C# Solution

public static string encryption(string s)
{
    s.Replace(" ", string.Empty);
    string encryptedString = string.Empty;
    int columns = (int)Math.Ceiling(Math.Sqrt(s.Length));

    for(int i = 0; i < columns; i++)
    {
        for(int j = i; j < s.Length; j += columns)
            encryptedString += s[j];

        encryptedString += " ";
    }
    return encryptedString;
}