A microfluidic viscometer assembly, the assembly comprising: a) a microfluidic cartridge comprising at least two microfluidic circuits , each microfluidic circuit comprising: i) an inlet ; ii) at least one microfluidic channel comprising: a first channel section having a first diameter and an end in direct fluidic communication with the inlet, and a second channel section having an optically clear channel subsection, or being completely optically clear, wherein the second section is in direct fluidic communication with the first section at an end opposite the inlet , and iii) an outlet in fluid communication with the second section at an end opposite the first channel section , wherein the first diameter of the first channel section is less than the second diameter of the second channel section such that the first channel section provides a resistance to a fluid traversing said microfluidic channel that is at least 80% and less than 100% of a combined resistance of the first and second channel sections, and/or of the microfluidic circuit ; b) an image recording system optically connected to the optically clear channel section of the at least two microfluidic circuits and configured to image a fluid-air interface between said fluid and air within the at least two microfluidic circuits ; and c) a pressure control unit configured to deliver fluid into each of the at least two microfluidic circuits . a) a microfluidic cartridge comprising at least two microfluidic circuits , each microfluidic circuit comprising: i) an inlet ; ii) at least one microfluidic channel comprising: a first channel section having a first diameter and an end in direct fluidic communication with the inlet, and a second channel section having an optically clear channel subsection, or being completely optically clear, wherein the second section is in direct fluidic communication with the first section at an end opposite the inlet , and iii) an outlet in fluid communication with the second section at an end opposite the first channel section , wherein the first diameter of the first channel section is less than the second diameter of the second channel section such that the first channel section provides a resistance to a fluid traversing said microfluidic channel that is at least 80% and less than 100% of a combined resistance of the first and second channel sections, and/or of the microfluidic circuit ; i) an inlet ; ii) at least one microfluidic channel comprising: a first channel section having a first diameter and an end in direct fluidic communication with the inlet, and a second channel section having an optically clear channel subsection, or being completely optically clear, wherein the second section is in direct fluidic communication with the first section at an end opposite the inlet , and a first channel section having a first diameter and an end in direct fluidic communication with the inlet, and a second channel section having an optically clear channel subsection, or being completely optically clear, wherein the second section is in direct fluidic communication with the first section at an end opposite the inlet , and iii) an outlet in fluid communication with the second section at an end opposite the first channel section , b) an image recording system optically connected to the optically clear channel section of the at least two microfluidic circuits and configured to image a fluid-air interface between said fluid and air within the at least two microfluidic circuits ; and c) a pressure control unit configured to deliver fluid into each of the at least two microfluidic circuits . <EOS>