was observed within the channel when the height H was below 20 µm. This was indicated by a high OD value, resul�ng from a dark background in the images. Contrarily, when the aspect ra�o 𝜂𝜂 = 𝐻𝐻/𝑊𝑊 exceeded 0.67, significantly lower OD values were observed, which were atributed to the bright birefringence signal falling within the range of 0.7 to 0.85. This revealed the disrup�on of the pris�ne homeotropic alignment over a large distance. A similar effect was also observed in the corresponding experiment where we expanded the width (W) while maintaining a constant height (H) of 30 µm (right). The mean OD remained in the range of 0.5 to 0.6 when W was below 50 µm but increased two to three-fold once the width exceeded this threshold. The findings from this second experiment further verified that a PDMS channel with η > 0.60 was expected to induce planar monodomains. The same conclusions were reached from the computer simula�ons by plo�ng the Full Width at (intensity) Half Maximum (FWHM) as a func�on of the aspect ra�o (Figure ). For η < 0.75 the FWHM/W increased sharply, indica�ng the existence of a region with pris�ne alignment and no intensity. Above this threshold, the FHWM/W dropped asympto�cally to 0, implying planar alignment. We believe that this threshold matches the experimentally obtained cri�cal value of η = 0.60. The remaining differences mainly result from uncertain�es in the thermotropic and elas�c parameters of the Landau de Gennes poten�al, as well as from the frequency dependence of the ordinary and extraordinary refrac�ve indices.