To capture a smooth, silky effect on flowing water in broad daylight, what external filter type must be used in conjunction with a slow shutter speed on the iPhone?

To capture a smooth, silky effect on flowing water in broad daylight using a slow shutter speed on an iPhone, a Neutral Density (ND) filter must be used. A slow shutter speed is the duration for which a camera's sensor is exposed to light, allowing moving subjects like water to blur into a silky texture over time. However, in broad daylight, there is an abundance of light. If a slow shutter speed is used in such bright conditions without intervention, the camera sensor will collect too much light, resulting in an overexposed image, which appears completely white or washed out. The purpose of a Neutral Density filter, often referred to as an ND filter, is to reduce the overall amount of light entering the camera lens without altering the color or contrast of the scene. It acts like sunglasses for the camera, uniformly blocking a specific amount of light. By reducing the light intensity, the ND filter allows the iPhone's camera to maintain a significantly slower shutter speed than would otherwise be possible in bright sunlight, preventing overexposure while simultaneously enabling the long exposure needed to achieve the desired smooth, silky motion blur of the flowing water.