for the hydrophobically modified cellulose ether is primarily caused by the disruption of a transient elastic network that is initially formed by intermolecular association of hydrophobic stickers. This extensional thinning behavior, in conjunction with the low extensibility of the hydrophobically modified cellulose ether additives, makes these rheology modifiers ideal for controlling the extensional rheology in formulations that require jetting or spraying, with minimal residual stringiness or stranding.

Design and operation of a Rayleigh Ohnesorge jetting extensional rheometer (ROJER) to study extensional properties of low viscosity polymer solutions

“…The solvent quality factor, ν, is estimated as 0.588 for EHEC [13], and ν = 0.550 for PEO [5]). Knowing the solvent quality parameter allows us to extract the intrinsic viscosity via the Mark-Houwink-Sakurada equation,…”

“…The ROJER setup used was substantially similar to the designs by Sharma et al [13] and Keshavarz et al [12]. However, rather than using a stroboscopic technique a high speeed camera was used in order to observe the real-time break-up behaviour of particular sections of the jet.…”

“…An alternative approach that can potentially overcome these restrictions is the Rayleigh Ohnesorge Jetting Extensional Rheometer (ROJER), a relatively new technique developed by McKinley and colleagues [12,13] that measures the thinning liquid bridges of a continuous jet. The most significant advantage of the ROJER over CaBER is the reduction in the characteristic length scale, without the complication of imparted fluid inertia.…”

Extensional rheometry of cellulose ether solutions: flow instability

“…CaBER measurements are relatively time-efficient and require less sample volume (\1 ml) compared to the techniques such as capillary extrusion (Cogswell 2003), opposed jet flow (Patruyo et al 2002) and filament stretching rheometry (FiSER) (Bhardwaj et al 2007). It is noteworthy that Cross-slot rheometry also offers some of the abovementioned advantages of CaBER, and has been used for cellulosic fluids (Sharma et al 2015) and hyaluronic acid solutions (Haward et al 2013;Haward 2014). From the perspective of processability, breaking up of an initially stable fluid filament is a process which has commercial applications such as spraying and atomization of pesticides, applying paints and adhesives, coating and food processing operations e.g.…”

“…Boger fluids (Plog et al 2005;Campo-Deano et al 2011;Anna and McKinley 2001), and yield stress fluids (McKinley 2005a, b;Niedzwiedz et al 2009) CaBER measurements for a number of systems containing biopolymers and their derivatives in aqueous media have been reported in the past literature, most notably for xanthan (Stelter et al 2002), hydroxypropyl ether guar (Duxenneuner et al 2008), methyl hydroxyethyl cellulose (HEC) (Plog et al 2005), alginate (Storz et al 2010;Rodriguez-Rivero et al 2014), scleroglucan (Japper-Jaafar et al 2009), cellulose acetate ? PEO solutions (Chen et al 2008), cellulose in an ionic liquid 1-ethyl-3-methylimidazolium acetate (Haward et al 2012) (Michud et al 2015), DNA (Chevallier 2009), cornstarch-water suspension (Bischoff White et al 2010), galactomannan (Bourbon et al 2010), casein/waxy maize starch (Chan et al 2009) and dilute ethyl HEC solutions (Sharma et al 2015). In the present study, extensional rheology of HEC solutions has been investigated with the help of CaBER.…”