Convective Patterns in Solution‐Casted Films from Acylated Hyaluronan

Abstract

The evaporation of thin solution layers is an important process taking place in fields such as distillation, coating, or thin film manufacturing. Under suitable conditions, evaporation can induce convective flow by creating gradients in surface tension (Bénard–Marangoni instability) or density. These phenomena have been studied by many authors, but detailed and illustrative visualizations of real experiments are surprisingly scarce compared to simulation results and drawn schemes. Therefore, detailed and representative visualizations of regular Bénard–Marangoni convective cell patterns are provided in solution‐casted films from acylated hyaluronan. The visualizations agree with predictions made in the literature, thus providing important experimental validation. Most importantly, structures resulting from both horizontal and vertical convective flow are clearly visible. In a subsequent parametric study, the effects of selected parameters on the size of the convective cells are elucidated. The results suggest that the cell size increases with increasing solution layer height and increasing polymer concentration, while no statistically significant effect is observed for the degree of hyaluronan substitution.

Transient Three-Dimensional Flow Field Measurements by Means of 3D µPTV in Drying Poly(Vinyl Acetate)-Methanol Thin Films Subject to Short-Scale Marangoni Instabilities

“…Solutally induced Marangoni convection was investigated theoretically in various configurations [ 15 , 26 , 27 , 28 , 29 , 30 ]. Available experimental work on convective Marangoni instabilities in drying polymer films appear to be governed primarily by a deliberate use of Marangoni convection as means for self-assembly [ 18 , 31 , 32 , 33 ], including the transition between different modes of instability and lateral patterns of convection cells [ 17 , 18 , 34 , 35 , 36 , 37 , 38 , 39 ]. A quantitative assessment of the stability threshold of solutally induced Marangoni convection in drying polymer films based on experimental findings, as reported in [ 17 , 18 ], can be found but appears to be less frequent.…”

“…Although the authors did not provide viscosity values, the dip-coating process implies a rather low initial viscosity [ 40 , 41 ]. Other authors reported less regular cells with different polygonal cells and large lateral size distributions having a standard deviation in the order of to [ 17 , 18 , 37 , 39 ]. In addition, it was found that the lateral cell size increased with increasing initial polymer concentration [ 32 , 39 ] and increasing initial film thickness [ 17 , 18 , 39 ].…”

“…Other authors reported less regular cells with different polygonal cells and large lateral size distributions having a standard deviation in the order of to [ 17 , 18 , 37 , 39 ]. In addition, it was found that the lateral cell size increased with increasing initial polymer concentration [ 32 , 39 ] and increasing initial film thickness [ 17 , 18 , 39 ]. Minarik et al investigated the impact of different molecular weight of 2-hydroxyethyl cellulose in aqueous solutions and found that for low molecular weight and narrow distributions fairly regular polygonal cells emerged, whereas for large and broad distributions very irregular patterns were found.…”

“…Experimental data on the stability threshold of Marangoni instabilities in drying polymer films are, however, scarce. Several authors have acknowledged this in the past [ 39 , 43 , 44 ]. In a recent comprehensive review on multiphase Marangoni convection, Wang et al explicitly stress the need for new quantitative experiments regarding convective instabilities in drying polymer films [ 44 ].…”

Critical Solutal Marangoni Number Correlation for Short-Scale Convective Instabilities in Drying Poly(vinyl acetate)-Methanol Thin Films

“…Existing experimental work regarding convective instabilities in drying polymeric films appears to be governed primarily by deliberate patterning, utilizing Marangoni convection as a means for self-assembly [57][58][59][60]. The transition between different modes of instability and lateral size and form of convection cells was also investigated [58,59,[61][62][63][64][65][66][67]. When thermal and solutal gradients arise simultaneously, several authors conclude that the solutal effect is dominant [48,49,68].…”