An optical flow diagnostic technique was developed to detect surface flow direction in weakly separated flows. The technique makes use of thermochromic liquid crystals applied to a surface that is first coated with flat-black paint. A thin reflective material is affixed to the test surface. The surface is heated uniformly from above with infrared heaters. The reflective surface remains cool relative to the nearby black surface. A thermal tuft of relatively cool air (having just passed over the reflector) is advected in the direction of the surface flow thereby creating a “comet tail” of liquid crystal color response. The “comet tail” points in the direction of the surface flow. The “cool” thermal tuft successfully provides a means for detecting surface flow direction. The advantages of this method are the following: it is sensitive enough to detect the presence of weakly separated flows; it is non-obtrusive; it is reversible so does not require clean-up between runs; and it can be applied to vertical surfaces because there is nothina to drip.